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/.