Manpage of ARCH_PRCTL
ARCH_PRCTLSection: Linux Programmer's Manual (2)
NAMEarch_prctl - set architecture-specific thread state
#include <sys/prctl.h>int arch_prctl(int code, unsigned long addr);int arch_prctl(int code, unsigned long *addr);
DESCRIPTIONThe arch_prctl() function sets architecture-specific process or thread state. codeselects a subfunction and passes argument addrto it; addris interpreted as either an unsigned longfor the "set" operations, or as an unsigned long *, for the "get" operations.
Subfunctions for x86-64 are:
- Set the 64-bit base for the FSregister to addr.
- Return the 64-bit base value for the FSregister of the current thread in the unsigned longpointed to by addr.
- Set the 64-bit base for the GSregister to addr.
- Return the 64-bit base value for the GSregister of the current thread in the unsigned longpointed to by addr.
RETURN VALUEOn success, arch_prctl() returns 0; on error, -1 is returned, and errnois set to indicate the error.
- addrpoints to an unmapped address or is outside the process address space.
- codeis not a valid subcommand.
- addris outside the process address space.
CONFORMING TOarch_prctl() is a Linux/x86-64 extension and should not be used in programs intended to be portable.
NOTESarch_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_GSis 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 2GB of address space can be allocated by using mmap(2) with the MAP_32BITflag.
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.
SEE ALSOmmap(2), modify_ldt(2), prctl(2), set_thread_area(2)
AMD X86-64 Programmer's manual
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