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 dynamic
- A string identifying the real platform; may differ from AT_PLATFORM
- 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 kernel.
- 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 capabilities.
- 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 value.
- 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
- 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 auxiliary
() 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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