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 4.10 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				  2016-07-17			  GETAUXVAL(3)