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getauxval — retrieve a value from the auxiliary vector
#include <sys/auxv.h>
unsigned long
getauxval( |
unsigned long type); |
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_BASEThe base address of the program interpreter (usually, the dynamic linker).
AT_BASE_PLATFORMA string identifying the real platform; may differ
from AT_PLATFORM (PowerPC
only).
AT_CLKTCKThe frequency with which times(2) counts. This
value can also be obtained via sysconf(_SC_CLK_TCK).
AT_DCACHEBSIZEThe data cache block size.
AT_EGIDThe effective group ID of the thread.
AT_ENTRYThe entry address of the executable.
AT_EUIDThe effective user ID of the thread.
AT_EXECFDFile descriptor of program.
AT_EXECFNPathname used to execute program.
AT_FLAGSFlags (unused).
AT_FPUCWUsed FPU control word (SuperH architecture only). This gives some information about the FPU initialization performed by the kernel.
AT_GIDThe real group ID of the thread.
AT_HWCAPA 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_ICACHEBSIZEThe instruction cache block size.
AT_PAGESZThe system page size (the same value returned by
sysconf(_SC_PAGESIZE)).
AT_PHDRThe address of the program headers of the executable.
AT_PHENTThe size of program header entry.
AT_PHNUMThe number of program headers.
AT_PLATFORMA 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_RANDOMThe address of sixteen bytes containing a random value.
AT_SECUREHas 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 program; 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_SYSINFOThe entry point to the system call function in the VDSO. Not present/needed on all architectures (e.g., absent on x86-64).
AT_SYSINFO_EHDRThe 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_UCACHEBSIZEThe unified cache block size.
AT_UIDThe real user ID of the thread.
On success, getauxval()
returns the value corresponding to type. If type is not found, 0 is
returned.
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.
This page is part of release 3.52 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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Copyright 2012 Michael Kerrisk <mtk.manpagesgmail.com> %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END See also https://lwn.net/Articles/519085/ |