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)