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Manual Reference Pages  - ARCH_PRCTL (2)

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arch_prctl - set architecture specific thread state

CONTENTS

Synopsis
Description
Notes
Errors
Author

#include <asm/prctl.h>

#include <sys/prctl.h>

int arch_prctl(int code, unsigned long addr)

The arch_prctl() function sets architecture specific process or thread state. code selects a subfunction and passes argument addr to it.

Sub functions for x86-64 are:

ARCH_SET_FS
	Set the 64bit base for the FS register to addr.
ARCH_GET_FS
	Return the 64bit base value for the FS register of the current thread in the unsigned long pointed to by the address parameter
ARCH_SET_GS
	Set the 64bit base for the GS register to addr.
ARCH_GET_GS
	Return the 64bit base value for the GS register of the current thread in the unsigned long pointed to by the address parameter.

arch_prctl() is only supported on Linux/x86-64 for 64bit programs currently.

The 64bit base changes when a new 32bit segment selector is loaded.

ARCH_SET_GS is disabled in some kernels.

Context switches for 64bit segment bases are rather expensive. It may be a faster alternative to set a 32bit base using a segment selector by setting up an LDT with modify_ldt(2) or using the set_thread_area(2) system call in a 2.5 kernel. 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 2.3.5, 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.

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.

Man page written by Andi Kleen.

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

mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

AMD X86-64 Programmer’s manual