ARCH_PRCTL(2) Linux Programmer's Manual ARCH_PRCTL(2)NAMEarch_prctl - set architecture-specific thread state
SYNOPSIS
#include <asm/prctl.h>
#include <sys/prctl.h>
int arch_prctl(int code, unsigned long addr);
int arch_prctl(int code, unsigned long *addr);
DESCRIPTIONarch_prctl() sets architecture-specific process or thread state. code
selects a subfunction and passes argument addr to it; addr is inter‐
preted as either an unsigned long for the "set" operations, or as an
unsigned long *, for the "get" operations.
Subfunctions for x86-64 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 current
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 current
thread in the unsigned long pointed to by addr.
RETURN VALUE
On success, arch_prctl() returns 0; on error, -1 is returned, and errno
is set to indicate the error.
ERRORS
EFAULT addr points to an unmapped address or is outside the process
address space.
EINVAL code is not a valid subcommand.
EPERM addr is outside the process address space.
CONFORMING TOarch_prctl() is a Linux/x86-64 extension and should not be used in pro‐
grams intended to be portable.
NOTESarch_prctl() is supported only on Linux/x86-64 for 64-bit programs cur‐
rently.
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 over‐
write each other's TLS entries.
As of version 2.7, glibc provides no prototype for arch_prctl(). You
have to declare it yourself for now. This may be fixed in future glibc
versions.
FS may be already used by the threading library. Programs that use
ARCH_SET_FS directly are very likely to crash.
SEE ALSOmmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
AMD X86-64 Programmer's manual
COLOPHON
This page is part of release 4.14 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 ARCH_PRCTL(2)
