• 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 3.19 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/.
  

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