ARCH_PRCTL(2)              Linux Programmer's Manual             ARCH_PRCTL(2)

       arch_prctl - set architecture-specific thread state

       #include <asm/prctl.h>
       #include <sys/prctl.h>

       int arch_prctl(int code, unsigned long addr);
       int arch_prctl(int code, unsigned long *addr);

       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:

              Set the 64-bit base for the FS register to addr.

              Return the 64-bit base value for the FS register of the  current
              thread in the unsigned long pointed to by addr.

              Set the 64-bit base for the GS register to addr.

              Return  the 64-bit base value for the GS register of the current
              thread in the unsigned long pointed to by addr.

       On success, arch_prctl() returns 0; on error, -1 is returned, and errno
       is set to indicate the error.

       EFAULT addr points to an unmapped address or is outside the process ad-
              dress space.

       EINVAL code is not a valid subcommand.

       EPERM  addr is outside the process address space.

       arch_prctl() is a Linux/x86-64 extension and should not be used in pro-
       grams intended to be portable.

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

       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

       FS  may  be  already  used by the threading library.  Programs that use
       ARCH_SET_FS directly are very likely to crash.

       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

       AMD X86-64 Programmer's manual

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       description  of  the project, information about reporting bugs, and the
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Linux                             2017-09-15                     ARCH_PRCTL(2)
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