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