# c99

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding Linux manual page for details of Linux behavior), or
           the interface may not be implemented on Linux.
    


## NAME



           c99 — compile standard C programs
    


## SYNOPSIS



           c99 **[**_options_...**]**_pathname_ **[[**_pathname_**] [** -I _directory_**]**
               **[** -L _directory_**] [** -l _library_**]]**...
    


## DESCRIPTION



           The _c99_ utility is an interface to the standard C compilation
           system; it shall accept source code conforming to the ISO C
           standard. The system conceptually consists of a compiler and link
           editor. The input files referenced by _pathname_ operands and **-l**
           option-arguments shall be compiled and linked to produce an
           executable file. (It is unspecified whether the linking occurs
           entirely within the operation of _c99_ ; some implementations may
           produce objects that are not fully resolved until the file is
           executed.)
    
           If the **-c** option is specified, for all pathname operands of the
           form _file_**.c** , the files:
    
               $(basename _pathname_ .c).o
    
           shall be created as the result of successful compilation. If the
           **-c** option is not specified, it is unspecified whether such **.o**
           files are created or deleted for the _file_**.c** operands.
    
           If there are no options that prevent link editing (such as **-c** or
           **-E**), and all input files compile and link without error, the
           resulting executable file shall be written according to the **-o**
           _outfile_ option (if present) or to the file **a.out**.
    
           The executable file shall be created as specified in _Section_
           _1.1.1.4_ , _File Read_ , _Write_ , _and Creation_ , except that the file
           permission bits shall be set to: S_IRWXO | S_IRWXG | S_IRWXU
    
           and the bits specified by the _umask_ of the process shall be
           cleared.
    


## OPTIONS



           The _c99_ utility shall conform to the Base Definitions volume of
           POSIX.1‐2017, _Section 12.2_ , _Utility Syntax Guidelines_ , except
           that:
    
            *  Options can be interspersed with operands.
    
            *  The order of specifying the **-L** and **-l** options, and the order
               of specifying **-l** options with respect to _pathname_ operands is
               significant.
    
            *  Conforming applications shall specify each option separately;
               that is, grouping option letters (for example, **-cO**) need not
               be recognized by all implementations.
    
           The following options shall be supported:
    
           **-c** Suppress the link-edit phase of the compilation, and do
                     not remove any object files that are produced.
    
           **-D** _name_**[=**_value_**]**
                     Define _name_ as if by a C-language **#define** directive. If
                     no =_value_ is given, a value of 1 shall be used. The **-D**
                     option has lower precedence than the **-U** option. That is,
                     if _name_ is used in both a **-U** and a **-D** option, _name_ shall
                     be undefined regardless of the order of the options.
                     Additional implementation-defined _name_ s may be provided
                     by the compiler. Implementations shall support at least
                     2048 bytes of **-D** definitions and 256 _names_.
    
           **-E** Copy C-language source files to standard output,
                     executing all preprocessor directives; no compilation
                     shall be performed. If any operand is not a text file,
                     the effects are unspecified.
    
           **-g** Produce symbolic information in the object or executable
                     files; the nature of this information is unspecified,
                     and may be modified by implementation-defined
                     interactions with other options.
    
           **-I** _directory_
                     Change the algorithm for searching for headers whose
                     names are not absolute pathnames to look in the
                     directory named by the _directory_ pathname before looking
                     in the usual places. Thus, headers whose names are
                     enclosed in double-quotes (**""**) shall be searched for
                     first in the directory of the file with the **#include**
                     line, then in directories named in **-I** options, and last
                     in the usual places. For headers whose names are
                     enclosed in angle brackets (**" <>"**), the header shall be
                     searched for only in directories named in **-I** options and
                     then in the usual places. Directories named in **-I**
                     options shall be searched in the order specified. If the
                     **-I** option is used to specify a directory that is one of
                     the usual places searched by default, the results are
                     unspecified. Implementations shall support at least ten
                     instances of this option in a single _c99_ command
                     invocation.
    
           **-L** _directory_
                     Change the algorithm of searching for the libraries
                     named in the **-l** objects to look in the directory named
                     by the _directory_ pathname before looking in the usual
                     places. Directories named in **-L** options shall be
                     searched in the order specified. If the **-L** option is
                     used to specify a directory that is one of the usual
                     places searched by default, the results are unspecified.
                     Implementations shall support at least ten instances of
                     this option in a single _c99_ command invocation. If a
                     directory specified by a **-L** option contains files with
                     names starting with any of the strings **"libc."** , **"libl."** ,
                     **"libpthread."** , **"libm."** , **"librt."** , **"libtrace."** ,
                     **"libxnet."** , or **"liby."** , the results are unspecified.
    
           **-l** _library_
                     Search the library named **liblibrary.a**.  A library shall
                     be searched when its name is encountered, so the
                     placement of a **-l** option is significant. Several
                     standard libraries can be specified in this manner, as
                     described in the EXTENDED DESCRIPTION section.
                     Implementations may recognize implementation-defined
                     suffixes other than **.a** as denoting libraries.
    
           **-O** _optlevel_
                     Specify the level of code optimization. If the _optlevel_
                     option-argument is the digit **'0'** , all special code
                     optimizations shall be disabled. If it is the digit **'1'** ,
                     the nature of the optimization is unspecified. If the **-O**
                     option is omitted, the nature of the system's default
                     optimization is unspecified. It is unspecified whether
                     code generated in the presence of the **-O** 0 option is the
                     same as that generated when **-O** is omitted. Other
                     _optlevel_ values may be supported.
    
           **-o** _outfile_
                     Use the pathname _outfile_ , instead of the default **a.out** ,
                     for the executable file produced. If the **-o** option is
                     present with **-c** or **-E** , the result is unspecified.
    
           **-s** Produce object or executable files, or both, from which
                     symbolic and other information not required for proper
                     execution using the _exec_ family defined in the System
                     Interfaces volume of POSIX.1‐2017 has been removed
                     (stripped). If both **-g** and **-s** options are present, the
                     action taken is unspecified.
    
           **-U** _name_   Remove any initial definition of _name_.
    
           Multiple instances of the **-D** , **-I** , **-L** , **-l** , and **-U** options can be
           specified.
    


## OPERANDS



           The application shall ensure that at least one _pathname_ operand is
           specified. The following forms for _pathname_ operands shall be
           supported:
    
           _file._**c** A C-language source file to be compiled and optionally
                     linked. The application shall ensure that the operand is
                     of this form if the **-c** option is used.
    
           _file._**a** A library of object files typically produced by the _ar_
                     utility, and passed directly to the link editor.
                     Implementations may recognize implementation-defined
                     suffixes other than **.a** as denoting object file
                     libraries.
    
           _file._**o** An object file produced by _c99_ **-c** and passed directly to
                     the link editor. Implementations may recognize
                     implementation-defined suffixes other than **.o** as
                     denoting object files.
    
           The processing of other files is implementation-defined.
    


## STDIN



           Not used.
    


## INPUT FILES



           Each input file shall be one of the following: a text file
           containing a C-language source program, an object file in the
           format produced by _c99_ **-c** , or a library of object files, in the
           format produced by archiving zero or more object files, using _ar_.
           Implementations may supply additional utilities that produce files
           in these formats. Additional input file formats are
           implementation-defined.
    


## ENVIRONMENT VARIABLES



           The following environment variables shall affect the execution of
           _c99_ :
    
           _LANG_      Provide a default value for the internationalization
                     variables that are unset or null. (See the Base
                     Definitions volume of POSIX.1‐2017, _Section 8.2_ ,
                     _Internationalization Variables_ for the precedence of
                     internationalization variables used to determine the
                     values of locale categories.)
    
           _LC_ALL_    If set to a non-empty string value, override the values
                     of all the other internationalization variables.
    
           _LC_CTYPE_  Determine the locale for the interpretation of sequences
                     of bytes of text data as characters (for example,
                     single-byte as opposed to multi-byte characters in
                     arguments and input files).
    
           _LC_MESSAGES_
                     Determine the locale that should be used to affect the
                     format and contents of diagnostic messages written to
                     standard error.
    
           _NLSPATH_   Determine the location of message catalogs for the
                     processing of _LC_MESSAGES_.
    
           _TMPDIR_    Provide a pathname that should override the default
                     directory for temporary files, if any.  On XSI-
                     conforming systems, provide a pathname that shall
                     override the default directory for temporary files, if
                     any.
    


## ASYNCHRONOUS EVENTS



           Default.
    


## STDOUT



           If more than one _pathname_ operand ending in **.c**(or possibly other
           unspecified suffixes) is given, for each such file:
    
               "%s:\n", <_pathname_ >
    
           may be written. These messages, if written, shall precede the
           processing of each input file; they shall not be written to the
           standard output if they are written to the standard error, as
           described in the STDERR section.
    
           If the **-E** option is specified, the standard output shall be a text
           file that represents the results of the preprocessing stage of the
           language; it may contain extra information appropriate for
           subsequent compilation passes.
    


## STDERR



           The standard error shall be used only for diagnostic messages.  If
           more than one _pathname_ operand ending in **.c**(or possibly other
           unspecified suffixes) is given, for each such file:
    
               "%s:\n", <_pathname_ >
    
           may be written to allow identification of the diagnostic and
           warning messages with the appropriate input file. These messages,
           if written, shall precede the processing of each input file; they
           shall not be written to the standard error if they are written to
           the standard output, as described in the STDOUT section.
    
           This utility may produce warning messages about certain conditions
           that do not warrant returning an error (non-zero) exit value.
    


## OUTPUT FILES



           Object files or executable files or both are produced in
           unspecified formats. If the pathname of an object file or
           executable file to be created by _c99_ resolves to an existing
           directory entry for a file that is not a regular file, it is
           unspecified whether _c99_ shall attempt to create the file or shall
           issue a diagnostic and exit with a non-zero exit status.
    


## EXTENDED DESCRIPTION



       **Standard Libraries**
           The _c99_ utility shall recognize the following **-l** options for
           standard libraries:
    
           **-l c** This option shall make available all interfaces
                     referenced in the System Interfaces volume of
                     POSIX.1‐2017, with the possible exception of those
                     interfaces listed as residing in _< aio.h>_, _< arpa/inet.h>_,
                     _< complex.h>_, _< fenv.h>_, _< math.h>_, _< mqueue.h>_, _< netdb.h>_,
                     _< net/if.h>_, _< netinet/in.h>_, _< pthread.h>_, _< sched.h>_,
                     _< semaphore.h>_, _< spawn.h>_, _< sys/socket.h>_,
                     _pthread_kill_(), and _pthread_sigmask_() in _< signal.h>_,
                     _< trace.h>_, interfaces marked as optional in
                     _< sys/mman.h>_, interfaces marked as ADV (Advisory
                     Information) in _< fcntl.h>_, and interfaces beginning with
                     the prefix clock_ or timer_ in _< time.h>_.  This option
                     shall not be required to be present to cause a search of
                     this library.
    
           **-l l** This option shall make available all interfaces required
                     by the C-language output of _lex_ that are not made
                     available through the **-l c** option.
    
           **-l pthread**
                     This option shall make available all interfaces
                     referenced in _< pthread.h>_ and _pthread_kill_() and
                     _pthread_sigmask_() referenced in _< signal.h>_.  An
                     implementation may search this library in the absence of
                     this option.
    
           **-l m** This option shall make available all interfaces
                     referenced in _< math.h>_, _< complex.h>_, and _< fenv.h>_.  An
                     implementation may search this library in the absence of
                     this option.
    
           **-l rt** This option shall make available all interfaces
                     referenced in _< aio.h>_, _< mqueue.h>_, _< sched.h>_,
                     _< semaphore.h>_, and _< spawn.h>_, interfaces marked as
                     optional in _< sys/mman.h>_, interfaces marked as ADV
                     (Advisory Information) in _< fcntl.h>_, and interfaces
                     beginning with the prefix clock_ and timer_ in _< time.h>_.
                     An implementation may search this library in the absence
                     of this option.
    
           **-l trace** This option shall make available all interfaces
                     referenced in _< trace.h>_.  An implementation may search
                     this library in the absence of this option.
    
           **-l xnet** This option shall make available all interfaces
                     referenced in _< arpa/inet.h>_, _< netdb.h>_, _< net/if.h>_,
                     _< netinet/in.h>_, and _< sys/socket.h>_.  An implementation
                     may search this library in the absence of this option.
    
           **-l y** This option shall make available all interfaces required
                     by the C-language output of _yacc_ that are not made
                     available through the **-l c** option.
    
           In the absence of options that inhibit invocation of the link
           editor, such as **-c** or **-E** , the _c99_ utility shall cause the
           equivalent of a **-l c** option to be passed to the link editor after
           the last _pathname_ operand or **-l** option, causing it to be searched
           after all other object files and libraries are loaded.
    
           It is unspecified whether the libraries **libc.a** , **libl.a** , **libm.a** ,
           **libpthread.a** , **librt.a** , **libtrace.a** , **libxnet.a** , or **liby.a** exist as
           regular files. The implementation may accept as **-l** option-
           arguments names of objects that do not exist as regular files.
    
       **External Symbols**
           The C compiler and link editor shall support the significance of
           external symbols up to a length of at least 31 bytes; the action
           taken upon encountering symbols exceeding the implementation-
           defined maximum symbol length is unspecified.
    
           The compiler and link editor shall support a minimum of 511
           external symbols per source or object file, and a minimum of 4095
           external symbols in total. A diagnostic message shall be written
           to the standard output if the implementation-defined limit is
           exceeded; other actions are unspecified.
    
       **Header Search**
           If a file with the same name as one of the standard headers
           defined in the Base Definitions volume of POSIX.1‐2017, _Chapter_
           _13_ , _Headers_ , not provided as part of the implementation, is placed
           in any of the usual places that are searched by default for
           headers, the results are unspecified.
    
       **Programming Environments**
           All implementations shall support one of the following programming
           environments as a default. Implementations may support more than
           one of the following programming environments. Applications can
           use _sysconf_() or _getconf_ to determine which programming
           environments are supported.
    
                    **Table 4-4: Programming Environments: Type Sizes**
          ┌─────────────────────────┬─────────┬─────────┬─────────┬─────────┐
          │ **Programming Environment** │ **Bits in** │ **Bits in** │ **Bits in** │ **Bits in** │
          │      _getconf_ **Name** │   **int** │  **long** │ **pointer** │  **off_t** │
          ├─────────────────────────┼─────────┼─────────┼─────────┼─────────┤
          │ _POSIX_V7_ILP32_OFF32   │    32   │    32   │    32   │    32   │
          │ _POSIX_V7_ILP32_OFFBIG  │    32   │    32   │    32   │   ≥64   │
          │ _POSIX_V7_LP64_OFF64    │    32   │    64   │    64   │    64   │
          │ _POSIX_V7_LPBIG_OFFBIG  │   ≥32   │   ≥64   │   ≥64   │   ≥64   │
          └─────────────────────────┴─────────┴─────────┴─────────┴─────────┘
    
           All implementations shall support one or more environments where
           the widths of the following types are no greater than the width of
           type **long** :
                           **blksize_t   ptrdiff_t     tcflag_t**
                           **cc_t        size_t        wchar_t**
                           **mode_t      speed_t       wint_t**
                           **nfds_t      ssize_t**
                           **pid_t       suseconds_t**
    
           The executable files created when these environments are selected
           shall be in a proper format for execution by the _exec_ family of
           functions. Each environment may be one of the ones in _Table 4-4,_
           _Programming Environments: Type Sizes_ , or it may be another
           environment. The names for the environments that meet this
           requirement shall be output by a _getconf_ command using the
           POSIX_V7_WIDTH_RESTRICTED_ENVS argument, as a <newline>-separated
           list of names suitable for use with the _getconf_ **-v** option. If more
           than one environment meets the requirement, the names of all such
           environments shall be output on separate lines. Any of these names
           can then be used in a subsequent _getconf_ command to obtain the
           flags specific to that environment with the following suffixes
           added as appropriate:
    
           _CFLAGS   To get the C compiler flags.
    
           _LDFLAGS  To get the linker/loader flags.
    
           _LIBS     To get the libraries.
    
           This requirement may be removed in a future version.
    
           When this utility processes a file containing a function called
           _main_(), it shall be defined with a return type equivalent to **int**.
           Using return from the initial call to _main_() shall be equivalent
           (other than with respect to language scope issues) to calling
           _exit_() with the returned value. Reaching the end of the initial
           call to _main_() shall be equivalent to calling **exit**(0).  The
           implementation shall not declare a prototype for this function.
    
           Implementations provide configuration strings for C compiler
           flags, linker/loader flags, and libraries for each supported
           environment.  When an application needs to use a specific
           programming environment rather than the implementation default
           programming environment while compiling, the application shall
           first verify that the implementation supports the desired
           environment. If the desired programming environment is supported,
           the application shall then invoke _c99_ with the appropriate C
           compiler flags as the first options for the compile, the
           appropriate linker/loader flags after any other options except **-l**
           but before any operands or **-l** options, and the appropriate
           libraries at the end of the operands and **-l** options.
    
           Conforming applications shall not attempt to link together object
           files compiled for different programming models. Applications
           shall also be aware that binary data placed in shared memory or in
           files might not be recognized by applications built for other
           programming models.
    
                   **Table 4-5: Programming Environments:**_c99_ **Arguments**
    ┌─────────────────────────┬─────────────────────┬───────────────────────────────┐
    │ **Programming Environment** │                     │         _c99_ **Arguments** │
    │      _getconf_ **Name** │         **Use** │         _getconf_ **Name** │
    ├─────────────────────────┼─────────────────────┼───────────────────────────────┤
    │ _POSIX_V7_ILP32_OFF32   │ C Compiler Flags    │ POSIX_V7_ILP32_OFF32_CFLAGS   │
    │                         │ Linker/Loader Flags │ POSIX_V7_ILP32_OFF32_LDFLAGS  │
    │                         │ Libraries           │ POSIX_V7_ILP32_OFF32_LIBS     │
    ├─────────────────────────┼─────────────────────┼───────────────────────────────┤
    │ _POSIX_V7_ILP32_OFFBIG  │ C Compiler Flags    │ POSIX_V7_ILP32_OFFBIG_CFLAGS  │
    │                         │ Linker/Loader Flags │ POSIX_V7_ILP32_OFFBIG_LDFLAGS │
    │                         │ Libraries           │ POSIX_V7_ILP32_OFFBIG_LIBS    │
    ├─────────────────────────┼─────────────────────┼───────────────────────────────┤
    │ _POSIX_V7_LP64_OFF64    │ C Compiler Flags    │ POSIX_V7_LP64_OFF64_CFLAGS    │
    │                         │ Linker/Loader Flags │ POSIX_V7_LP64_OFF64_LDFLAGS   │
    │                         │ Libraries           │ POSIX_V7_LP64_OFF64_LIBS      │
    ├─────────────────────────┼─────────────────────┼───────────────────────────────┤
    │ _POSIX_V7_LPBIG_OFFBIG  │ C Compiler Flags    │ POSIX_V7_LPBIG_OFFBIG_CFLAGS  │
    │                         │ Linker/Loader Flags │ POSIX_V7_LPBIG_OFFBIG_LDFLAGS │
    │                         │ Libraries           │ POSIX_V7_LPBIG_OFFBIG_LIBS    │
    └─────────────────────────┴─────────────────────┴───────────────────────────────┘
    
           In addition to the type size programming environments above, all
           implementations also support a multi-threaded programming
           environment that is orthogonal to all of the programming
           environments listed above.  The _getconf_ utility can be used to get
           flags for the threaded programming environment, as indicated in
           _Table 4-6, Threaded Programming Environment: c99 Arguments_.
    
               **Table 4-6: Threaded Programming Environment:**_c99_ **Arguments**
      ┌─────────────────────────┬─────────────────────┬──────────────────────────┐
      │ **Programming Environment** │                     │      _c99_ **Arguments** │
      │      _getconf_ **Name** │         **Use** │       _getconf_ **Name** │
      ├─────────────────────────┼─────────────────────┼──────────────────────────┤
      │ _POSIX_THREADS          │ C Compiler Flags    │ POSIX_V7_THREADS_CFLAGS  │
      │                         │ Linker/Loader Flags │ POSIX_V7_THREADS_LDFLAGS │
      └─────────────────────────┴─────────────────────┴──────────────────────────┘
    
           These programming environment flags may be used in conjunction
           with any of the type size programming environments supported by
           the implementation.
    


## EXIT STATUS



           The following exit values shall be returned:
    
            0    Successful compilation or link edit.
    
           >0    An error occurred.
    


## CONSEQUENCES OF ERRORS



           When _c99_ encounters a compilation error that causes an object file
           not to be created, it shall write a diagnostic to standard error
           and continue to compile other source code operands, but it shall
           not perform the link phase and it shall return a non-zero exit
           status. If the link edit is unsuccessful, a diagnostic message
           shall be written to standard error and _c99_ exits with a non-zero
           status. A conforming application shall rely on the exit status of
           _c99_ , rather than on the existence or mode of the executable file.
    
           _The following sections are informative._
    


## APPLICATION USAGE



           Since the _c99_ utility usually creates files in the current
           directory during the compilation process, it is typically
           necessary to run the _c99_ utility in a directory in which a file
           can be created.
    
           On systems providing POSIX Conformance (see the Base Definitions
           volume of POSIX.1‐2017, _Chapter 2_ , _Conformance_), _c99_ is required
           only with the C-Language Development option; XSI-conformant
           systems always provide _c99_.
    
           Some historical implementations have created **.o** files when **-c** is
           not specified and more than one source file is given. Since this
           area is left unspecified, the application cannot rely on **.o** files
           being created, but it also must be prepared for any related **.o**
           files that already exist being deleted at the completion of the
           link edit.
    
           There is the possible implication that if a user supplies versions
           of the standard functions (before they would be encountered by an
           implicit **-l c** or explicit **-l m**), that those versions would be used
           in place of the standard versions.  There are various reasons this
           might not be true (functions defined as macros, manipulations for
           clean name space, and so on), so the existence of files named in
           the same manner as the standard libraries within the **-L**
           directories is explicitly stated to produce unspecified behavior.
    
           All of the functions specified in the System Interfaces volume of
           POSIX.1‐2017 may be made visible by implementations when the
           Standard C Library is searched. Conforming applications must
           explicitly request searching the other standard libraries when
           functions made visible by those libraries are used.
    
           In the ISO C standard the mapping from physical source characters
           to the C source character set is implementation-defined.
           Implementations may strip white-space characters before the
           terminating <newline> of a (physical) line as part of this mapping
           and, as a consequence of this, one or more white-space characters
           (and no other characters) between a <backslash> character and the
           <newline> character that terminates the line produces
           implementation-defined results. Portable applications should not
           use such constructs.
    
           Some _c99_ compilers not conforming to POSIX.1‐2008 do not support
           trigraphs by default.
    


## EXAMPLES



            1. The following usage example compiles **foo.c** and creates the
               executable file **foo** :
    
                   c99 -o foo foo.c
    
               The following usage example compiles **foo.c** and creates the
               object file **foo.o** :
    
                   c99 -c foo.c
    
               The following usage example compiles **foo.c** and creates the
               executable file **a.out** :
    
                   c99 foo.c
    
               The following usage example compiles **foo.c** , links it with
               **bar.o** , and creates the executable file **a.out**.  It may also
               create and leave **foo.o** :
    
                   c99 foo.c bar.o
    
            2. The following example shows how an application using threads
               interfaces can test for support of and use a programming
               environment supporting 32-bit **int** , **long** , and **pointer** types and
               an **off_t** type using at least 64 bits:
    
                   offbig_env=$(getconf _POSIX_V7_ILP32_OFFBIG)
                   if [ $offbig_env != "-1" ] && [ $offbig_env != "undefined" ]
                   then
                       c99 $(getconf POSIX_V7_ILP32_OFFBIG_CFLAGS) \
                       $(getconf POSIX_V7_THREADS_CFLAGS) -D_XOPEN_SOURCE=700 \
                       $(getconf POSIX_V7_ILP32_OFFBIG_LDFLAGS) \
                       $(getconf POSIX_V7_THREADS_LDFLAGS) foo.c -o foo \
                       $(getconf POSIX_V7_ILP32_OFFBIG_LIBS) \
                       -l pthread
                   else
                       echo ILP32_OFFBIG programming environment not supported
                       exit 1
                   fi
    
            3. The following examples clarify the use and interactions of **-L**
               and **-l** options.
    
               Consider the case in which module **a.c** calls function _f_() in
               library **libQ.a** , and module **b.c** calls function _g_() in library
               **libp.a**.  Assume that both libraries reside in **/a/b/c**.  The
               command line to compile and link in the desired way is:
    
                   c99 -L /a/b/c main.o a.c -l Q b.c -l p
    
               In this case the **-L** option need only precede the first **-l**
               option, since both **libQ.a** and **libp.a** reside in the same
               directory.
    
               Multiple **-L** options can be used when library name collisions
               occur. Building on the previous example, suppose that the user
               wants to use a new **libp.a** , in **/a/a/a** , but still wants _f_() from
               **/a/b/c/libQ.a** :
    
                   c99 -L /a/a/a -L /a/b/c main.o a.c -l Q b.c -l p
    
               In this example, the linker searches the **-L** options in the
               order specified, and finds **/a/a/a/libp.a** before **/a/b/c/libp.a**
               when resolving references for **b.c**.  The order of the **-l**
               options is still important, however.
    
            4. The following example shows how an application can use a
               programming environment where the widths of the following
               types: **blksize_t** , **cc_t** , **mode_t** , **nfds_t** , **pid_t** , **ptrdiff_t** ,
               **size_t** , **speed_t** , **ssize_t** , **suseconds_t** , **tcflag_t** , **wchar_t** ,
               **wint_t**
    
               are no greater than the width of type **long** :
    
                   # First choose one of the listed environments ...
    
                   # ... if there are no additional constraints, the first one will do:
                   CENV=$(getconf POSIX_V7_WIDTH_RESTRICTED_ENVS | head -n l)
    
                   # ... or, if an environment that supports large files is preferred,
                   # look for names that contain "OFF64" or "OFFBIG". (This chooses
                   # the last one in the list if none match.)
                   for CENV in $(getconf POSIX_V7_WIDTH_RESTRICTED_ENVS)
                   do
                       case $CENV in
                       *OFF64*|*OFFBIG*) break ;;
                       esac
                   done
    
                   # The chosen environment name can now be used like this:
    
                   c99 $(getconf ${CENV}_CFLAGS) -D _POSIX_C_SOURCE=200809L \
                   $(getconf ${CENV}_LDFLAGS) foo.c -o foo \
                   $(getconf ${CENV}_LIBS)
    


## RATIONALE



           The _c99_ utility is based on the _c89_ utility originally introduced
           in the ISO POSIX‐2:1993 standard.
    
           Some of the changes from _c89_ include the ability to intersperse
           options and operands (which many _c89_ implementations allowed
           despite it not being specified), the description of **-l** as an
           option instead of an operand, and the modification to the contents
           of the Standard Libraries section to account for new headers and
           options; for example, _< spawn.h>_ added to the description of **-l rt** ,
           and **-l trace** added for the Tracing option.
    
           POSIX.1‐2008 specifies that the _c99_ utility must be able to use
           regular files for ***.o** files and for **a.out** files. Implementations
           are free to overwrite existing files of other types when
           attempting to create object files and executable files, but are
           not required to do so. If something other than a regular file is
           specified and using it fails for any reason, _c99_ is required to
           issue a diagnostic message and exit with a non-zero exit status.
           But for some file types, the problem may not be noticed for a long
           time. For example, if a FIFO named **a.out** exists in the current
           directory, _c99_ may attempt to open **a.out** and will hang in the
           _open_() call until another process opens the FIFO for reading. Then
           _c99_ may write most of the **a.out** to the FIFO and fail when it tries
           to seek back close to the start of the file to insert a timestamp
           (FIFOs are not seekable files). The _c99_ utility is also allowed to
           issue a diagnostic immediately if it encounters an **a.out** or ***.o**
           file that is not a regular file. For portable use, applications
           should ensure that any **a.out** , **-o** option-argument, or ***.o** files
           corresponding to any ***.c** files do not conflict with names already
           in use that are not regular files or symbolic links that point to
           regular files.
    
           On many systems, multi-threaded applications run in a programming
           environment that is distinct from that used by single-threaded
           applications. This multi-threaded programming environment (in
           addition to needing to specify **-l pthread** at link time) may
           require additional flags to be set when headers are processed at
           compile time (**-D_REENTRANT** being common). This programming
           environment is orthogonal to the type size programming
           environments discussed above and listed in _Table 4-4, Programming_
           _Environments: Type Sizes_.  This version of the standard adds
           _getconf_ utility calls to provide the C compiler flags and
           linker/loader flags needed to support multi-threaded applications.
           Note that on a system where single-threaded applications are a
           special case of a multi-threaded application, both of these
           _getconf_ calls may return NULL strings; on other implementations
           both of these strings may be non-NULL strings.
    
           The C standardization committee invented trigraphs (e.g., **"??!"** to
           represent **'|'**) to address character portability problems in
           development environments based on national variants of the 7-bit
           ISO/IEC 646:1991 standard character set. However, these
           environments were already obsolete by the time the first ISO C
           standard was published, and in practice trigraphs have not been
           used for their intended purpose, and usually are intended to have
           their original meaning in K&R C.  For example, in practice a C-
           language source string like **"What??!"** is usually intended to end
           in two <question-mark> characters and an <exclamation-mark>, not
           in **'|'**.
    
           When the **-E** option is used, execution of some **#pragma** preprocessor
           directives may simply result in a copy of the directive being
           included in the output as part of the allowed extra information
           used by subsequent compilation passes (see STDOUT).
    


## FUTURE DIRECTIONS



           Unlike all of the other non-OB-shaded utilities in this standard,
           a utility by this name probably will not appear in the next
           version of this standard.  This utility's name is tied to the
           current revision of the ISO C standard at the time this standard
           is approved. Since the ISO C standard and this standard are
           maintained by different organizations on different schedules, we
           cannot predict what the compiler will be named in the next version
           of the standard.
    


* * *