arch_prctl(2) — Linux manual page


arch_prctl(2)              System Calls Manual             arch_prctl(2)

NAME         top

       arch_prctl - set architecture-specific thread state

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <asm/prctl.h>        /* Definition of ARCH_* constants */
       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       int syscall(SYS_arch_prctl, int op, unsigned long addr);
       int syscall(SYS_arch_prctl, int op, unsigned long *addr);

       Note: glibc provides no wrapper for arch_prctl(), necessitating
       the use of syscall(2).

DESCRIPTION         top

       arch_prctl() sets architecture-specific process or thread state.
       op selects an operation 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 both x86 and x86-64 are:

       ARCH_SET_CPUID (since Linux 4.12)
              Enable (addr != 0) or disable (addr == 0) the cpuid
              instruction for the calling thread.  The instruction is
              enabled by default.  If disabled, any execution of a cpuid
              instruction will instead generate a SIGSEGV signal.  This
              feature can be used to emulate cpuid results that differ
              from what the underlying hardware would have produced
              (e.g., in a paravirtualization setting).

              The ARCH_SET_CPUID setting is preserved across fork(2) and
              clone(2) but reset to the default (i.e., cpuid enabled) on

       ARCH_GET_CPUID (since Linux 4.12)
              Return the setting of the flag manipulated by
              ARCH_SET_CPUID as the result of the system call (1 for
              enabled, 0 for disabled).  addr is ignored.

       Subfunctions for x86-64 only are:

              Set the 64-bit base for the FS register to addr.

              Return the 64-bit base value for the FS register of the
              calling 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
              calling thread in the unsigned long pointed to by addr.

RETURN VALUE         top

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

ERRORS         top

       EFAULT addr points to an unmapped address or is outside the
              process address space.

       EINVAL op is not a valid operation.

       ENODEV ARCH_SET_CPUID was requested, but the underlying hardware
              does not support CPUID faulting.

       EPERM  addr is outside the process address space.

STANDARDS         top


NOTES         top

       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

       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 if set_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 using mmap(2) with the MAP_32BIT flag.

       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.

       FS may be already used by the threading library.  Programs that
       use ARCH_SET_FS directly are very likely to crash.

SEE ALSO         top

       mmap(2), modify_ldt(2), prctl(2), set_thread_area(2)

       AMD X86-64 Programmer's manual

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                  arch_prctl(2)

Pages that refer to this page: clone(2)modify_ldt(2)set_thread_area(2)syscalls(2)