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.

       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.  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 needed only 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 3.54 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/.