getauxval

GETAUXVAL(3)               Linux Programmer's Manual              GETAUXVAL(3)

NAME
       getauxval - retrieve a value from the auxiliary vector

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  in-
       formation 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  dy-
              namic linker).

       AT_BASE_PLATFORM
              A  pointer to a string identifying the real platform; may differ
              from AT_PLATFORM (PowerPC only).

       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  indi-
              cate  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_PA-
              GESIZE)).

       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 se-
              curely.  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  an-
              other),  or  that  it  gained capabilities by executing a binary
              file that  has  capabilities  (see  capabilities(7)).   Alterna-
              tively,  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  se-
              cure_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  Ob-
              ject (vDSO) that the kernel creates in order to provide fast im-
              plementations 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.

VERSIONS
       The getauxval() function was added to glibc in version 2.16.

ATTRIBUTES
       For an  explanation  of  the  terms  used  in  this  section,  see  at-
       tributes(7).

       +------------+---------------+---------+
       |Interface   | Attribute     | Value   |
       +------------+---------------+---------+
       |getauxval() | Thread safety | MT-Safe |
       +------------+---------------+---------+
CONFORMING TO
       This function is a nonstandard glibc extension.

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  environ-
       ment  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.

SEE ALSO
       secure_getenv(3), vdso(7), ld-linux.so(8)

COLOPHON
       This page is part of release 5.05 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       https://www.kernel.org/doc/man-pages/.

GNU                               2019-10-10                      GETAUXVAL(3)