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ARCH_PRCTL(2) Linux Programmer's Manual ARCH_PRCTL(2) NAME arch_prctl - set architecture specific thread state SYNOPSIS #include <asm/prctl.h> #include <sys/prctl.h> int arch_prctl(int code, unsigned long addr); DESCRIPTION The arch_prctl() function sets architecture specific process or thread state. code selects a subfunction and passes argument addr to it. Sub functions 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 the address parameter 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 the address parameter. RETURN VALUE On sucess, arch_prctl() returns 0; on error, -1 is returned, and errno is set to indicate the error. ERRORS 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. CONFORMING TO arch_prctl() is a Linux/x86-64 extension and should not be used in pro- grams intended to be portable. NOTES arch_prctl() is only supported on Linux/x86-64 for 64-bit programs cur- rently. 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 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. As of version 2.3.5, 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. SEE ALSO mmap(2), modify_ldt(2), prctl(2), set_thread_area(2) AMD X86-64 Programmer's manual Linux 2003-02-02 ARCH_PRCTL(2)