# getauxval

## NAME



           getauxval - retrieve a value from the auxiliary vector
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/auxv.h>**
    
           **unsigned long getauxval(unsigned long** _type_**);**
    


## DESCRIPTION



           The **getauxval**() function retrieves values from the auxiliary
           vector, a mechanism that the kernel's ELF binary loader uses to
           pass certain information to user space when a program is executed.
    
           Each entry in the auxiliary vector consists of a pair of values: a
           type that identifies what this entry represents, and a value for
           that type.  Given the argument _type_ , **getauxval**() returns the
           corresponding value.
    
           The value returned for each _type_ is given in the following list.
           Not all _type_ values are present on all architectures.
    
           **AT_BASE**
                  The base address of the program interpreter (usually, the
                  dynamic linker).
    
           **AT_BASE_PLATFORM**
                  A pointer to a string (PowerPC and MIPS only).  On PowerPC,
                  this identifies the real platform; may differ from
                  **AT_PLATFORM**.  On MIPS, this identifies the ISA level (since
                  Linux 5.7).
    
           **AT_CLKTCK**
                  The frequency with which times(2) counts.  This value can
                  also be obtained via _sysconf(_SC_CLK_TCK)_.
    
           **AT_DCACHEBSIZE**
                  The data cache block size.
    
           **AT_EGID**
                  The effective group ID of the thread.
    
           **AT_ENTRY**
                  The entry address of the executable.
    
           **AT_EUID**
                  The effective user ID of the thread.
    
           **AT_EXECFD**
                  File descriptor of program.
    
           **AT_EXECFN**
                  A pointer to a string containing the pathname used to
                  execute the program.
    
           **AT_FLAGS**
                  Flags (unused).
    
           **AT_FPUCW**
                  Used FPU control word (SuperH architecture only).  This
                  gives some information about the FPU initialization
                  performed by the kernel.
    
           **AT_GID** The real group ID of the thread.
    
           **AT_HWCAP**
                  An architecture and ABI dependent bit-mask whose settings
                  indicate detailed processor capabilities.  The contents of
                  the bit mask are hardware dependent (for example, see the
                  kernel source file _arch/x86/include/asm/cpufeature.h_ for
                  details relating to the Intel x86 architecture; the value
                  returned is the first 32-bit word of the array described
                  there).  A human-readable version of the same information
                  is available via _/proc/cpuinfo_.
    
           **AT_HWCAP2**(since glibc 2.18)
                  Further machine-dependent hints about processor
                  capabilities.
    
           **AT_ICACHEBSIZE**
                  The instruction cache block size.
    
           **AT_L1D_CACHEGEOMETRY**
                  Geometry of the L1 data cache, encoded with the cache line
                  size in bytes in the bottom 16 bits and the cache
                  associativity in the next 16 bits.  The associativity is
                  such that if N is the 16-bit value, the cache is N-way set
                  associative.
    
           **AT_L1D_CACHESIZE**
                  The L1 data cache size.
    
           **AT_L1I_CACHEGEOMETRY**
                  Geometry of the L1 instruction cache, encoded as for
                  **AT_L1D_CACHEGEOMETRY**.
    
           **AT_L1I_CACHESIZE**
                  The L1 instruction cache size.
    
           **AT_L2_CACHEGEOMETRY**
                  Geometry of the L2 cache, encoded as for
                  **AT_L1D_CACHEGEOMETRY**.
    
           **AT_L2_CACHESIZE**
                  The L2 cache size.
    
           **AT_L3_CACHEGEOMETRY**
                  Geometry of the L3 cache, encoded as for
                  **AT_L1D_CACHEGEOMETRY**.
    
           **AT_L3_CACHESIZE**
                  The L3 cache size.
    
           **AT_PAGESZ**
                  The system page size (the same value returned by
                  _sysconf(_SC_PAGESIZE)_).
    
           **AT_PHDR**
                  The address of the program headers of the executable.
    
           **AT_PHENT**
                  The size of program header entry.
    
           **AT_PHNUM**
                  The number of program headers.
    
           **AT_PLATFORM**
                  A pointer to a string that identifies the hardware platform
                  that the program is running on.  The dynamic linker uses
                  this in the interpretation of _rpath_ values.
    
           **AT_RANDOM**
                  The address of sixteen bytes containing a random value.
    
           **AT_SECURE**
                  Has a nonzero value if this executable should be treated
                  securely.  Most commonly, a nonzero value indicates that
                  the process is executing a set-user-ID or set-group-ID
                  binary (so that its real and effective UIDs or GIDs differ
                  from one another), or that it gained capabilities by
                  executing a binary file that has capabilities (see
                  capabilities(7)).  Alternatively, a nonzero value may be
                  triggered by a Linux Security Module.  When this value is
                  nonzero, the dynamic linker disables the use of certain
                  environment variables (see ld-linux.so(8)) and glibc
                  changes other aspects of its behavior.  (See also
                  secure_getenv(3).)
    
           **AT_SYSINFO**
                  The entry point to the system call function in the vDSO.
                  Not present/needed on all architectures (e.g., absent on
                  x86-64).
    
           **AT_SYSINFO_EHDR**
                  The address of a page containing the virtual Dynamic Shared
                  Object (vDSO) that the kernel creates in order to provide
                  fast implementations of certain system calls.
    
           **AT_UCACHEBSIZE**
                  The unified cache block size.
    
           **AT_UID** The real user ID of the thread.
    


## RETURN VALUE



           On success, **getauxval**() returns the value corresponding to _type_.
           If _type_ is not found, 0 is returned.
    


## ERRORS



           **ENOENT**(since glibc 2.19)
                  No entry corresponding to _type_ could be found in the
                  auxiliary vector.
    


## ATTRIBUTES



           For an explanation of the terms used in this section, see
           attributes(7).
           ┌──────────────────────────────────────┬───────────────┬─────────┐
           │ **Interface** │ **Attribute** │ **Value** │
           ├──────────────────────────────────────┼───────────────┼─────────┤
           │ **getauxval**()                          │ Thread safety │ MT-Safe │
           └──────────────────────────────────────┴───────────────┴─────────┘
    


## STANDARDS



           GNU.
    


## NOTES



           The primary consumer of the information in the auxiliary vector is
           the dynamic linker, ld-linux.so(8).  The auxiliary vector is a
           convenient and efficient shortcut that allows the kernel to
           communicate a certain set of standard information that the dynamic
           linker usually or always needs.  In some cases, the same
           information could be obtained by system calls, but using the
           auxiliary vector is cheaper.
    
           The auxiliary vector resides just above the argument list and
           environment in the process address space.  The auxiliary vector
           supplied to a program can be viewed by setting the **LD_SHOW_AUXV**
           environment variable when running a program:
    
               $ LD_SHOW_AUXV=1 sleep 1
    
           The auxiliary vector of any process can (subject to file
           permissions) be obtained via _/proc/_ pid _/auxv_ ; see proc(5) for more
           information.
    


## BUGS



           Before the addition of the **ENOENT** error in glibc 2.19, there was
           no way to unambiguously distinguish the case where _type_ could not
           be found from the case where the value corresponding to _type_ was
           zero.
    


* * *