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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.

       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
       programs intended to be portable.

NOTES

       arch_prctl()  is  only  supported  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.  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 Programmer’s manual

COLOPHON

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