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man: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 4.16 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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