set_tid_address(2) — Linux manual page


set_tid_address(2)         System Calls Manual        set_tid_address(2)

NAME         top

       set_tid_address - set pointer to thread ID

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

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

       pid_t syscall(SYS_set_tid_address, int *tidptr);

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

DESCRIPTION         top

       For each thread, the kernel maintains two attributes (addresses)
       called set_child_tid and clear_child_tid.  These two attributes
       contain the value NULL by default.

              If a thread is started using clone(2) with the
              CLONE_CHILD_SETTID flag, set_child_tid is set to the value
              passed in the ctid argument of that system call.

              When set_child_tid is set, the very first thing the new
              thread does is to write its thread ID at this address.

              If a thread is started using clone(2) with the
              CLONE_CHILD_CLEARTID flag, clear_child_tid is set to the
              value passed in the ctid argument of that system call.

       The system call set_tid_address() sets the clear_child_tid value
       for the calling thread to tidptr.

       When a thread whose clear_child_tid is not NULL terminates, then,
       if the thread is sharing memory with other threads, then 0 is
       written at the address specified in clear_child_tid and the
       kernel performs the following operation:

           futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);

       The effect of this operation is to wake a single thread that is
       performing a futex wait on the memory location.  Errors from the
       futex wake operation are ignored.

RETURN VALUE         top

       set_tid_address() always returns the caller's thread ID.

ERRORS         top

       set_tid_address() always succeeds.

STANDARDS         top


HISTORY         top

       Linux 2.5.48.

       Details as given here are valid since Linux 2.5.49.

SEE ALSO         top

       clone(2), futex(2), gettid(2)

Linux man-pages 6.04           2023-03-30             set_tid_address(2)

Pages that refer to this page: clone(2)prctl(2)syscalls(2)futex(7)