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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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arch_prctl(2) System Calls Manual arch_prctl(2)
arch_prctl - set architecture-specific thread state
Standard C library (libc, -lc)
#include <asm/prctl.h> /* Definition of ARCH_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_arch_prctl, int op, unsigned long addr);
int syscall(SYS_arch_prctl, int op, unsigned long *addr);
Note: glibc provides no wrapper for arch_prctl(), necessitating
the use of syscall(2).
arch_prctl() sets architecture-specific process or thread state.
op selects an operation and passes argument addr to it; addr is
interpreted as either an unsigned long for the "set" operations,
or as an unsigned long *, for the "get" operations.
Subfunctions for both x86 and x86-64 are:
ARCH_SET_CPUID (since Linux 4.12)
Enable (addr != 0) or disable (addr == 0) the cpuid
instruction for the calling thread. The instruction is
enabled by default. If disabled, any execution of a cpuid
instruction will instead generate a SIGSEGV signal. This
feature can be used to emulate cpuid results that differ
from what the underlying hardware would have produced
(e.g., in a paravirtualization setting).
The ARCH_SET_CPUID setting is preserved across fork(2) and
clone(2) but reset to the default (i.e., cpuid enabled) on
execve(2).
ARCH_GET_CPUID (since Linux 4.12)
Return the setting of the flag manipulated by
ARCH_SET_CPUID as the result of the system call (1 for
enabled, 0 for disabled). addr is ignored.
Subfunctions for x86-64 only are:
ARCH_SET_FS
Set the 64-bit base for the FS register to addr.
ARCH_GET_FS
Return the 64-bit base value for the FS register of the
calling thread in the unsigned long pointed to by addr.
ARCH_SET_GS
Set the 64-bit base for the GS register to addr.
ARCH_GET_GS
Return the 64-bit base value for the GS register of the
calling thread in the unsigned long pointed to by addr.
On success, arch_prctl() returns 0; on error, -1 is returned, and
errno is set to indicate the error.
EFAULT addr points to an unmapped address or is outside the
process address space.
EINVAL op is not a valid operation.
ENODEV ARCH_SET_CPUID was requested, but the underlying hardware
does not support CPUID faulting.
EPERM addr is outside the process address space.
Linux/x86-64.
arch_prctl() is supported only on Linux/x86-64 for 64-bit programs
currently.
The 64-bit base changes when a new 32-bit segment selector is
loaded.
ARCH_SET_GS is disabled in some kernels.
Context switches for 64-bit segment bases are rather expensive.
As an optimization, if a 32-bit TLS base address is used,
arch_prctl() may use a real TLS entry as if set_thread_area(2) had
been called, instead of manipulating the segment base register
directly. Memory in the first 2 GB of address space can be
allocated by using mmap(2) with the MAP_32BIT flag.
Because of the aforementioned optimization, using arch_prctl() and
set_thread_area(2) in the same thread is dangerous, as they may
overwrite each other's TLS entries.
FS may be already used by the threading library. Programs that
use ARCH_SET_FS directly are very likely to crash.
mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
AMD X86-64 Programmer's manual
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 arch_prctl(2)
Pages that refer to this page: clone(2), modify_ldt(2), set_thread_area(2), syscalls(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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