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 string identifying the real platform; may differ from AT_PLAT-
FORM (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 indi-
cate 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)). Alterna-
tively, 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 environ-
ment 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 4.15 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 2017-09-15 GETAUXVAL(3)
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