ARCH_PRCTL(2) Linux Programmer s Manual ARCH_PRCTL(2)

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)

DESCRIPTION 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.

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

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.

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.

AUTHOR Man page written by Andi Kleen.

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

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

AMD X86-64 Programmers manual

Linux 2.4.20 2003-02-02 ARCH_PRCTL(2)