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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| getauxval () |
Thread safety |
MT-Safe |
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)