getauxval - retrieve a value from the auxiliary vector
unsigned long getauxval(unsigned long type);
() 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
() returns the corresponding value.
The value returned for each type
is given in the following list. Not all
values are present on all architectures.
- The base address of the program interpreter (usually, the
- A string identifying the real platform; may differ from
AT_PLATFORM (PowerPC only).
- The frequency with which times(2) counts. This value
can also be obtained via sysconf(_SC_CLK_TCK).
- The data cache block size.
- The effective group ID of the thread.
- The entry address of the executable.
- The effective user ID of the thread.
- File descriptor of program.
- Pathname used to execute program.
- Flags (unused).
- Used FPU control word (SuperH architecture only). This
gives some information about the FPU initialization performed by the
- The real group ID of the thread.
- 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
- The instruction cache block size.
- The system page size (the same value returned by
- The address of the program headers of the executable.
- The size of program header entry.
- The number of program headers.
- 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.
- The address of sixteen bytes containing a random
- 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).)
- The entry point to the system call function in the vDSO.
Not present/needed on all architectures (e.g., absent on x86-64).
- 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.
- The unified cache block size.
- The real user ID of the thread.
On success, getauxval
() returns the value corresponding to type
is not found, 0 is returned.
- ENOENT (since glibc 2.19)
- No entry corresponding to type could be found in the
() function was added to glibc in version 2.16.
For an explanation of the terms used in this section, see attributes(7)
This function is a nonstandard glibc extension.
The primary consumer of the information in the auxiliary vector is the dynamic
. 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
$ 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
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