getauxval - retrieve a value from the auxiliary vector
Standard C library (
libc,
-lc)
#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_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.
On success,
getauxval() returns the value corresponding to
type.
If
type is not found, 0 is returned.
-
ENOENT (since glibc 2.19)
- No entry corresponding to type could be found in the
auxiliary vector.
The
getauxval() function was added in glibc 2.16.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
getauxval () |
Thread safety |
MT-Safe |
This function is a nonstandard glibc extension.
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.
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.
execve(2),
secure_getenv(3),
vdso(7),
ld-linux.so(8)