GETAUXVAL(3)              (2020-06-09)               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 pointer to a string (PowerPC and MIPS only).  On Pow-
               erPC, 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.

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          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 set-
               tings 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 capa-
               bilities.

          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

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               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 indi-
               cates 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 Mod-
               ule.  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).

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          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).  allbox; lb lb lb l l l.
          Interface Attribute Value T{ getauxval() T}   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 informa-
          tion 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.

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     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 correspond-
          ing to type was zero.

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

     COLOPHON
          This page is part of release 5.10 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/.

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