NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       getauxval - retrieve a value from the auxiliary vector

SYNOPSIS         top

       #include <sys/auxv.h>

       unsigned long getauxval(unsigned long type);

DESCRIPTION         top

       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_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
              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
              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_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         top

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

ERRORS         top

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

VERSIONS         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌────────────┬───────────────┬─────────┐
       │Interface   Attribute     Value   │
       ├────────────┼───────────────┼─────────┤
       │getauxval() │ Thread safety │ MT-Safe │
       └────────────┴───────────────┴─────────┘

CONFORMING TO         top

       This function is a nonstandard glibc extension.

NOTES         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.13 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)

Pages that refer to this page: getunwind(2)getenv(3)proc(5)libc(7)random(7)vdso(7)ld.so(8)