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
       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 string identifying the real platform; may differ from AT_PLAT-
              FORM (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
              Pathname used to execute 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_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)).  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
              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.

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

ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(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 4.15 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                               2017-09-15                      GETAUXVAL(3)
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