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

       arch_prctl() sets architecture-specific process or thread state.  code selects a subfunction and passes argument addr to it; addr is inter-
       preted 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.	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 2 GB 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 4.15 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								    2017-09-15							     ARCH_PRCTL(2)