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ARCH_PRCTL(2)              Linux Programmer's Manual             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
              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 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2007-12-26                     ARCH_PRCTL(2)