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 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
       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 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
       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
       execve(2), secure_getenv(3), vdso(7), ld-linux.so(8)

COLOPHON
       This page is part of release 5.13 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                                2021-08-27                       GETAUXVAL(3)