oracular (3) getauxval.3.gz

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NAME

       getauxval - retrieve a value from the auxiliary vector

LIBRARY

       Standard C library (libc, -lc)

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.

ATTRIBUTES

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

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

STANDARDS

       GNU.

HISTORY

       glibc 2.16.

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)