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);
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 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 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.

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

AMD X86-64 Programmer's manual

COLOPHON
This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.