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.
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.
On success, arch_prctl() returns 0; on error, -1 is returned, and errno is set to indicate the error.
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.
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. 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 needed only 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.
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