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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_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

       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).

       ┌────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├────────────┼───────────────┼─────────┤
       │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

       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/.