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SET_TID_ADDRESS(2)                                                      Linux Programmer's Manual                                                     SET_TID_ADDRESS(2)

NAME
       set_tid_address - set pointer to thread ID

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 de‐
       fault.

       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  ig‐
       nored.

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

ERRORS
       set_tid_address() always succeeds.

VERSIONS
       This call is present since Linux 2.5.48.  Details as given here are valid since Linux 2.5.49.

CONFORMING TO
       This system call is Linux-specific.

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

Linux                                                                          2021-06-20                                                             SET_TID_ADDRESS(2)