getauxval(3) — Linux manual page

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 pointer to a string (PowerPC and MIPS only).  On
              PowerPC, this identifies the real platform; may differ
              from AT_PLATFORM.  On MIPS, this identifies the ISA level
              (since Linux 5.7).

       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
              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_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_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 5.11 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                            2021-03-22                   GETAUXVAL(3)

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