ARCH_PRCTL(2)		   Linux Programmers Manual		ARCH_PRCTL(2)



NAME
       arch_prctl - set architecture-specific thread state

SYNOPSIS
       #include 
       #include 

       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 pro
       grams intended to be portable.

NOTES
       arch_prctl() is only supported on Linux/x86-64 for 64-bit programs cur
       rently.

       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 Programmers manual

COLOPHON
       This page is part of release 3.05 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/.



Linux				  2007-12-26			 ARCH_PRCTL(2)