arch_prctl — set architecture-specific thread state


#include <asm/prctl.h>
#include <sys/prctl.h>
int arch_prctl( int code,
  unsigned long addr);
int arch_prctl( int code,
  unsigned long *addr);


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:


Set the 64-bit base for the FS register to addr.


Return the 64-bit base value for the FS register of the current thread in the unsigned long pointed to by addr.


Set the 64-bit base for the GS register to addr.


Return the 64-bit base value for the GS register of the current thread in the unsigned long pointed to by addr.


On success, arch_prctl() returns 0; on error, −1 is returned, and errno is set to indicate the error.



addr points to an unmapped address or is outside the process address space.


code is not a valid subcommand.


addr is outside the process address space.


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


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.


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

AMD X86-64 Programmer's manual


This page is part of release 4.16 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−pages/.

  Copyright (C) 2003 Andi Kleen

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