Ubuntu Manpage: getauxval - retrieve a value from the auxiliary vector

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
A pointer to a multibyte mask of bits 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). 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 it's 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 zer0.

COLOPHON

       This page is part of release 4.04 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
       http://www.kernel.org/doc/man-pages/.