set_tid_address(2)            System Calls Manual           set_tid_address(2)

NAME
       set_tid_address - set pointer to thread ID

LIBRARY
       Standard C library (libc, -lc)

SYNOPSIS
       #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
       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.

       set_child_tid
              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.

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

ERRORS
       set_tid_address() always succeeds.

STANDARDS
       Linux.

HISTORY
       Linux 2.5.48.

       Details as given here are valid since Linux 2.5.49.

SEE ALSO
       clone(2), futex(2), gettid(2)

Linux man-pages 6.7               2023-10-31                set_tid_address(2)