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ARCH_PRCTL(2)              Linux Programmer's ManualARCH_PRCTL(2)NAME       arch_prctl - set architecture-specific thread stateSYNOPSIS       #include <asm/prctl.h>       #include <sys/prctl.h>       int arch_prctl(int code, unsigned long addr);       int arch_prctl(int code, unsigned long *addr);DESCRIPTION       arch_prctl()  sets architecture-specific process or thread state.  code       selects a subfunction and passes argument addr to it;  addr  is  inter-       preted  as  either  an unsigned long for the "set" operations, or as an       unsigned long *, for the "get" operations.       Subfunctions 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 addr.       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 addr.RETURN VALUE       On success, 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 ad-              dress 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 supported only 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.  As  an       optimization,  if  a  32-bit TLS base address is used, arch_prctl() may       use a real TLS entry as ifset_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 usingmmap(2) with  the       MAP_32BIT flag.       Because  of  the  aforementioned  optimization,  using arch_prctl() andset_thread_area(2) in the same thread is dangerous, as they  may  over-       write 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.SEE ALSOmmap(2),modify_ldt(2),prctl(2),set_thread_area(2)       AMD X86-64 Programmer's manualCOLOPHON       This page is part of release 5.05 of the Linux  man-pages  project.   A       description  of  the project, information about reporting bugs, and the       latest    version    of    this    page,    can     be     found     at       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15ARCH_PRCTL(2)