|ARCH_PRCTL(2)||Linux Programmer's Manual||ARCH_PRCTL(2)|
#include <asm/prctl.h> #include <sys/prctl.h>
int arch_prctl(int code, unsigned long addr); int arch_prctl(int code, unsigned long *addr);
Subfunctions for x86-64 are:
- Set the 64-bit base for the FS register to addr.
- Return the 64-bit base value for the FS register of the current thread in the unsigned long pointed to by addr.
- Set the 64-bit base for the GS register to addr.
- Return the 64-bit base value for the GS register of the current thread in the unsigned long pointed to by addr.
- addr points to an unmapped address or is outside the process address space.
- code is not a valid subcommand.
- addr is outside the process address space.
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.
AMD X86-64 Programmer's manual