SET_TID_ADDRESS(2)

NAME
set_tid_address - set pointer to thread ID

SYNOPSIS
#include <linux/unistd.h>

long set_tid_address(int *tidptr);

DESCRIPTION
The kernel keeps for each process two values called set_child_tid and clear_child_tid that are NULL by default.

  set_child_tid
If a process is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to child_tidptr, the fifth parameter of that system call.

When set_child_tid is set, the very first thing the new process does is writing its PID at this address.

  clear_child_tid
If a process is started using clone(2) with the CLONE_CHILD_CLEARTID flag, clear_child_tid is set to child_tidptr, the fifth parameter of that system call.

The system call set_tid_address() sets the clear_child_tid value for the calling process to tidptr.

When clear_child_tid is set, and the process exits, and the process was sharing memory with other processes or threads, then 0 is written at this address, and a futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL, 0); call is done. (That is, wake a single process waiting on this futex.) Errors are ignored.

RETURN VALUE
set_tid_address() returns the PID of the current process.

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

SEE ALSO
clone(2), futex(2)