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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.