getauxval(3) Library Functions Manual getauxval(3)
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 in-
formation 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 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 ver-
sion 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_PA-
GESIZE)).
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 se-
curely. 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 an-
other), 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 se-
cure_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 Ob-
ject (vDSO) that the kernel creates in order to provide fast im-
plementations 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).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ 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 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
execve(2), secure_getenv(3), vdso(7), ld-linux.so(8)
Linux man-pages 6.9.1 2024-05-02 getauxval(3)
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