arch_prctl (2)

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Copyright (C) 2003 Andi Kleen

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Since the Linux kernel and li...

(The comments found at the beginning of the groff file "man2/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.

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 programs intended to be portable.

NOTES

arch_prctl() is supported only 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. As an optimization, if a 32-bit TLS base address is used, arch_prctl(2) 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 2GB of address space can be allocated by using mmap(2) with the MAP_32BIT flag.
Because of the aforementioned optimization, using arch_prctl(2) and set_thread_area(2) in the same thread is dangerous, as they may overwrite 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.

SEE ALSO

mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
AMD X86-64 Programmer's manual

COLOPHON

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