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 );
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 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 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 address 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 programs intended to be portable.
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.
mmap (2) modify_ldt (2) prctl (2) set_thread_area (2)
AMD X86-64 Programmer's manual
