arch_prctl is only supported on Linux/x86-64 for 64bit programs currently.
The 64bit base changes when a new 32bit segment selector is loaded.
ARCH_SET_GS is disabled in some kernels.
Context switches for 64bit segment bases are rather expensive. It may be a faster alternative to set a 32bit base using a segment selector by setting up an LDT with modify_ldt(2) or using the set_thread_area(2) system call in a 2.5 kernel. arch_prctl is only needed 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.
No prototype for arch_prctl in glibc 2.2. You have to declare it yourself for now. This will be fixed in future glibc versions.
FS may be already used by the threading library.