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 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 pro- grams intended to be portable.

NOTES arch_prctl() is only supported 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. 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 Programmers manual

COLOPHON This page is part of release 3.22 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux 2007-12-26 ARCH_PRCTL(2)