arch_prctl(2) - Linux man page

Name

arch_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);

Description

The arch_prctl() function sets architecture-specific process or thread state. code selects a subfunction 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.

Sub functions 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 to

arch_prctl() is a Linux/x86-64 extension and should not be used in programs intended to be portable.

Notes

arch_prctl() is only supported 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. It may be a faster alternative to set a 32-bit base using a segment selector by setting up an LDT with modify_ldt(2) or using the set_thread_area(2) system call in kernel 2.5 or later. arch_prctl() is only needed when you want to set bases that are larger than 4GB. Memory in the first 2GB of address space can be allocated by using mmap(2) with the MAP_32BIT flag.

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.