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.

       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 pro-
       grams intended to be portable.

NOTES
       arch_prctl() is supported only 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.	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 2GB 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	 over-
       write 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 4.10 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/.



Linux				  2016-10-08			 ARCH_PRCTL(2)