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
arch_prctl() 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.

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

NOTES
arch_prctl() is supported only 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. 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 overwrite 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 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 4.15 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/.