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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
       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 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 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)