SET_TID_ADDRESS(2)        (2020-12-21)         SET_TID_ADDRESS(2)

     NAME
          set_tid_address - set pointer to thread ID

     SYNOPSIS
          #include <linux/unistd.h>

          pid_t 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 opera-
          tion:

              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.

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     SET_TID_ADDRESS(2)        (2020-12-21)         SET_TID_ADDRESS(2)

     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 5.10 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/.

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