I

Executable and Linkable Format (ELF)Contents

Preface

1

OBJECT FILES

2PROGRAM LOADING AND DYNAMIC LINKING

3C LIBRARY

I

Index

Introduction
ELF Header
Sections
String Table
Symbol Table
Relocation

1-1
1-3
1-8
1-16
1-17
1-21

Introduction
Program Header
Program Loading
Dynamic Linking

2-1
2-2
2-7
2-10

C Library

3-1

Index

Tool Interface Standards (TIS)

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Portable Formats Specification, Version 1.1iELF: Executable and Linkable Format

ii

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)

Figures and Tables

Figure 1-1: Object File Format
Figure 1-2: 32-Bit Data Types
Figure 1-3: ELF Header
Figure 1-4: e_ident[ ] Identification Indexes
Figure 1-5: Data Encoding ELFDATA2LSB
Figure 1-6: Data Encoding ELFDATA2MSB
Figure 1-7: 32-bit Intel Architecture Identification, e_ident
Figure 1-8: Special Section Indexes
Figure 1-9: Section Header
Figure 1-10: Section Types, sh_type
Figure 1-11: Section Header Table Entry: Index 0
Figure 1-12: Section Attribute Flags, sh_flags
Figure 1-13: sh_link and sh_info Interpretation
Figure 1-14: Special Sections
Figure 1-15: String Table Indexes
Figure 1-16: Symbol Table Entry
Figure 1-17: Symbol Binding, ELF32_ST_BIND
Figure 1-18: Symbol Types, ELF32_ST_TYPE
Figure 1-19: Symbol Table Entry: Index 0
Figure 1-20: Relocation Entries
Figure 1-21: Relocatable Fields
Figure 1-22: Relocation Types
Figure 2-1: Program Header
Figure 2-2: Segment Types, p_type
Figure 2-3: Note Information
Figure 2-4: Example Note Segment
Figure 2-5: Executable File
Figure 2-6: Program Header Segments
Figure 2-7: Process Image Segments
Figure 2-8: Example Shared Object Segment Addresses
Figure 2-9: Dynamic Structure
Figure 2-10: Dynamic Array Tags, d_tag
Figure 2-11: Global Offset Table
Figure 2-12: Absolute Procedure Linkage Table
Figure 2-13: Position-Independent Procedure Linkage Table
Figure 2-14: Symbol Hash Table
Figure 2-15: Hashing Function
Figure 3-1: libc Contents, Names without Synonyms
Figure 3-2: libc Contents, Names with Synonyms
Figure 3-3: libc Contents, Global External Data Symbols

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iii

Preface
ELF: Executable and Linking Format
The Executable and Linking Format was originally developed and published by UNIX System Laboratories (USL) as part of the Application Binary Interface (ABI). The Tool Interface Standards committee(TIS) has selected the evolving ELF standard as a portable object file format that works on 32-bit Intel
Architecture environments for a variety of operating systems.
The ELF standard is intended to streamline software development by providing developers with a set ofbinary interface definitions that extend across multiple operating environments. This should reduce thenumber of different interface implementations, thereby reducing the need for recoding and recompilingcode.

About This Document
This document is intended for developers who are creating object or executable files on various 32-bit
environment operating systems. It is divided into the following three parts:
Part 1, ‘‘Object Files’’ describes the ELF object file format for the three main types of object files.
Part 2, ‘‘Program Loading and Dynamic Linking’’ describes the object file information and system
actions that create running programs.
Part 3, ‘‘C Library’’ lists the symbols contained in libsys, the standard ANSI C and libc routines,
and the global data symbols required by the libc routines.
References to X86 architecture have been changed to Intel Architecture.
NOTE

Tool Interface Standards (TIS)

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1

1

OBJECT FILES

Introduction
File Format
Data Representation

ELF Header

1-1
1-1
1-2

1-3
1-5
1-7

ELF Identification
Machine Information

Sections
Special Sections

1-8
1-13

String Table

1-16

Symbol Table

1-17
1-20

Symbol Values

Relocation

1-21
1-22

Relocation Types

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1iIntroduction
Part 1 describes the iABI object file format, called ELF (Executable and Linking Format). There are threemain types of object files.
A relocatable file holds code and data suitable for linking with other object files to create an executable or a shared object file.
An executable file holds a program suitable for execution; the file specifies how exec(BA_OS) createsa program’s process image.
A shared object file holds code and data suitable for linking in two contexts. First, the link editor [seeld(SD_CMD)] may process it with other relocatable and shared object files to create another object
file. Second, the dynamic linker combines it with an executable file and other shared objects tocreate a process image.
Created by the assembler and link editor, object files are binary representations of programs intended to
execute directly on a processor. Programs that require other abstract machines, such as shell scripts, areexcluded.
After the introductory material, Part 1 focuses on the file format and how it pertains to building programs. Part 2 also describes parts of the object file, concentrating on the information necessary to executea program.

File Format
Object files participate in program linking (building a program) and program execution (running a program). For convenience and efficiency, the object file format provides parallel views of a file’s contents,
reflecting the differing needs of these activities. Figure 1-1 shows an object file’s organization.
Figure 1-1: Object File Format

Linking View______________________
______________________

ELF header
 Program header table 
optional______________________
______________________

Section 1...

______________________
Section n
______________________...
______________________



...
______________________ Section header table 
______________________

Execution View_______________________
_______________________

ELF header
 Program header table 
_______________________
Segment 1

_______________________
Segment 2
_______________________



...
_______________________ Section header table 
_______________________

optional

An ELF header resides at the beginning and holds a ‘‘road map’’ describing the file’s organization. Sections hold the bulk of object file information for the linking view: instructions, data, symbol table, relocation information, and so on. Descriptions of special sections appear later in Part 1. Part 2 discusses segments and the program execution view of the file.

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ELF: Executable and Linkable Format

A program header table, if present, tells the system how to create a process image. Files used to build a process image (execute a program) must have a program header table; relocatable files do not need one. A
section header table contains information describing the file’s sections. Every section has an entry in thetable; each entry gives information such as the section name, the section size, etc. Files used during linking must have a section header table; other object files may or may not have one.
NOTE

Although the figure shows the program header table immediately after the ELF header, and the section
header table following the sections, actual files may differ. Moreover, sections and segments have no
specified order. Only the ELF header has a fixed position in the file.

Data Representation
As described here, the object file format supports various processors with 8-bit bytes and 32-bit architectures. Nevertheless, it is intended to be extensible to larger (or smaller) architectures. Object files therefore represent some control data with a machine-independent format, making it possible to identify
object files and interpret their contents in a common way. Remaining data in an object file use the encoding of the target processor, regardless of the machine on which the file was created.
Figure 1-2: 32-Bit Data Types

Name
Size
Alignment
Purpose_____________________________________________________________
 4 
 Unsigned program address
Elf32_Addr
4
 2 
 Unsigned medium integer
Elf32_Half2
 4 
 Unsigned file offset
Elf32_Off4
Elf32_Sword4
 4 
 Signed large integer
Elf32_Word44 Unsigned large integer
 Unsigned small integer
unsigned char  1 1_____________________________________________________________

All data structures that the object file format defines follow the ‘‘natural’’ size and alignment guidelines
for the relevant class. If necessary, data structures contain explicit padding to ensure 4-byte alignment for
4-byte objects, to force structure sizes to a multiple of 4, etc. Data also have suitable alignment from the
beginning of the file. Thus, for example, a structure containing an Elf32_Addr member will be aligned
on a 4-byte boundary within the file.
For portability reasons, ELF uses no bit-fields.

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Tool Interface Standards (TIS)ELF Header
Some object file control structures can grow, because the ELF header contains their actual sizes. If theobject file format changes, a program may encounter control structures that are larger or smaller than
expected. Programs might therefore ignore ‘‘extra’’ information. The treatment of ‘‘missing’’ information depends on context and will be specified when and if extensions are defined.
Figure 1-3: ELF Header#define EI_NIDENT

16

typedef struct {unsigned charElf32_Half
Elf32_Half
Elf32_Word
Elf32_Addr
Elf32_Off
Elf32_Off
Elf32_Word
Elf32_Half
Elf32_Half
Elf32_Half
Elf32_Half
Elf32_Half
Elf32_Half} Elf32_Ehdr;

e_ident[EI_NIDENT];
e_type;e_machine;e_version;
e_entry;e_phoff;e_shoff;
e_flags;
e_ehsize;e_phentsize;
e_phnum;
e_shentsize;e_shnum;
e_shstrndx;

e_ident

The initial bytes mark the file as an object file and provide machine-independent datawith which to decode and interpret the file’s contents. Complete descriptions appear
below, in ‘‘ELF Identification.’’e_typeThis member identifies the object file type.
Name
Value
Meaning________________________________________

ET_NONE
0  No file type
ET_REL1  Relocatable file
ET_EXEC
2  Executable file

ET_DYN
3  Shared object file
ET_CORE
4  Core file
ET_LOPROC  0xff00  Processor-specific
ET_HIPROC  0xffff  Processor-specific________________________________________

Although the core file contents are unspecified, type ET_CORE is reserved to mark the
file. Values from ET_LOPROC through ET_HIPROC (inclusive) are reserved forprocessor-specific semantics. Other values are reserved and will be assigned to new
object file types as necessary.

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ELF: Executable and Linkable Format

e_machine

This member’s value specifies the required architecture for an individual file.
Name
Value
Meaning____________________________________
 No machine
EM_NONE  0
 1
 AT&T WE 32100
EM_M32 SPARC
EM_SPARC2

EM_386
 3
 Intel 80386
EM_68K
 4
 Motorola 68000
 5
 Motorola 88000
EM_88K
 7
 Intel 80860
EM_860 MIPS RS3000
EM_MIPS
8____________________________________
Other values are reserved and will be assigned to new machines as necessary.
Processor-specific ELF names use the machine name to distinguish them. For example,
the flags mentioned below use the prefix EF_; a flag named WIDGET for the EM_XYZ
machine would be called EF_XYZ_WIDGET.

e_version

This member identifies the object file version.
Name
Value
Meaning______________________________________
 0
 Invalid version
EV_NONE Current version
EV_CURRENT  1______________________________________

The value 1 signifies the original file format; extensions will create new versions withhigher numbers. The value of EV_CURRENT, though given as 1 above, will change asnecessary to reflect the current version number.

e_entryThis member gives the virtual address to which the system first transfers control, thus
starting the process. If the file has no associated entry point, this member holds zero.

e_phoff

This member holds the program header table’s file offset in bytes. If the file has noprogram header table, this member holds zero.

e_shoff

This member holds the section header table’s file offset in bytes. If the file has no section header table, this member holds zero.

e_flags

This member holds processor-specific flags associated with the file. Flag names takethe form EF_machine_flag. See ‘‘Machine Information’’ for flag definitions.

e_ehsizeThis member holds the ELF header’s size in bytes.

e_phentsize

This member holds the size in bytes of one entry in the file’s program header table; allentries are the same size.

e_phnum

This member holds the number of entries in the program header table. Thus the product of e_phentsize and e_phnum gives the table’s size in bytes. If a file has no program header table, e_phnum holds the value zero.

e_shentsize

This member holds a section header’s size in bytes. A section header is one entry inthe section header table; all entries are the same size.

e_shnum

This member holds the number of entries in the section header table. Thus the productof e_shentsize and e_shnum gives the section header table’s size in bytes. If a file
has no section header table, e_shnum holds the value zero.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

e_shstrndx

This member holds the section header table index of the entry associated with the section name string table. If the file has no section name string table, this member holds
the value SHN_UNDEF. See ‘‘Sections’’ and ‘‘String Table’’ below for more information.

ELF Identification
As mentioned above, ELF provides an object file framework to support multiple processors, multiple data
encodings, and multiple classes of machines. To support this object file family, the initial bytes of the filespecify how to interpret the file, independent of the processor on which the inquiry is made and independent of the file’s remaining contents.
The initial bytes of an ELF header (and an object file) correspond to the e_ident member.
Figure 1-4: e_ident[ ] Identification Indexes

Name
Value
Purpose___________________________________________

EI_MAG0
0  File identification

EI_MAG1
1  File identification

EI_MAG2
2  File identification
EI_MAG3
3  File identification

EI_CLASS
4  File class
EI_DATA
5  Data encoding
EI_VERSION 
6  File version

EI_PAD
7  Start of padding bytes

EI_NIDENT  16  Size of e_ident[]___________________________________________


These indexes access bytes that hold the following values.
EI_MAG0 to EI_MAG3
A file’s first 4 bytes hold a ‘‘magic number,’’ identifying the file as an ELF object file.
Name
Value
Position_______________________________________
ELFMAG0  0x7f  e_ident[EI_MAG0]
ELFMAG1  ’E’  e_ident[EI_MAG1]
ELFMAG2  ’L’  e_ident[EI_MAG2]
ELFMAG3  ’F’  e_ident[EI_MAG3]_______________________________________



EI_CLASS

The next byte, e_ident[EI_CLASS], identifies the file’s class, or capacity.

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ELF: Executable and Linkable Format

Name
Value
Meaning______________________________________
 Invalid class
ELFCLASSNONE  0
 1
 32-bit objects
ELFCLASS32
 2
 64-bit objects
ELFCLASS64______________________________________

The file format is designed to be portable among machines of various sizes, without
imposing the sizes of the largest machine on the smallest. Class ELFCLASS32 supports
machines with files and virtual address spaces up to 4 gigabytes; it uses the basic types
defined above.
Class ELFCLASS64 is reserved for 64-bit architectures. Its appearance here shows how
the object file may change, but the 64-bit format is otherwise unspecified. Other classes
will be defined as necessary, with different basic types and sizes for object file data.
EI_DATA

Byte e_ident[EI_DATA] specifies the data encoding of the processor-specific data inthe object file. The following encodings are currently defined.
Name
Value
Meaning____________________________________________
 Invalid data encoding
ELFDATANONE  0
 See below
ELFDATA2LSB  1
 2
 See below
ELFDATA2MSB____________________________________________

More information on these encodings appears below. Other values are reserved andwill be assigned to new encodings as necessary.

EI_VERSION

Byte e_ident[EI_VERSION] specifies the ELF header version number. Currently, thisvalue must be EV_CURRENT, as explained above for e_version.

EI_PAD

This value marks the beginning of the unused bytes in e_ident. These bytes arereserved and set to zero; programs that read object files should ignore them. The valueof EI_PAD will change in the future if currently unused bytes are given meanings.

A file’s data encoding specifies how to interpret the basic objects in a file. As described above, class
ELFCLASS32 files use objects that occupy 1, 2, and 4 bytes. Under the defined encodings, objects are
represented as shown below. Byte numbers appear in the upper left corners.
Encoding ELFDATA2LSB specifies 2’s complement values, with the least significant byte occupying thelowest address.
Figure 1-5: Data Encoding ELFDATA2LSB

0x01
0x01020x01020304

1-6

0

0

0

01
02
04

1

1

01
03

2

02

3

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Encoding ELFDATA2MSB specifies 2’s complement values, with the most significant byte occupying thelowest address.
Figure 1-6: Data Encoding ELFDATA2MSB

0x01
0x01020x01020304

0

0

0

01
01
01

1

1

02
02

2

03

3

04

Machine Information
For file identification in e_ident, the 32-bit Intel Architecture requires the following values.
Figure 1-7: 32-bit Intel Architecture Identification, e_ident

Position
Value_____________________________________
e_ident[EI_CLASS]  ELFCLASS32
e_ident[EI_DATA]  ELFDATA2LSB_____________________________________


Processor identification resides in the ELF header’s e_machine member and must have the value
EM_386.
The ELF header’s e_flags member holds bit flags associated with the file. The 32-bit Intel Architecture
defines no flags; so this member contains zero.

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Sections
An object file’s section header table lets one locate all the file’s sections. The section header table is anarray of Elf32_Shdr structures as described below. A section header table index is a subscript into thisarray. The ELF header’s e_shoff member gives the byte offset from the beginning of the file to the section header table; e_shnum tells how many entries the section header table contains; e_shentsize
gives the size in bytes of each entry.
Some section header table indexes are reserved; an object file will not have sections for these special
indexes.
Figure 1-8: Special Section Indexes

Name
Value
__________________________

SHN_UNDEF
0
SHN_LORESERVE  0xff00
 0xff00
SHN_LOPROC

SHN_HIPROC
 0xff1f
SHN_ABS 0xfff1
 0xfff2
SHN_COMMONSHN_HIRESERVE  0xffff__________________________


SHN_UNDEFNOTEThis value marks an undefined, missing, irrelevant, or otherwise meaningless sectionreference. For example, a symbol ‘‘defined’’ relative to section number SHN_UNDEF
is an undefined symbol.

Although index 0 is reserved as the undefined value, the section header table contains an entry forindex 0. That is, if the e_shnum member of the ELF header says a file has 6 entries in the section
header table, they have the indexes 0 through 5. The contents of the initial entry are specified later inthis section.

SHN_LORESERVEThis value specifies the lower bound of the range of reserved indexes.

SHN_LOPROC through SHN_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.
SHN_ABSThis value specifies absolute values for the corresponding reference. For example,
symbols defined relative to section number SHN_ABS have absolute values and are
not affected by relocation.

SHN_COMMON

Symbols defined relative to this section are common symbols, such as FORTRANCOMMON or unallocated C external variables.

SHN_HIRESERVEThis value specifies the upper bound of the range of reserved indexes. The system
reserves indexes between SHN_LORESERVE and SHN_HIRESERVE, inclusive; thevalues do not reference the section header table. That is, the section header table
does not contain entries for the reserved indexes.

Sections contain all information in an object file, except the ELF header, the program header table, and thesection header table. Moreover, object files’ sections satisfy several conditions.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Every section in an object file has exactly one section header describing it. Section headers may
exist that do not have a section.
Each section occupies one contiguous (possibly empty) sequence of bytes within a file.
Sections in a file may not overlap. No byte in a file resides in more than one section.
An object file may have inactive space. The various headers and the sections might not ‘‘cover’’
every byte in an object file. The contents of the inactive data are unspecified.
A section header has the following structure.
Figure 1-9: Section Header

typedef struct {Elf32_Word
Elf32_Word
Elf32_Word
Elf32_Addr
Elf32_Off
Elf32_Word
Elf32_Word
Elf32_Word
Elf32_Word
Elf32_Word} Elf32_Shdr;

sh_name;sh_type;sh_flags;
sh_addr;sh_offset;
sh_size;
sh_link;
sh_info;sh_addralign;
sh_entsize;

sh_nameThis member specifies the name of the section. Its value is an index into the section
header string table section [see ‘‘String Table’’ below], giving the location of a nullterminated string.

sh_typeThis member categorizes the section’s contents and semantics. Section types and theirdescriptions appear below.

sh_flags

Sections support 1-bit flags that describe miscellaneous attributes. Flag definitions
appear below.

sh_addrIf the section will appear in the memory image of a process, this member gives theaddress at which the section’s first byte should reside. Otherwise, the member contains 0.

sh_offset

This member’s value gives the byte offset from the beginning of the file to the first
byte in the section. One section type, SHT_NOBITS described below, occupies nospace in the file, and its sh_offset member locates the conceptual placement in thefile.

sh_size

This member gives the section’s size in bytes. Unless the section type is
SHT_NOBITS, the section occupies sh_size bytes in the file. A section of type
SHT_NOBITS may have a non-zero size, but it occupies no space in the file.

sh_link

This member holds a section header table index link, whose interpretation depends
on the section type. A table below describes the values.

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sh_infoThis member holds extra information, whose interpretation depends on the section
type. A table below describes the values.

sh_addralignSome sections have address alignment constraints. For example, if a section holds a
doubleword, the system must ensure doubleword alignment for the entire section.
That is, the value of sh_addr must be congruent to 0, modulo the value of
sh_addralign. Currently, only 0 and positive integral powers of two are allowed.
Values 0 and 1 mean the section has no alignment constraints.

sh_entsize

Some sections hold a table of fixed-size entries, such as a symbol table. For such a section, this member gives the size in bytes of each entry. The member contains 0 if thesection does not hold a table of fixed-size entries.

A section header’s sh_type member specifies the section’s semantics.
Figure 1-10: Section Types, sh_type

Name
Value
______________________________

SHT_NULL0
SHT_PROGBITS 
1
SHT_SYMTAB2SHT_STRTAB3

SHT_RELA4
SHT_HASH
5
SHT_DYNAMIC 
6
SHT_NOTE
7
SHT_NOBITS8SHT_REL9
SHT_SHLIB
10SHT_DYNSYM
11
 0x70000000
SHT_LOPROC
SHT_HIPROC
 0x7fffffff
SHT_LOUSER
 0x80000000
 0xffffffff
SHT_HIUSER______________________________


SHT_NULL

This value marks the section header as inactive; it does not have an associated section.
Other members of the section header have undefined values.

SHT_PROGBITSThe section holds information defined by the program, whose format and meaning aredetermined solely by the program.

SHT_SYMTAB and SHT_DYNSYM
These sections hold a symbol table. Currently, an object file may have only one section of each type, but this restriction may be relaxed in the future. Typically,
SHT_SYMTAB provides symbols for link editing, though it may also be used for
dynamic linking. As a complete symbol table, it may contain many symbols unnecessary for dynamic linking. Consequently, an object file may also contain a
SHT_DYNSYM section, which holds a minimal set of dynamic linking symbols, to save
space. See ‘‘Symbol Table’’ below for details.

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SHT_STRTAB

The section holds a string table. An object file may have multiple string table sections.
See ‘‘String Table’’ below for details.

SHT_RELA

The section holds relocation entries with explicit addends, such as type Elf32_Rela
for the 32-bit class of object files. An object file may have multiple relocation sections.
See ‘‘Relocation’’ below for details.

SHT_HASH

The section holds a symbol hash table. All objects participating in dynamic linking
must contain a symbol hash table. Currently, an object file may have only one hashtable, but this restriction may be relaxed in the future. See ‘‘Hash Table’’ in Part 2 fordetails.

SHT_DYNAMIC

The section holds information for dynamic linking. Currently, an object file may have
only one dynamic section, but this restriction may be relaxed in the future. See‘‘Dynamic Section’’ in Part 2 for details.

SHT_NOTE

The section holds information that marks the file in some way. See ‘‘Note Section’’ inPart 2 for details.

SHT_NOBITS

A section of this type occupies no space in the file but otherwise resembles
SHT_PROGBITS. Although this section contains no bytes, the sh_offset member
contains the conceptual file offset.

SHT_REL

The section holds relocation entries without explicit addends, such as typeElf32_Rel for the 32-bit class of object files. An object file may have multiple relocation sections. See ‘‘Relocation’’ below for details.

SHT_SHLIBThis section type is reserved but has unspecified semantics. Programs that contain asection of this type do not conform to the ABI.

SHT_LOPROC through SHT_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.
SHT_LOUSER

This value specifies the lower bound of the range of indexes reserved for application
programs.

SHT_HIUSER

This value specifies the upper bound of the range of indexes reserved for application
programs. Section types between SHT_LOUSER and SHT_HIUSER may be used by
the application, without conflicting with current or future system-defined section
types.

Other section type values are reserved. As mentioned before, the section header for index 0(SHN_UNDEF) exists, even though the index marks undefined section references. This entry holds the following.
Figure 1-11: Section Header Table Entry: Index 0

Name
Value
Note
______________________________________________________
 No namesh_name0
 SHT_NULL  Inactive
sh_type No flags
sh_flags0
sh_addr0 No address
sh_offset0 No file offset
sh_size0 No size

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-11ELF: Executable and Linkable Format

Figure 1-11: Section Header Table Entry: Index 0 (continued )sh_link SHN_UNDEF  No link information
 No auxiliary information
sh_info0
 No alignment
sh_addralign 0 No entries
sh_entsize
0______________________________________________________


A section header’s sh_flags member holds 1-bit flags that describe the section’s attributes. Definedvalues appear below; other values are reserved.
Figure 1-12: Section Attribute Flags, sh_flags

Name
Value
_______________________________
SHF_WRITE
0x1
SHF_ALLOC
0x2
SHF_EXECINSTR 
0x4
SHF_MASKPROC  0xf0000000_______________________________


If a flag bit is set in sh_flags, the attribute is ‘‘on’’ for the section. Otherwise, the attribute is ‘‘off’’ ordoes not apply. Undefined attributes are set to zero.
SHF_WRITE

The section contains data that should be writable during process execution.

SHF_ALLOC

The section occupies memory during process execution. Some control sections do
not reside in the memory image of an object file; this attribute is off for those sections.

SHF_EXECINSTR

The section contains executable machine instructions.

SHF_MASKPROC

All bits included in this mask are reserved for processor-specific semantics.

Two members in the section header, sh_link and sh_info, hold special information, depending on
section type.

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Figure 1-13: sh_link and sh_info Interpretation

sh_typesh_link
sh_info______________________________________________________________________SHT_DYNAMIC  The section header index of  0
 the string table used by entries in the section.
______________________________________________________________________
 The section header index of  0
SHT_HASH
 the symbol table to which the hash table applies.
______________________________________________________________________
 The section header index of  The section header index of
SHT_REL the associated symbol table.  the section to which the
SHT_RELA relocation applies.
______________________________________________________________________
SHT_SYMTAB  The section header index of  One greater than the sym bol table index of the last
SHT_DYNSYM  the associated string table.
 local symbol (binding STB_LOCAL).
______________________________________________________________________
 0
 SHN_UNDEF
other______________________________________________________________________


Special Sections
Various sections hold program and control information. Sections in the list below are used by the systemand have the indicated types and attributes.
Figure 1-14: Special Sections

Name
Type
Attributes____________________________________________________________
 SHT_NOBITS SHF_ALLOC + SHF_WRITE.bss.comment  SHT_PROGBITS  none
 SHT_PROGBITS  SHF_ALLOC + SHF_WRITE
.data


.data1 SHT_PROGBITS  SHF_ALLOC + SHF_WRITE.debug
 SHT_PROGBITS  none
.dynamic  SHT_DYNAMIC  see below
 SHT_STRTAB SHF_ALLOC.dynstr
 SHT_DYNSYM SHF_ALLOC.dynsym.fini SHT_PROGBITS  SHF_ALLOC + SHF_EXECINSTR.got
 SHT_PROGBITS  see below
 SHT_HASH SHF_ALLOC.hash SHT_PROGBITS  SHF_ALLOC + SHF_EXECINSTR.init
 SHT_PROGBITS  see below.interp.line SHT_PROGBITS  none.note
 SHT_NOTE
 none SHT_PROGBITS  see below.plt SHT_REL
 see below.relname

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-13ELF: Executable and Linkable Format

Figure 1-14: Special Sections (continued ).relaname  SHT_RELA see below
 SHT_PROGBITS  SHF_ALLOC.rodata.rodata1  SHT_PROGBITS  SHF_ALLOC none.shstrtab  SHT_STRTAB
.strtab SHT_STRTAB
 see below.symtab SHT_SYMTAB
 see below
 SHT_PROGBITS  SHF_ALLOC + SHF_EXECINSTR.text____________________________________________________________



.bss

This section holds uninitialized data that contribute to the program’s memory image. By
definition, the system initializes the data with zeros when the program begins to run. The
section occupies no file space, as indicated by the section type, SHT_NOBITS.

.comment

This section holds version control information.

.data and .data1
These sections hold initialized data that contribute to the program’s memory image.
.debugThis section holds information for symbolic debugging. The contents are unspecified.

.dynamic

This section holds dynamic linking information. The section’s attributes will include the
SHF_ALLOC bit. Whether the SHF_WRITE bit is set is processor specific. See Part 2 formore information.

.dynstr

This section holds strings needed for dynamic linking, most commonly the strings that
represent the names associated with symbol table entries. See Part 2 for more information.

.dynsymThis section holds the dynamic linking symbol table, as ‘‘Symbol Table’’ describes. See
Part 2 for more information.

.finiThis section holds executable instructions that contribute to the process termination code.
That is, when a program exits normally, the system arranges to execute the code in thissection.

.got

This section holds the global offset table. See ‘‘Special Sections’’ in Part 1 and ‘‘Global
Offset Table’’ in Part 2 for more information.

.hashThis section holds a symbol hash table. See ‘‘Hash Table’’ in Part 2 for more information.

.initThis section holds executable instructions that contribute to the process initialization code.
That is, when a program starts to run, the system arranges to execute the code in this section before calling the main program entry point (called main for C programs).

.interp

This section holds the path name of a program interpreter. If the file has a loadable segment that includes the section, the section’s attributes will include the SHF_ALLOC bit; otherwise, that bit will be off. See Part 2 for more information.

.line

This section holds line number information for symbolic debugging, which describes thecorrespondence between the source program and the machine code. The contents areunspecified.

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format.noteThis section holds information in the format that ‘‘Note Section’’ in Part 2 describes.

.pltThis section holds the procedure linkage table. See ‘‘Special Sections’’ in Part 1 and ‘‘Procedure Linkage Table’’ in Part 2 for more information.

.relname and .relaname
These sections hold relocation information, as ‘‘Relocation’’ below describes. If the file hasa loadable segment that includes relocation, the sections’ attributes will include the
SHF_ALLOC bit; otherwise, that bit will be off. Conventionally, name is supplied by thesection to which the relocations apply. Thus a relocation section for .text normally
would have the name .rel.text or .rela.text.
.rodata and .rodata1
These sections hold read-only data that typically contribute to a non-writable segment in
the process image. See ‘‘Program Header’’ in Part 2 for more information.
.shstrtab

This section holds section names.

.strtabThis section holds strings, most commonly the strings that represent the names associatedwith symbol table entries. If the file has a loadable segment that includes the symbol stringtable, the section’s attributes will include the SHF_ALLOC bit; otherwise, that bit will be off.

.symtabThis section holds a symbol table, as ‘‘Symbol Table’’ in this section describes. If the file
has a loadable segment that includes the symbol table, the section’s attributes will include
the SHF_ALLOC bit; otherwise, that bit will be off.

.text

This section holds the ‘‘text,’’ or executable instructions, of a program.

Section names with a dot (.) prefix are reserved for the system, although applications may use these sections if their existing meanings are satisfactory. Applications may use names without the prefix to avoid
conflicts with system sections. The object file format lets one define sections not in the list above. An
object file may have more than one section with the same name.
Section names reserved for a processor architecture are formed by placing an abbreviation of the architecture name ahead of the section name. The name should be taken from the architecture names used fore_machine. For instance .FOO.psect is the psect section defined by the FOO architecture. Existing
extensions are called by their historical names.
Pre-existing Extensions________________________.sdata.sbss.lit8.gptab.conflict

Tool Interface Standards (TIS).tdesc.lit4.reginfo.liblist

Portable Formats Specification, Version 1.1

1-15

String Table
String table sections hold null-terminated character sequences, commonly called strings. The object fileuses these strings to represent symbol and section names. One references a string as an index into thestring table section. The first byte, which is index zero, is defined to hold a null character. Likewise, astring table’s last byte is defined to hold a null character, ensuring null termination for all strings. A
string whose index is zero specifies either no name or a null name, depending on the context. An empty
string table section is permitted; its section header’s sh_size member would contain zero. Non-zeroindexes are invalid for an empty string table.
A section header’s sh_name member holds an index into the section header string table section, as designated by the e_shstrndx member of the ELF header. The following figures show a string table with 25
bytes and the strings associated with various indexes.
Index
0
10
20

+0
+1
+2
+3
+4
+5
+6
+7
+8
+9
______________________________________________________
______________________________________________________\0  n  a  m  e  .  \0  V  a  r 
 i  a  b  l  e  \0  a  b  l  e 
______________________________________________________










\0  \0  x  x  \0 
______________________________________________________






Figure 1-15: String Table Indexes

Index
String__________________
0  none1  name.
7  Variable
11  able
16  able
24  null string__________________As the example shows, a string table index may refer to any byte in the section. A string may appear
more than once; references to substrings may exist; and a single string may be referenced multiple times.
Unreferenced strings also are allowed.

1-16

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)Symbol Table
An object file’s symbol table holds information needed to locate and relocate a program’s symbolic
definitions and references. A symbol table index is a subscript into this array. Index 0 both designates
the first entry in the table and serves as the undefined symbol index. The contents of the initial entry are
specified later in this section.
Name
Value
___________________

STN_UNDEF
0___________________
A symbol table entry has the following format.
Figure 1-16: Symbol Table Entry

typedef struct {Elf32_Word
Elf32_Addr
Elf32_Word
unsigned char
unsigned charElf32_Half} Elf32_Sym;

st_namest_name;st_value;
st_size;
st_info;st_other;st_shndx;

This member holds an index into the object file’s symbol string table, which holds thecharacter representations of the symbol names. If the value is non-zero, it represents astring table index that gives the symbol name. Otherwise, the symbol table entry has no
name.

External C symbols have the same names in C and object files’ symbol tables.
NOTE

st_value

This member gives the value of the associated symbol. Depending on the context, thismay be an absolute value, an address, etc.; details appear below.

st_size

Many symbols have associated sizes. For example, a data object’s size is the number ofbytes contained in the object. This member holds 0 if the symbol has no size or an
unknown size.

st_infoThis member specifies the symbol’s type and binding attributes. A list of the values andmeanings appears below. The following code shows how to manipulate the values.

#define ELF32_ST_BIND(i)((i)>>4)#define ELF32_ST_TYPE(i)((i)&0xf)#define ELF32_ST_INFO(b,t) (((b)<<4)+((t)&0xf))

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-17ELF: Executable and Linkable Format

st_otherThis member currently holds 0 and has no defined meaning.

st_shndx

Every symbol table entry is ‘‘defined’’ in relation to some section; this member holds therelevant section header table index. As Figure 1-7 and the related text describe, some
section indexes indicate special meanings.

A symbol’s binding determines the linkage visibility and behavior.
Figure 1-17: Symbol Binding, ELF32_ST_BIND

Name
Value
_____________________
STB_LOCAL 
0
STB_GLOBAL 
1
STB_WEAK2
STB_LOPROC  13
STB_HIPROC  15_____________________


STB_LOCAL

Local symbols are not visible outside the object file containing their definition. Local
symbols of the same name may exist in multiple files without interfering with eachother.

STB_GLOBAL

Global symbols are visible to all object files being combined. One file’s definition of a
global symbol will satisfy another file’s undefined reference to the same global symbol.

STB_WEAK

Weak symbols resemble global symbols, but their definitions have lower precedence.

STB_LOPROC through STB_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.
Global and weak symbols differ in two major ways.
When the link editor combines several relocatable object files, it does not allow multiple definitionsof STB_GLOBAL symbols with the same name. On the other hand, if a defined global symbol
exists, the appearance of a weak symbol with the same name will not cause an error. The link editor honors the global definition and ignores the weak ones. Similarly, if a common symbol exists(i.e., a symbol whose st_shndx field holds SHN_COMMON), the appearance of a weak symbol withthe same name will not cause an error. The link editor honors the common definition and ignores
the weak ones.
When the link editor searches archive libraries, it extracts archive members that contain definitionsof undefined global symbols. The member’s definition may be either a global or a weak symbol.
The link editor does not extract archive members to resolve undefined weak symbols. Unresolved
weak symbols have a zero value.
In each symbol table, all symbols with STB_LOCAL binding precede the weak and global symbols. As‘‘Sections’’ above describes, a symbol table section’s sh_info section header member holds the symbol
table index for the first non-local symbol.

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

A symbol’s type provides a general classification for the associated entity.
Figure 1-18: Symbol Types, ELF32_ST_TYPE

Name
Value
______________________
STT_NOTYPE 
0
STT_OBJECT 
1
STT_FUNC
2

STT_SECTION 
3
STT_FILE
4
STT_LOPROC  13
STT_HIPROC  15______________________


STT_NOTYPE

The symbol’s type is not specified.

STT_OBJECT

The symbol is associated with a data object, such as a variable, an array, etc.

STT_FUNC

The symbol is associated with a function or other executable code.

STT_SECTION

The symbol is associated with a section. Symbol table entries of this type exist primarily for relocation and normally have STB_LOCAL binding.

STT_FILE

Conventionally, the symbol’s name gives the name of the source file associated with theobject file. A file symbol has STB_LOCAL binding, its section index is SHN_ABS, and it
precedes the other STB_LOCAL symbols for the file, if it is present.

STT_LOPROC through STT_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.
Function symbols (those with type STT_FUNC) in shared object files have special significance. Whenanother object file references a function from a shared object, the link editor automatically creates a procedure linkage table entry for the referenced symbol. Shared object symbols with types other than
STT_FUNC will not be referenced automatically through the procedure linkage table.
If a symbol’s value refers to a specific location within a section, its section index member, st_shndx,
holds an index into the section header table. As the section moves during relocation, the symbol’s valuechanges as well, and references to the symbol continue to ‘‘point’’ to the same location in the program.
Some special section index values give other semantics.
SHN_ABSThe symbol has an absolute value that will not change because of relocation.

SHN_COMMON

The symbol labels a common block that has not yet been allocated. The symbol’s value
gives alignment constraints, similar to a section’s sh_addralign member. That is, thelink editor will allocate the storage for the symbol at an address that is a multiple ofst_value. The symbol’s size tells how many bytes are required.

SHN_UNDEF

This section table index means the symbol is undefined. When the link editor combines
this object file with another that defines the indicated symbol, this file’s references to thesymbol will be linked to the actual definition.

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-19ELF: Executable and Linkable Format

As mentioned above, the symbol table entry for index 0 (STN_UNDEF) is reserved; it holds the following.
Figure 1-19: Symbol Table Entry: Index 0

Name
Value
Note
_______________________________________________
 No namest_name 0 Zero value
st_value 0 No size
st_size0
st_info 
0 No type, local binding
st_other 0st_shndx  SHN_UNDEF  No section_______________________________________________
Symbol Values
Symbol table entries for different object file types have slightly different interpretations for thest_value member.
In relocatable files, st_value holds alignment constraints for a symbol whose section index is
SHN_COMMON.
In relocatable files, st_value holds a section offset for a defined symbol. That is, st_value is anoffset from the beginning of the section that st_shndx identifies.
In executable and shared object files, st_value holds a virtual address. To make these files’ symbols more useful for the dynamic linker, the section offset (file interpretation) gives way to a virtual
address (memory interpretation) for which the section number is irrelevant.
Although the symbol table values have similar meanings for different object files, the data allow efficient
access by the appropriate programs.

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)Relocation
Relocation is the process of connecting symbolic references with symbolic definitions. For example, whena program calls a function, the associated call instruction must transfer control to the proper destination
address at execution. In other words, relocatable files must have information that describes how tomodify their section contents, thus allowing executable and shared object files to hold the right information for a process’s program image. Relocation entries are these data.
Figure 1-20: Relocation Entries

typedef struct {Elf32_Addr
Elf32_Word} Elf32_Rel;

r_offset;
r_info;typedef struct {Elf32_Addr
Elf32_Word
Elf32_Sword} Elf32_Rela;r_offset;
r_info;r_addend;

r_offsetThis member gives the location at which to apply the relocation action. For a relocatable
file, the value is the byte offset from the beginning of the section to the storage unit affectedby the relocation. For an executable file or a shared object, the value is the virtual address ofthe storage unit affected by the relocation.

r_infoThis member gives both the symbol table index with respect to which the relocation must be
made, and the type of relocation to apply. For example, a call instruction’s relocation entry
would hold the symbol table index of the function being called. If the index is STN_UNDEF,
the undefined symbol index, the relocation uses 0 as the ‘‘symbol value.’’ Relocation types
are processor-specific. When the text refers to a relocation entry’s relocation type or symbol
table index, it means the result of applying ELF32_R_TYPE or ELF32_R_SYM, respectively,
to the entry’s r_info member.

#define ELF32_R_SYM(i)((i)>>8)#define ELF32_R_TYPE(i)((unsigned char)(i))#define ELF32_R_INFO(s,t) (((s)<<8)+(unsigned char)(t))

r_addend

This member specifies a constant addend used to compute the value to be stored into therelocatable field.

As shown above, only Elf32_Rela entries contain an explicit addend. Entries of type Elf32_Rel store
an implicit addend in the location to be modified. Depending on the processor architecture, one form or
the other might be necessary or more convenient. Consequently, an implementation for a particularmachine may use one form exclusively or either form depending on context.

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-21ELF: Executable and Linkable Format

A relocation section references two other sections: a symbol table and a section to modify. The section
header’s sh_info and sh_link members, described in ‘‘Sections’’ above, specify these relationships.
Relocation entries for different object files have slightly different interpretations for the r_offset
member.
In relocatable files, r_offset holds a section offset. That is, the relocation section itself describes
how to modify another section in the file; relocation offsets designate a storage unit within thesecond section.
In executable and shared object files, r_offset holds a virtual address. To make these files’ relocation entries more useful for the dynamic linker, the section offset (file interpretation) gives way toa virtual address (memory interpretation).
Although the interpretation of r_offset changes for different object files to allow efficient access by therelevant programs, the relocation types’ meanings stay the same.

Relocation Types
Relocation entries describe how to alter the following instruction and data fields (bit numbers appear in
the lower box corners).
Figure 1-21: Relocatable Fields

word3231

word320This specifies a 32-bit field occupying 4 bytes with arbitrary byte alignment. These values use
the same byte order as other word values in the 32-bit Intel Architecture.

0x01020304

3

01

2

02

1

03

31

0

04
0

Calculations below assume the actions are transforming a relocatable file into either an executable or ashared object file. Conceptually, the link editor merges one or more relocatable files to form the output.
It first decides how to combine and locate the input files, then updates the symbol values, and finally performs the relocation. Relocations applied to executable or shared object files are similar and accomplish
the same result. Descriptions below use the following notation.
A

This means the addend used to compute the value of the relocatable field.

B

This means the base address at which a shared object has been loaded into memory duringexecution. Generally, a shared object file is built with a 0 base virtual address, but the execution address will be different.

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

G

This means the offset into the global offset table at which the address of the relocation entry’s
symbol will reside during execution. See ‘‘Global Offset Table’’ in Part 2 for more information.

GOTThis means the address of the global offset table. See ‘‘Global Offset Table’’ in Part 2 for more
information.

L

This means the place (section offset or address) of the procedure linkage table entry for a symbol. A procedure linkage table entry redirects a function call to the proper destination. The
link editor builds the initial procedure linkage table, and the dynamic linker modifies theentries during execution. See ‘‘Procedure Linkage Table’’ in Part 2 for more information.

P

This means the place (section offset or address) of the storage unit being relocated (computed
using r_offset).

S

This means the value of the symbol whose index resides in the relocation entry.

A relocation entry’s r_offset value designates the offset or virtual address of the first byte of theaffected storage unit. The relocation type specifies which bits to change and how to calculate their values.
The SYSTEM V architecture uses only Elf32_Rel relocation entries, the field to be relocated holds theaddend. In all cases, the addend and the computed result use the same byte order.
Figure 1-22: Relocation Types

Name
Value
Field
Calculation___________________________________________________
 noneR_386_NONE
0  none

R_386_32
1  word32  S + A

R_386_PC32
2  word32  S + A - P
R_386_GOT32
3  word32  G + A - P

R_386_PLT32
4  word32  L + A - P noneR_386_COPY
5  none
R_386_GLOB_DAT 
6  word32  S
R_386_JMP_SLOT 
7  word32  S
R_386_RELATIVE 
8  word32  B + A
R_386_GOTOFF
9  word32  S + A - GOT 10  word32  GOT + A - P
R_386_GOTPC___________________________________________________


Some relocation types have semantics beyond simple calculation.
R_386_GOT32

This relocation type computes the distance from the base of the global offset
table to the symbol’s global offset table entry. It additionally instructs the link
editor to build a global offset table.

R_386_PLT32

This relocation type computes the address of the symbol’s procedure linkage
table entry and additionally instructs the link editor to build a procedure linkage
table.

R_386_COPY

The link editor creates this relocation type for dynamic linking. Its offset
member refers to a location in a writable segment. The symbol table index
specifies a symbol that should exist both in the current object file and in a sharedobject. During execution, the dynamic linker copies data associated with theshared object’s symbol to the location specified by the offset.

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

1-23ELF: Executable and Linkable Format

R_386_GLOB_DATThis relocation type is used to set a global offset table entry to the address of thespecified symbol. The special relocation type allows one to determine thecorrespondence between symbols and global offset table entries.

R_3862_JMP_SLOT

The link editor creates this relocation type for dynamic linking. Its offset
member gives the location of a procedure linkage table entry. The dynamic
linker modifies the procedure linkage table entry to transfer control to the designated symbol’s address [see ‘‘Procedure Linkage Table’’ in Part 2].

R_386_RELATIVE

The link editor creates this relocation type for dynamic linking. Its offset
member gives a location within a shared object that contains a value representing a relative address. The dynamic linker computes the corresponding virtual
address by adding the virtual address at which the shared object was loaded to
the relative address. Relocation entries for this type must specify 0 for the symbol table index.

R_386_GOTOFF

This relocation type computes the difference between a symbol’s value and theaddress of the global offset table. It additionally instructs the link editor to build
the global offset table.

R_386_GOTPC

This relocation type resembles R_386_PC32, except it uses the address of the
global offset table in its calculation. The symbol referenced in this relocation
normally is _GLOBAL_OFFSET_TABLE_, which additionally instructs the link
editor to build the global offset table.

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Tool Interface Standards (TIS)2

PROGRAM LOADING AND DYNAMIC LINKING

Introduction

2-1

Program Header
Base Address
Note Section

2-2
2-4
2-4

Program Loading

2-7

Dynamic Linking

2-10
2-10
2-10
2-11
2-15
2-16
2-17
2-19
2-20

Program Interpreter
Dynamic Linker
Dynamic Section
Shared Object Dependencies
Global Offset Table
Procedure Linkage Table
Hash Table
Initialization and Termination Functions

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1iIntroduction
Part 2 describes the object file information and system actions that create running programs. Some information here applies to all systems; other information is processor-specific.
Executable and shared object files statically represent programs. To execute such programs, the system
uses the files to create dynamic program representations, or process images. A process image has segments that hold its text, data, stack, and so on. The major sections in this part discuss the following.
Program header. This section complements Part 1, describing object file structures that relate directly
to program execution. The primary data structure, a program header table, locates segment images
within the file and contains other information necessary to create the memory image for the program.
Program loading. Given an object file, the system must load it into memory for the program to run.
Dynamic linking. After the system loads the program, it must complete the process image by resolving symbolic references among the object files that compose the process.

NOTE

There are naming conventions for ELF constants that have specified processor ranges. Names such as
DT_, PT_, for processor-specific extensions, incorporate the name of the processor:
DT_M32_SPECIAL, for example. Pre–existing processor extensions not using this convention will besupported.

Extensions_Pre-existing____________________
DT_JMP_REL

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2-1

Program Header
An executable or shared object file’s program header table is an array of structures, each describing a segment or other information the system needs to prepare the program for execution. An object file segment
contains one or more sections, as ‘‘Segment Contents’’ describes below. Program headers are meaningful
only for executable and shared object files. A file specifies its own program header size with the ELF
header’s e_phentsize and e_phnum members [see ‘‘ELF Header’’ in Part 1].
Figure 2-1: Program Header

typedef struct {Elf32_Word
Elf32_Off
Elf32_Addr
Elf32_Addr
Elf32_Word
Elf32_Word
Elf32_Word
Elf32_Word} Elf32_Phdr;

p_type;p_offset;
p_vaddr;p_paddr;p_filesz;
p_memsz;
p_flags;p_align;p_typeThis member tells what kind of segment this array element describes or how to interpretthe array element’s information. Type values and their meanings appear below.

p_offsetThis member gives the offset from the beginning of the file at which the first byte of thesegment resides.

p_vaddrThis member gives the virtual address at which the first byte of the segment resides inmemory.

p_paddr

On systems for which physical addressing is relevant, this member is reserved for thesegment’s physical address. Because System V ignores physical addressing for application programs, this member has unspecified contents for executable files and sharedobjects.

p_filesz

This member gives the number of bytes in the file image of the segment; it may be zero.

p_memsz

This member gives the number of bytes in the memory image of the segment; it may be
zero.

p_flags

This member gives flags relevant to the segment. Defined flag values appear below.

p_alignAs ‘‘Program Loading’’ later in this part describes, loadable process segments must havecongruent values for p_vaddr and p_offset, modulo the page size. This member
gives the value to which the segments are aligned in memory and in the file. Values 0
and 1 mean no alignment is required. Otherwise, p_align should be a positive, integral
power of 2, and p_vaddr should equal p_offset, modulo p_align.

Some entries describe process segments; others give supplementary information and do not contribute to
the process image. Segment entries may appear in any order, except as explicitly noted below. Defined
type values follow; other values are reserved for future use.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Figure 2-2: Segment Types, p_type

Name
Value
___________________________

PT_NULL
0
PT_LOAD1
PT_DYNAMIC 2
PT_INTERP 
3
PT_NOTE4
PT_SHLIB
5
PT_PHDR
6
PT_LOPROC  0x70000000

PT_HIPROC  0x7fffffff___________________________


PT_NULL

The array element is unused; other members’ values are undefined. This type lets the
program header table have ignored entries.

PT_LOAD

The array element specifies a loadable segment, described by p_filesz and p_memsz.
The bytes from the file are mapped to the beginning of the memory segment. If thesegment’s memory size (p_memsz) is larger than the file size (p_filesz), the ‘‘extra’’bytes are defined to hold the value 0 and to follow the segment’s initialized area. The filesize may not be larger than the memory size. Loadable segment entries in the program
header table appear in ascending order, sorted on the p_vaddr member.

PT_DYNAMIC

The array element specifies dynamic linking information. See ‘‘Dynamic Section’’ below
for more information.

PT_INTERP

The array element specifies the location and size of a null-terminated path name to
invoke as an interpreter. This segment type is meaningful only for executable files(though it may occur for shared objects); it may not occur more than once in a file. If it ispresent, it must precede any loadable segment entry. See ‘‘Program Interpreter’’ below
for further information.

PT_NOTE

The array element specifies the location and size of auxiliary information. See ‘‘Note Section’’ below for details.

PT_SHLIB

This segment type is reserved but has unspecified semantics. Programs that contain anarray element of this type do not conform to the ABI.

PT_PHDR

The array element, if present, specifies the location and size of the program header table
itself, both in the file and in the memory image of the program. This segment type may
not occur more than once in a file. Moreover, it may occur only if the program header
table is part of the memory image of the program. If it is present, it must precede any
loadable segment entry. See ‘‘Program Interpreter’’ below for further information.

PT_LOPROC through PT_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.

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ELF: Executable and Linkable Format

NOTE

Unless specifically required elsewhere, all program header segment types are optional. That is, a file’s
program header table may contain only those elements relevant to its contents.

Base Address
Executable and shared object files have a base address, which is the lowest virtual address associated withthe memory image of the program’s object file. One use of the base address is to relocate the memory
image of the program during dynamic linking.
An executable or shared object file’s base address is calculated during execution from three values: thememory load address, the maximum page size, and the lowest virtual address of a program’s loadable
segment. As ‘‘Program Loading’’
in this chapter describes, the virtual addresses in the program headers might not represent the actual virtual addresses of the program’s memory image. To compute the base address, one determines thememory address associated with the lowest p_vaddr value for a PT_LOAD segment. One then obtains
the base address by truncating the memory address to the nearest multiple of the maximum page size.
Depending on the kind of file being loaded into memory, the memory address might or might not match
the p_vaddr values.
As ‘‘Sections’’ in Part 1 describes, the .bss section has the type SHT_NOBITS. Although it occupies nospace in the file, it contributes to the segment’s memory image. Normally, these uninitialized data resideat the end of the segment, thereby making p_memsz larger than p_filesz in the associated program
header element.

Note Section
Sometimes a vendor or system builder needs to mark an object file with special information that otherprograms will check for conformance, compatibility, etc. Sections of type SHT_NOTE and program
header elements of type PT_NOTE can be used for this purpose. The note information in sections andprogram header elements holds any number of entries, each of which is an array of 4-byte words in theformat of the target processor. Labels appear below to help explain note information organization, butthey are not part of the specification.
Figure 2-3: Note Information

2-4__________
__________
namesz  descsz __________
type 
__________
 name  . . . 
__________ desc . . . __________


Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

namesz and name
The first namesz bytes in name contain a null-terminated character representation of theentry’s owner or originator. There is no formal mechanism for avoiding name conflicts. By
convention, vendors use their own name, such as ‘‘XYZ Computer Company,’’ as theidentifier. If no name is present, namesz contains 0. Padding is present, if necessary, toensure 4-byte alignment for the descriptor. Such padding is not included in namesz.
descsz and desc
The first descsz bytes in desc hold the note descriptor. The ABI places no constraints on a
descriptor’s contents. If no descriptor is present, descsz contains 0. Padding is present, if
necessary, to ensure 4-byte alignment for the next note entry. Such padding is not included
in descsz.
typeThis word gives the interpretation of the descriptor. Each originator controls its own types;
multiple interpretations of a single type value may exist. Thus, a program must recognize
both the name and the type to ‘‘understand’’ a descriptor. Types currently must be nonnegative. The ABI does not define what descriptors mean.

To illustrate, the following note segment holds two entries.
Figure 2-4: Example Note Segment

nameszdescsztype
namenameszdescsz
type
name
desc

NOTE

+0
+1
+2
+3
______________________
______________________
7 No descriptor0
______________________
1
______________________
______________________X  Y  Z 





C  o  \0  pad _____________________
_______________________
______________________

7


8
______________________
3
______________________
______________________X  Y  Z 





C  o  \0  pad 
______________________
______________________
word 0word
1
______________________

The system reserves note information with no name (namesz= =0) and with a zero-length name(name[0]= =’\0’) but currently defines no types. All other names must have at least one non-nullcharacter.

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Portable Formats Specification, Version 1.1

2-5

ELF: Executable and Linkable Format

NOTE

2-6

Note information is optional. The presence of note information does not affect a program’s ABI conformance, provided the information does not affect the program’s execution behavior. Otherwise, the program does not conform to the ABI and has undefined behavior.

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)Program Loading
As the system creates or augments a process image, it logically copies a file’s segment to a virtual
memory segment. When—and if—the system physically reads the file depends on the program’s execution behavior, system load, etc. A process does not require a physical page unless it references the logical
page during execution, and processes commonly leave many pages unreferenced. Therefore delaying
physical reads frequently obviates them, improving system performance. To obtain this efficiency inpractice, executable and shared object files must have segment images whose file offsets and virtual
addresses are congruent, modulo the page size.
Virtual addresses and file offsets for the SYSTEM V architecture segments are congruent modulo 4 KB
(0x1000) or larger powers of 2. Because 4 KB is the maximum page size, the files will be suitable for paging regardless of physical page size.
Figure 2-5: Executable File

File Offset
0
Program header table
0x100

0x2bf00

0x30d00

File
Virtual Address____________________
____________________

ELF header
____________________
Other information 
____________________
 0x8048100
Text segment


...


0x2be00 bytes  0x8073eff
____________________
 Data segment
 0x8074f00
...
 0x4e00 bytes
 0x8079cff
____________________ Other information ...
____________________


Figure 2-6: Program Header Segments

Member
Text
Data______________________________________________
p_typePT_LOAD 
PT_LOADp_offset 
0x100 0x2bf00
p_vaddr  0x8048100 
0x8074f00

p_paddr  unspecified 
unspecified
p_filesz 
0x2be00 
0x4e00p_memsz 
0x2be00 
0x5e24p_flags  PF_R + PF_X  PF_R + PF_W + PF_X
p_align 
0x1000 
0x1000______________________________________________


Although the example’s file offsets and virtual addresses are congruent modulo 4 KB for both text anddata, up to four file pages hold impure text or data (depending on page size and file system block size).
The first text page contains the ELF header, the program header table, and other information.

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ELF: Executable and Linkable Format

The last text page holds a copy of the beginning of data.
The first data page has a copy of the end of text.
The last data page may contain file information not relevant to the running process.
Logically, the system enforces the memory permissions as if each segment were complete and separate;
segments’ addresses are adjusted to ensure each logical page in the address space has a single set of permissions. In the example above, the region of the file holding the end of text and the beginning of data
will be mapped twice: at one virtual address for text and at a different virtual address for data.
The end of the data segment requires special handling for uninitialized data, which the system defines tobegin with zero values. Thus if a file’s last data page includes information not in the logical memory
page, the extraneous data must be set to zero, not the unknown contents of the executable file. ‘‘Impurities’’ in the other three pages are not logically part of the process image; whether the system expunges
them is unspecified. The memory image for this program follows, assuming 4 KB (0x1000) pages.
Figure 2-7: Process Image Segments

Virtual Address ____________________
Contents
Segment
0x8048000
Header padding
0x100 bytes____________________
 Text0x8048100 
Text segment




...




0x2be00 bytes 
____________________

0x8073f00 
Data padding
____________________

0x100
bytes


____________________

0x8074000 
Text padding
____________________

0xf00 bytes

0x8074f00 Data segment




...




0x4e00bytes
____________________
0x8079d00  Uninitialized data 
____________________
0x1024 zero bytes 
0x807ad24 Page padding
0x2dc zero bytes 
____________________

Data

One aspect of segment loading differs between executable files and shared objects. Executable file segments typically contain absolute code. To let the process execute correctly, the segments must reside atthe virtual addresses used to build the executable file. Thus the system uses the p_vaddr values
unchanged as virtual addresses.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

On the other hand, shared object segments typically contain position-independent code. This lets a
segment’s virtual address change from one process to another, without invalidating execution behavior.
Though the system chooses virtual addresses for individual processes, it maintains the segments’ relative
positions. Because position-independent code uses relative addressing between segments, the difference
between virtual addresses in memory must match the difference between virtual addresses in the file.
The following table shows possible shared object virtual address assignments for several processes, illustrating constant relative positioning. The table also illustrates the base address computations.
Figure 2-8: Example Shared Object Segment Addresses

Sourc
Text
Data
Base Address______________________________________________________
File
0x200 0x2a400 
0x0
Process 1  0x80000200  0x8002a400  0x80000000
Process 2  0x80081200  0x800ab400  0x80081000

Process 3  0x900c0200  0x900ea400  0x900c0000
4  0x900c6200  0x900f0400  0x900c6000_Process_____________________________________________________

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Dynamic Linking
Program Interpreter
An executable file may have one PT_INTERP program header element. During exec(BA_OS), the system retrieves a path name from the PT_INTERP segment and creates the initial process image from theinterpreter file’s segments. That is, instead of using the original executable file’s segment images, the system composes a memory image for the interpreter. It then is the interpreter’s responsibility to receive
control from the system and provide an environment for the application program.
The interpreter receives control in one of two ways. First, it may receive a file descriptor to read the executable file, positioned at the beginning. It can use this file descriptor to read and/or map the executable
file’s segments into memory. Second, depending on the executable file format, the system may load theexecutable file into memory instead of giving the interpreter an open file descriptor. With the possible
exception of the file descriptor, the interpreter’s initial process state matches what the executable filewould have received. The interpreter itself may not require a second interpreter. An interpreter may beeither a shared object or an executable file.
A shared object (the normal case) is loaded as position-independent, with addresses that may varyfrom one process to another; the system creates its segments in the dynamic segment area used bymmap(KE_OS) and related services. Consequently, a shared object interpreter typically will notconflict with the original executable file’s original segment addresses.
An executable file is loaded at fixed addresses; the system creates its segments using the virtual
addresses from the program header table. Consequently, an executable file interpreter’s virtual
addresses may collide with the first executable file; the interpreter is responsible for resolving
conflicts.

Dynamic Linker
When building an executable file that uses dynamic linking, the link editor adds a program header element of type PT_INTERP to an executable file, telling the system to invoke the dynamic linker as the program interpreter.
The locations of the system provided dynamic linkers are processor–specific.
NOTE

Exec(BA_OS) and the dynamic linker cooperate to create the process image for the program, which
entails the following actions:
Adding the executable file’s memory segments to the process image;
Adding shared object memory segments to the process image;
Performing relocations for the executable file and its shared objects;
Closing the file descriptor that was used to read the executable file, if one was given to the dynamic
linker;Transferring control to the program, making it look as if the program had received control directlyfrom exec(BA_OS).

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The link editor also constructs various data that assist the dynamic linker for executable and shared object
files. As shown above in ‘‘Program Header,’’ these data reside in loadable segments, making them available during execution. (Once again, recall the exact segment contents are processor-specific. See the processor supplement for complete information.)A .dynamic section with type SHT_DYNAMIC holds various data. The structure residing at the
beginning of the section holds the addresses of other dynamic linking information.
The .hash section with type SHT_HASH holds a symbol hash table.
The .got and .plt sections with type SHT_PROGBITS hold two separate tables: the global offset
table and the procedure linkage table. Sections below explain how the dynamic linker uses andchanges the tables to create memory images for object files.
Because every ABI-conforming program imports the basic system services from a shared object library,
the dynamic linker participates in every ABI-conforming program execution.
As ‘‘Program Loading’’ explains in the processor supplement, shared objects may occupy virtual memory
addresses that are different from the addresses recorded in the file’s program header table. The dynamic
linker relocates the memory image, updating absolute addresses before the application gains control.
Although the absolute address values would be correct if the library were loaded at the addresses
specified in the program header table, this normally is not the case.
If the process environment [see exec(BA_OS)] contains a variable named LD_BIND_NOW with a non-nullvalue, the dynamic linker processes all relocation before transferring control to the program. For example, all the following environment entries would specify this behavior.
LD_BIND_NOW=1
LD_BIND_NOW=on
LD_BIND_NOW=off
Otherwise, LD_BIND_NOW either does not occur in the environment or has a null value. The dynamic
linker is permitted to evaluate procedure linkage table entries lazily, thus avoiding symbol resolution andrelocation overhead for functions that are not called. See ‘‘Procedure Linkage Table’’ in this part for more
information.

Dynamic Section
If an object file participates in dynamic linking, its program header table will have an element of type
PT_DYNAMIC. This ‘‘segment’’ contains the .dynamic section. A special symbol, _DYNAMIC, labels thesection, which contains an array of the following structures.

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ELF: Executable and Linkable Format

Figure 2-9: Dynamic Structure

typedef struct {Elf32_Swordd_tag;
union {
Elf32_Word
Elf32_Addr} d_un;} Elf32_Dyn;

d_val;
d_ptr;

extern Elf32_Dyn _DYNAMIC[];For each object with this type, d_tag controls the interpretation of d_un.
d_val

These Elf32_Word objects represent integer values with various interpretations.

d_ptr

These Elf32_Addr objects represent program virtual addresses. As mentioned previously,
a file’s virtual addresses might not match the memory virtual addresses during execution.
When interpreting addresses contained in the dynamic structure, the dynamic linker computes actual addresses, based on the original file value and the memory base address. For
consistency, files do not contain relocation entries to ‘‘correct’’ addresses in the dynamic
structure.

The following table summarizes the tag requirements for executable and shared object files. If a tag ismarked ‘‘mandatory,’’ then the dynamic linking array for an ABI-conforming file must have an entry ofthat type. Likewise, ‘‘optional’’ means an entry for the tag may appear but is not required.
Figure 2-10: Dynamic Array Tags, d_tag

Name
Value
d_un
Executable
Shared Object_______________________________________________________________________
 mandatory  mandatory
DT_NULL
0  ignored optional optional
DT_NEEDED
1  d_val optional
DT_PLTRELSZ
2d_val
optional
DT_PLTGOT
3  d_ptr
 optional optional
DT_HASH4  d_ptr
 mandatory  mandatory mandatory  mandatory
DT_STRTAB
5  d_ptr
 mandatory  mandatory
DT_SYMTAB
6  d_ptr mandatory  optional
DT_RELA
7
d_ptr




DT_RELASZ
8  d_val mandatory  optional
DT_RELAENT 
9  d_val
 mandatory  optional mandatory  mandatory
DT_STRSZ
10  d_val
 mandatory  mandatory
DT_SYMENT11  d_val optional optional
DT_INIT
12
d_ptr




DT_FINI13  d_ptr
 optional optional
DT_SONAME
14  d_val
 ignored optional optional ignored
DT_RPATH
15  d_val
 ignored optional
DT_SYMBOLIC 
16  ignored

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Figure 2-10: Dynamic Array Tags, d_tag (continued )

Name
Value
d_un
Executable
Shared Object_______________________________________________________________________
 mandatory  optional
DT_REL17  d_ptr mandatory  optional
DT_RELSZ
18  d_val
 mandatory  optional
DT_RELENT
19  d_val

DT_PLTREL
20  d_val optional optional
DT_DEBUG
21
d_ptr
optional ignored optional optional
DT_TEXTREL 
22  ignored optional optional
DT_JMPREL
23  d_ptr
 0x70000000  unspecified  unspecified  unspecified
DT_LOPROC
DT_HIPROC
 0x7fffffff  unspecified  unspecified  unspecified_______________________________________________________________________

DT_NULL

An entry with a DT_NULL tag marks the end of the _DYNAMIC array.

DT_NEEDEDThis element holds the string table offset of a null-terminated string, giving the name ofa needed library. The offset is an index into the table recorded in the DT_STRTAB
entry. See ‘‘Shared Object Dependencies’’ for more information about these names.
The dynamic array may contain multiple entries with this type. These entries’ relative
order is significant, though their relation to entries of other types is not.

DT_PLTRELSZThis element holds the total size, in bytes, of the relocation entries associated with theprocedure linkage table. If an entry of type DT_JMPREL is present, a DT_PLTRELSZ
must accompany it.

DT_PLTGOTThis element holds an address associated with the procedure linkage table and/or the
global offset table. See this section in the processor supplement for details.

DT_HASH

This element holds the address of the symbol hash table, described in ‘‘Hash Table.’’This hash table refers to the symbol table referenced by the DT_SYMTAB element.

DT_STRTAB

This element holds the address of the string table, described in Part 1. Symbol names,
library names, and other strings reside in this table.

DT_SYMTAB

This element holds the address of the symbol table, described in Part 1, with
Elf32_Sym entries for the 32-bit class of files.

DT_RELAThis element holds the address of a relocation table, described in Part 1. Entries in thetable have explicit addends, such as Elf32_Rela for the 32-bit file class. An object file
may have multiple relocation sections. When building the relocation table for an executable or shared object file, the link editor catenates those sections to form a single
table. Although the sections remain independent in the object file, the dynamic linker
sees a single table. When the dynamic linker creates the process image for an executable file or adds a shared object to the process image, it reads the relocation table andperforms the associated actions. If this element is present, the dynamic structure mustalso have DT_RELASZ and DT_RELAENT elements. When relocation is ‘‘mandatory’’for a file, either DT_RELA or DT_REL may occur (both are permitted but not required).

DT_RELASZThis element holds the total size, in bytes, of the DT_RELA relocation table.

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DT_RELAENTThis element holds the size, in bytes, of the DT_RELA relocation entry.

DT_STRSZThis element holds the size, in bytes, of the string table.

DT_SYMENTThis element holds the size, in bytes, of a symbol table entry.

DT_INIT

This element holds the address of the initialization function, discussed in ‘‘Initialization
and Termination Functions’’ below.

DT_FINI

This element holds the address of the termination function, discussed in ‘‘Initialization
and Termination Functions’’ below.

DT_SONAME

This element holds the string table offset of a null-terminated string, giving the name ofthe shared object. The offset is an index into the table recorded in the DT_STRTAB
entry. See ‘‘Shared Object Dependencies’’ below for more information about thesenames.

DT_RPATH

This element holds the string table offset of a null-terminated search library search pathstring, discussed in ‘‘Shared Object Dependencies.’’ The offset is an index into the table
recorded in the DT_STRTAB entry.

DT_SYMBOLIC

This element’s presence in a shared object library alters the dynamic linker’s symbol
resolution algorithm for references within the library. Instead of starting a symbol
search with the executable file, the dynamic linker starts from the shared object itself. If
the shared object fails to supply the referenced symbol, the dynamic linker thensearches the executable file and other shared objects as usual.

DT_REL

This element is similar to DT_RELA, except its table has implicit addends, such as
Elf32_Rel for the 32-bit file class. If this element is present, the dynamic structure
must also have DT_RELSZ and DT_RELENT elements.

DT_RELSZ

This element holds the total size, in bytes, of the DT_REL relocation table.

DT_RELENTThis element holds the size, in bytes, of the DT_REL relocation entry.

DT_PLTREL

This member specifies the type of relocation entry to which the procedure linkage tablerefers. The d_val member holds DT_REL or DT_RELA, as appropriate. All relocations
in a procedure linkage table must use the same relocation.

DT_DEBUG

This member is used for debugging. Its contents are not specified for the ABI; programs that access this entry are not ABI-conforming.

DT_TEXTRELThis member’s absence signifies that no relocation entry should cause a modification toa non-writable segment, as specified by the segment permissions in the program header
table. If this member is present, one or more relocation entries might request
modifications to a non-writable segment, and the dynamic linker can prepare accordingly.

DT_JMPREL

If present, this entries’s d_ptr member holds the address of relocation entries associated solely with the procedure linkage table. Separating these relocation entries lets the
dynamic linker ignore them during process initialization, if lazy binding is enabled. If
this entry is present, the related entries of types DT_PLTRELSZ and DT_PLTREL mustalso be present.

DT_LOPROC through DT_HIPROC
Values in this inclusive range are reserved for processor-specific semantics.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Except for the DT_NULL element at the end of the array, and the relative order of DT_NEEDED elements,
entries may appear in any order. Tag values not appearing in the table are reserved.

Shared Object Dependencies
When the link editor processes an archive library, it extracts library members and copies them into the
output object file. These statically linked services are available during execution without involving the
dynamic linker. Shared objects also provide services, and the dynamic linker must attach the propershared object files to the process image for execution. Thus executable and shared object files describe
their specific dependencies.
When the dynamic linker creates the memory segments for an object file, the dependencies (recorded in
DT_NEEDED entries of the dynamic structure) tell what shared objects are needed to supply the
program’s services. By repeatedly connecting referenced shared objects and their dependencies, thedynamic linker builds a complete process image. When resolving symbolic references, the dynamic
linker examines the symbol tables with a breadth-first search. That is, it first looks at the symbol table ofthe executable program itself, then at the symbol tables of the DT_NEEDED entries (in order), then at thesecond level DT_NEEDED entries, and so on. Shared object files must be readable by the process; otherpermissions are not required.
NOTE

Even when a shared object is referenced multiple times in the dependency list, the dynamic linker willconnect the object only once to the process.

Names in the dependency list are copies either of the DT_SONAME strings or the path names of the sharedobjects used to build the object file. For example, if the link editor builds an executable file using one
shared object with a DT_SONAME entry of lib1 and another shared object library with the path name/usr/lib/lib2, the executable file will contain lib1 and /usr/lib/lib2 in its dependency list.
If a shared object name has one or more slash (/) characters anywhere in the name, such as/usr/lib/lib2 above or directory/file, the dynamic linker uses that string directly as the path
name. If the name has no slashes, such as lib1 above, three facilities specify shared object path searching, with the following precedence.
First, the dynamic array tag DT_RPATH may give a string that holds a list of directories, separated
by colons (:). For example, the string /home/dir/lib:/home/dir2/lib: tells the dynamic
linker to search first the directory /home/dir/lib, then /home/dir2/lib, and then the current
directory to find dependencies.
Second, a variable called LD_LIBRARY_PATH in the process environment [see exec(BA_OS)] may
hold a list of directories as above, optionally followed by a semicolon (;) and another directory list.
The following values would be equivalent to the previous example:
LD_LIBRARY_PATH=/home/dir/lib:/home/dir2/lib:
LD_LIBRARY_PATH=/home/dir/lib;/home/dir2/lib:
LD_LIBRARY_PATH=/home/dir/lib:/home/dir2/lib:;
All LD_LIBRARY_PATH directories are searched after those from DT_RPATH. Although some programs (such as the link editor) treat the lists before and after the semicolon differently, the dynamic
linker does not. Nevertheless, the dynamic linker accepts the semicolon notation, with theTool Interface Standards (TIS)

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ELF: Executable and Linkable Format

semantics described above.
Finally, if the other two groups of directories fail to locate the desired library, the dynamic linker
searches /usr/lib.

NOTE

For security, the dynamic linker ignores environmental search specifications (such as
LD_LIBRARY_PATH) for set-user and set-group ID programs. It does, however, search DT_RPATH
directories and /usr/lib.

Global Offset Table
Position-independent code cannot, in general, contain absolute virtual addresses. Global offset tables
hold absolute addresses in private data, thus making the addresses available without compromising theposition-independence and sharability of a program’s text. A program references its global offset tableusing position-independent addressing and extracts absolute values, thus redirecting positionindependent references to absolute locations.
Initially, the global offset table holds information as required by its relocation entries [see ‘‘Relocation’’ inPart 1]. After the system creates memory segments for a loadable object file, the dynamic linker processes
the relocation entries, some of which will be type R_386_GLOB_DAT referring to the global offset table.
The dynamic linker determines the associated symbol values, calculates their absolute addresses, and sets
the appropriate memory table entries to the proper values. Although the absolute addresses are
unknown when the link editor builds an object file, the dynamic linker knows the addresses of all
memory segments and can thus calculate the absolute addresses of the symbols contained therein.
If a program requires direct access to the absolute address of a symbol, that symbol will have a global
offset table entry. Because the executable file and shared objects have separate global offset tables, asymbol’s address may appear in several tables. The dynamic linker processes all the global offset tablerelocations before giving control to any code in the process image, thus ensuring the absolute addresses
are available during execution.
The table’s entry zero is reserved to hold the address of the dynamic structure, referenced with the symbol _DYNAMIC. This allows a program, such as the dynamic linker, to find its own dynamic structure
without having yet processed its relocation entries. This is especially important for the dynamic linker,
because it must initialize itself without relying on other programs to relocate its memory image. On the
32-bit Intel Architecture, entries one and two in the global offset table also are reserved. ‘‘Procedure
Linkage Table’’ below describes them.
The system may choose different memory segment addresses for the same shared object in different programs; it may even choose different library addresses for different executions of the same program.
Nonetheless, memory segments do not change addresses once the process image is established. As longas a process exists, its memory segments reside at fixed virtual addresses.
A global offset table’s format and interpretation are processor-specific. For the 32-bit Intel Architecture,
the symbol _GLOBAL_OFFSET_TABLE_ may be used to access the table.

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Tool Interface Standards (TIS)ELF: Executable and Linkable Format

Figure 2-11: Global Offset Table

extern Elf32_Addr_GLOBAL_OFFSET_TABLE_[];The symbol _GLOBAL_OFFSET_TABLE_ may reside in the middle of the .got section, allowing both
negative and non-negative ‘‘subscripts’’ into the array of addresses.

Procedure Linkage Table
Much as the global offset table redirects position-independent address calculations to absolute locations,the procedure linkage table redirects position-independent function calls to absolute locations. The link
editor cannot resolve execution transfers (such as function calls) from one executable or shared object toanother. Consequently, the link editor arranges to have the program transfer control to entries in the procedure linkage table. On the SYSTEM V architecture, procedure linkage tables reside in shared text, butthey use addresses in the private global offset table. The dynamic linker determines the destinations’absolute addresses and modifies the global offset table’s memory image accordingly. The dynamic linker
thus can redirect the entries without compromising the position-independence and sharability of the
program’s text. Executable files and shared object files have separate procedure linkage tables.
Figure 2-12: Absolute Procedure Linkage Table.PLT0:pushl got_plus_4
jmp*got_plus_8
nop; nop
nop; nop.PLT1:jmp*name1_in_GOT
pushl $offset@PC.PLT2:jmp*name2_in_GOT
push $offset
jmp.PLT0@PC...

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ELF: Executable and Linkable Format

Figure 2-13: Position-Independent Procedure Linkage Table.PLT0:pushl 4(%ebx)
jmp*8(%ebx)
nop; nop
nop; nop.PLT1:jmp*name1@GOT(%ebx)
pushl $offset
jmp.PLT0@PC.PLT2:jmp*name2@GOT(%ebx)
pushl $offset
jmp.PLT0@PC...

NOTE

As the figures show, the procedure linkage table instructions use different operand addressing modes
for absolute code and for position-independent code. Nonetheless, their interfaces to the dynamic linker
are the same.

Following the steps below, the dynamic linker and the program ‘‘cooperate’’ to resolve symbolic references through the procedure linkage table and the global offset table.
1 . When first creating the memory image of the program, the dynamic linker sets the second and thethird entries in the global offset table to special values. Steps below explain more about thesevalues.
2 . If the procedure linkage table is position-independent, the address of the global offset table must
reside in %ebx. Each shared object file in the process image has its own procedure linkage table,
and control transfers to a procedure linkage table entry only from within the same object file. Consequently, the calling function is responsible for setting the global offset table base register before
calling the procedure linkage table entry.
3 . For illustration, assume the program calls name1, which transfers control to the label .PLT1.
4 . The first instruction jumps to the address in the global offset table entry for name1. Initially, theglobal offset table holds the address of the following pushl instruction, not the real address of
name1.
5 . Consequently, the program pushes a relocation offset (offset) on the stack. The relocation offset is a
32-bit, non-negative byte offset into the relocation table. The designated relocation entry will havetype R_386_JMP_SLOT, and its offset will specify the global offset table entry used in the previous
jmp instruction. The relocation entry also contains a symbol table index, thus telling the dynamic
linker what symbol is being referenced, name1 in this case.
6 . After pushing the relocation offset, the program then jumps to .PLT0, the first entry in the procedure linkage table. The pushl instruction places the value of the second global offset table entry(got_plus_4 or 4(%ebx)) on the stack, thus giving the dynamic linker one word of identifying
information. The program then jumps to the address in the third global offset table entry

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Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format(got_plus_8 or 8(%ebx)), which transfers control to the dynamic linker.
7 . When the dynamic linker receives control, it unwinds the stack, looks at the designated relocation
entry, finds the symbol’s value, stores the ‘‘real’’ address for name1 in its global offset table entry,
and transfers control to the desired destination.
8 . Subsequent executions of the procedure linkage table entry will transfer directly to name1, without
calling the dynamic linker a second time. That is, the jmp instruction at .PLT1 will transfer toname1, instead of ‘‘falling through’’ to the pushl instruction.
The LD_BIND_NOW environment variable can change dynamic linking behavior. If its value is non-null,
the dynamic linker evaluates procedure linkage table entries before transferring control to the program.
That is, the dynamic linker processes relocation entries of type R_386_JMP_SLOT during process initialization. Otherwise, the dynamic linker evaluates procedure linkage table entries lazily, delaying symbol
resolution and relocation until the first execution of a table entry.
NOTE

Lazy binding generally improves overall application performance, because unused symbols do not incur
the dynamic linking overhead. Nevertheless, two situations make lazy binding undesirable for some
applications. First, the initial reference to a shared object function takes longer than subsequent calls,
because the dynamic linker intercepts the call to resolve the symbol. Some applications cannot tolerate
this unpredictability. Second, if an error occurs and the dynamic linker cannot resolve the symbol, thedynamic linker will terminate the program. Under lazy binding, this might occur at arbitrary times. Onceagain, some applications cannot tolerate this unpredictability. By turning off lazy binding, the dynamic
linker forces the failure to occur during process initialization, before the application receives control.

Hash Table
A hash table of Elf32_Word objects supports symbol table access. Labels appear below to help explain
the hash table organization, but they are not part of the specification.
Figure 2-14: Symbol Hash Table________________________
________________________

nbucketnchain________________________
bucket[0]




. . .
________________________
bucket[nbucket - 1] chain[0]


. . .


________________________
chain[nchain - 1] The bucket array contains nbucket entries, and the chain array contains nchain entries; indexes
start at 0. Both bucket and chain hold symbol table indexes. Chain table entries parallel the symbol
table. The number of symbol table entries should equal nchain; so symbol table indexes also selectchain table entries. A hashing function (shown below) accepts a symbol name and returns a value that
may be used to compute a bucket index. Consequently, if the hashing function returns the value x forsome name, bucket[x%nbucket] gives an index, y, into both the symbol table and the chain table. If
the symbol table entry is not the one desired, chain[y] gives the next symbol table entry with the samehash value. One can follow the chain links until either the selected symbol table entry holds the desired

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ELF: Executable and Linkable Format

name or the chain entry contains the value STN_UNDEF.
Figure 2-15: Hashing Function

unsigned long
elf_hash(const unsigned char *name){unsigned longh = 0, g;while (*name){h = (h << 4) + *name++;if (g = h & 0xf0000000)
h ^= g >> 24;
h &= ~g;}return h;}Initialization and Termination Functions
After the dynamic linker has built the process image and performed the relocations, each shared object
gets the opportunity to execute some initialization code. These initialization functions are called in no
specified order, but all shared object initializations happen before the executable file gains control.
Similarly, shared objects may have termination functions, which are executed with the atexit(BA_OS)
mechanism after the base process begins its termination sequence. Once again, the order in which the
dynamic linker calls termination functions is unspecified.
Shared objects designate their initialization and termination functions through the DT_INIT and
DT_FINI entries in the dynamic structure, described in ‘‘Dynamic Section’’ above. Typically, the codefor these functions resides in the .init and .fini sections, mentioned in ‘‘Sections’’ of Part 1.
NOTE

2-20

Although the atexit(BA_OS) termination processing normally will be done, it is not guaranteed tohave executed upon process death. In particular, the process will not execute the termination processing if it calls _exit [see exit(BA_OS)] or if the process dies because it received a signal that it neither caught nor ignored.

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)3

C LIBRARY

C Library

3-1
3-2

Global Data Symbols

Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1iC Library
The C library, libc, contains all of the symbols contained in libsys, and, in addition, contains the routines listed in the following two tables. The first table lists routines from the ANSI C standard.
Figure 3-1: libc Contents, Names without Synonyms

abort
abs
asctime
atof
atoi
atol
bsearchclearerr
clock
ctime
difftime
div
fclosefeof
ferrorfflushfgetc
fgetpos
fgets
fopen
fprintf

fputc
fputs
fread
freopen
frexp
fscanffseek
fsetpos
ftell
fwritegetc
getchargetenvgets
gmtimeisalnum
isalphaiscntrl
isdigitisgraphislower

isprint
ispunct
isspaceisupper
isxdigit
labs
ldexp
ldiv
localtime
longjmpmblen
mbstowcs
mbtowcmemchrmemcmpmemcpymemmovememsetmktimeperrorprintfputc
putchar
puts
qsort
raise
rand
rewindscanf
setbufsetjmpsetvbuf
sprintf
srandsscanfstrcatstrchrstrcmpstrcpystrcspnstrlenstrncatstrncmpstrncpy
strpbrk
strrchrstrspnstrstrstrtodstrtokstrtolstrtoul
tmpfiletmpnamtolower
toupper
ungetcvfprintf
vprintf
vsprintf
wcstombs
wctombAdditionally, libc holds the following services.
Figure 3-2: libc Contents, Names with Synonyms_ _assert
cfgetispeed
cfgetospeed
cfsetispeed
cfsetospeed
ctermid
cuserid
dup2
fdopen_ _filbuf
fileno_ _flsbuf
fmtmsg †getdategetoptgetpass
getsubopt
getw
hcreatehdestroyhsearchisascii
isattyisnan
isnand †lfind

lockf †
lsearchmemccpy
mkfifomktempmonitornftw
nl_langinfo
pclosepopen
putenv
putw
setlabel

sleep
strdupswab
tcdraintcflow
tcflushtcgetattr
tcgetpgrp
tcgetsid
tcsendbreaktcsetattr
tcsetpgrp
tdeletetell †
tempnamtfind
toascii_tolowertsearch_toupper
twalk
tzset
_xftw

† Function is at Level 2 in the SVID Issue 3 and therefore at Level 2 in the ABI.

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Besides the symbols listed in the With Synonyms table above, synonyms of the form _name exist for nameentries that are not listed with a leading underscore prepended to their name. Thus libc contains bothgetopt and _getopt, for example.
Of the routines listed above, the following are not defined elsewhere.
int _ _filbuf(FILE *f);
This function returns the next input character for f, filling its buffer as appropriate. It
returns EOF if an error occurs.
int _ _flsbuf(int x, FILE *f);
This function flushes the output characters for f as if putc(x,f) had been called and thenappends the value of x to the resulting output stream. It returns EOF if an error occurs andx otherwise.
int _xftw(int, char *, int (*)(char *, struct stat *, int), int);
Calls to the ftw(BA_LIB) function are mapped to this function when applications are compiled. This function is identical to ftw(BA_LIB), except that _xftw() takes an interposed
first argument, which must have the value 2.
See this chapter’s other library sections for more SVID, ANSI C, and POSIX facilities. See ‘‘System Data
Interfaces’’ later in this chapter for more information.

Global Data Symbols
The libc library requires that some global external data symbols be defined for its routines to work
properly. All the data symbols required for the libsys library must be provided by libc, as well as thedata symbols listed in the table below.
For formal declarations of the data objects represented by these symbols, see the System V Interface
Definition, Third Edition or the ‘‘Data Definitions’’ section of Chapter 6 in the appropriate processor supplement to the System V ABI.
For entries in the following table that are in name - _name form, both symbols in each pair represent thesame data. The underscore synonyms are provided to satisfy the ANSI C standard.
Figure 3-3: libc Contents, Global External Data Symbols

getdate_err_getdate_err_ _iob3-2

Tool Interface Standards (TIS)

optargopterroptindoptoptPortable Formats Specification, Version 1.1

I

Index

Index

Tool Interface Standards (TIS)

I-1

Portable Formats Specification, Version 1.1iIndex
2’s complement

1: 6

A
ABI conformance 1: 11, 2: 3, 6, 12, 14
abort 3: 1
abs 3: 1
absolute code 2: 9
absolute symbols 1: 8
address, virtual 2: 7
addseverity 3: 1
alignment
executable file 2: 7
section 1: 10
ANSI C 3: 2
archive file 1: 18, 2: 15
asctime 3: 1
assembler 1: 1
symbol names 1: 17_ _assert 3: 1
atexit(BA_OS) 2: 20
atof 3: 1
atoi 3: 1
atol 3: 1

Bbase address 1: 22, 2: 9, 12
definition 2: 4
bsearch 3: 1
byte order 1: 6

CC language
assembly names 1: 17
library (see library)
C library 3: 1
cfgetispeed 3: 1
cfgetospeed 3: 1
cfsetispeed 3: 1
cfsetospeed 3: 1
clearerr 3: 1
clock 3: 1
common symbols 1: 8
core file 1: 3
ctermid 3: 1

Tool Interface Standards (TIS)

ctime 3: 1
cuserid 3: 1

Ddata, uninitialized 2: 8
data representation 1: 2, 6difftime 3: 1
div 3: 1
dup2 3: 1_DYNAMIC 2: 11
see also dynamic linking 2: 11
dynamic library (see shared object file)
dynamic linker 1: 1, 2: 10–11
see also dynamic linking 2: 10
see also link editor 2: 10
see also shared object file 2: 10
dynamic linking 2: 10
base address 2: 4
_DYNAMIC 2: 11
environment 2: 11, 15, 19
hash function 2: 19
initialization function 2: 14, 20
lazy binding 2: 11, 19
LD_BIND_NOW 2: 11, 19
LD_LIBRARY_PATH 2: 15
relocation 2: 13, 16, 18
see also dynamic linker 2: 10
see also hash table 2: 13
see also procedure linkage table 2: 13
string table 2: 13
symbol resolution 2: 15
symbol table 1: 10, 14, 2: 13
termination function 2: 14, 20
dynamic segments 2: 9

E
ELF 1: 1
entry point (see process, entry point)
environment 2: 11, 15, 19
exec(BA_OS) 1: 1, 2: 10–11, 15
paging 2: 7
executable file 1: 1
segments 2: 9
exit 2: 20

Portable Formats Specification, Version 1.1

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ELF: Executable and Linkable Format

F

gmtimefclose 3: 1
fdopen 3: 1
feof 3: 1
ferror 3: 1
fflush 3: 1
fgetc 3: 1
fgetpos 3: 1
fgets 3: 1
_ _filbuf 3: 1–2
file, object (see object file)
file offset 2: 7
fileno 3: 1
_ _flsbuf 3: 1–2
fmtmsg 3: 1
fopen 3: 1
formats, object file 1: 1
FORTRAN 1: 8
fprintf 3: 1
fputc 3: 1
fputs 3: 1
fread 3: 1
freopen 3: 1
frexp 3: 1
fscanf 3: 1
fseek 3: 1
fsetpos 3: 1
ftell 3: 1
ftw(BA_LIB) 3: 2
fwrite 3: 1

Hhash function 2: 19
hash table 1: 12, 14, 2: 11, 13, 19
hcreate 3: 1
hdestroy 3: 1
hsearch 3: 1

I
interpreter, see program interpreter_ _iob 3: 2
isalnum 3: 1
isalpha 3: 1
isascii 3: 1
isatty 3: 1
iscntrl 3: 1
isdigit 3: 1
isgraph 3: 1
islower 3: 1
isnan 3: 1
isnand 3: 1
isprint 3: 1
ispunct 3: 1
isspace 3: 1
isupper 3: 1
isxdigit 3: 1

2: 10

J

G
getc 3: 1
getchar 3: 1
getdate 3: 1
_getdate_err 3: 2
getdate_err 3: 2
getenv 3: 1
getopt 3: 1
_getopt 3: 2
getopt 3: 2
getpass 3: 1
gets 3: 1
getsubopt 3: 1
getw 3: 1
global data symbols 3: 2
global offset table 1: 14, 23–24, 2: 11, 16

I-2

3: 1

jmp instruction

2: 17–18

Llabs 3: 1
lazy binding 2: 11, 19
LD_BIND_NOW 2: 11, 19
ldexp 3: 1
ldiv 3: 1
LD_LIBRARY_PATH 2: 15
ld(SD_CMD) (see link editor)
lfind 3: 1
libc 3: 0, 2
see also library 3: 0
libc contents 3: 1–2

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

library
dynamic (see shared object file)
see also libc 3: 0
shared (see shared object file)libsys 3: 1–2
link editor 1: 1, 18–19, 23, 2: 11, 13, 15–16
see also dynamic linker 2: 10
localtime 3: 1
lockf 3: 1
longjmp 3: 1
lsearch 3: 1

M
magic number 1: 4–5
main 1: 14
mblen 3: 1
mbstowcs 3: 1
mbtowc 3: 1
memccpy 3: 1
memchr 3: 1
memcmp 3: 1
memcpy 3: 1
memmove 3: 1
memset 3: 1
mkfifo 3: 1
mktemp 3: 1
mktime 3: 1
mmap(KE_OS) 2: 10
monitor 3: 1

Nnftw 3: 1
nl_langinfo

3: 1

O
object file 1: 1
archive file 1: 18
data representation 1: 2
data types 1: 2
ELF header 1: 1, 3
extensions 1: 4
format 1: 1
hash table 2: 11, 13, 19
program header 1: 2, 2: 2

Tool Interface Standards (TIS)

program loading 2: 2
relocation 1: 12, 21, 2: 13
section 1: 1, 8
section alignment 1: 10
section attributes 1: 12
section header 1: 2, 8
section names 1: 15
section types 1: 10
see also archive file 1: 1
see also dynamic linking 2: 10
see also executable file 1: 1
see also relocatable file 1: 1
see also shared object file 1: 1
segment 2: 1–2, 7
shared object file 2: 10
special sections 1: 13
string table 1: 12, 16–17
symbol table 1: 12, 17
type 1: 3
version 1: 4
optarg 3: 2
opterr 3: 2
optind 3: 2

Ppage size 2: 7
paging 2: 7
performance 2: 7
pclose 3: 1
performance, paging 2: 7
perror 3: 1
popen 3: 1
position-independent code 2: 9, 11
POSIX 3: 2
printf 3: 1
procedure linkage table 1: 15, 19, 23–24, 2: 11,
13–14, 17

process
entry point 1: 4, 14, 2: 20
image 1: 1, 2: 1–2
virtual addressing 2: 2
processor-specific 2: 10
processor-specific information

1: 4, 6–8, 11–12,
18–19, 21, 2: 1, 3, 7, 11, 14, 16–17, 19
program header 2: 2
program interpreter 1: 14, 2: 10
program loading 2: 1, 7

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ELF: Executable and Linkable Format

pushl instruction 2: 17–18
putc 3: 1
putc(BA_LIB) 3: 2
putchar 3: 1
putenv 3: 1
puts 3: 1
putw 3: 1

Q
qsort

3: 1

R
raise 3: 1
rand 3: 1
relocatable file 1: 1
relocation, see object filerewind 3: 1

1: 21

S
scanf 3: 1
section, object file 2: 7
segment
dynamic 2: 10–11
object file 2: 1–2
permissions 2: 8
process 2: 1, 7, 10, 15–16
program header 2: 2
setbuf 3: 1
setjmp 3: 1
set-user ID programs 2: 16
setvbuf 3: 1
shared library (see shared object file)shared object file 1: 1
functions 1: 19
see also dynamic linking 2: 10
see also object file 2: 10
segments 2: 9
shell scripts 1: 1
sleep 3: 1
sprintf 3: 1
srand 3: 1
sscanf 3: 1
strcat 3: 1
strchr 3: 1

I-4

strcmp 3: 1
strcpy 3: 1
strcspn 3: 1
strdup 3: 1
string table, see object file 1: 16
strlen 3: 1
strncat 3: 1
strncmp 3: 1
strncpy 3: 1
strpbrk 3: 1
strrchr 3: 1
strspn 3: 1
strstr 3: 1
strtod 3: 1
strtok 3: 1
strtol 3: 1
strtoul 3: 1
swab 3: 1
symbol names, C and assembly 1: 17
symbol table, see object file 1: 17
symbolsabsolute 1: 8
binding 1: 18
common 1: 8
see also hash table 1: 14
shared object file functions 1: 19
type 1: 18
undefined 1: 8
value 1: 18, 20
SYSTEM V 2: 7

T
tcdrain 3: 1
tcflow 3: 1
tcflush 3: 1
tcgetattr 3: 1
tcgetpgrp 3: 1
tcgetsid 3: 1
tcsendbreak 3: 1
tcsetattr 3: 1
tcsetpgrp 3: 1
tdelete 3: 1
tell 3: 1
tempnam 3: 1
tfind 3: 1
tmpfile 3: 1
tmpnam 3: 1

Portable Formats Specification, Version 1.1

Tool Interface Standards (TIS)ELF: Executable and Linkable Format

toascii 3: 1
_tolower 3: 1
tolower 3: 1
_toupper 3: 1
toupper 3: 1
tsearch 3: 1
twalk 3: 1
tzset 3: 1

U
undefined behavior 1: 10, 2: 6–7
undefined symbols 1: 8
ungetc 3: 1
uninitialized data 2: 8
unspecified property 1: 2–3, 9, 11, 14, 2: 2–3, 5, 7–8,
14, 20V
vfprintf 3: 1
virtual addressing
vprintf 3: 1
vsprintf 3: 1

2: 2

W
wcstombs 3: 1
wctomb 3: 1

X
_xftw

3: 1–2Z
zero, uninitialized data

2: 8Tool Interface Standards (TIS)

Portable Formats Specification, Version 1.1

I-5