set_tid_address

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



NAME
       set_tid_address - set pointer to thread ID

SYNOPSIS
       #include <linux/unistd.h>

       long set_tid_address(int *tidptr);

       Note: There is no glibc wrapper for this system call; see NOTES.

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.

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.

NOTES
       Glibc does not provide a wrapper for this system call; call it using
       syscall(2).

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

COLOPHON
       This page is part of release 5.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2017-09-15                 SET_TID_ADDRESS(2)