Provided by: manpages-dev_2.17-1_all
set_tid_address - set pointer to thread ID
long set_tid_address(int *tidptr);
The kernel keeps for each process two values called set_child_tid and
clear_child_tid that are NULL by default.
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
When set_child_tid is set, the very first thing the new process does is
writing its PID at this address.
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.
set_tid_address() returns the PID of the current process.
This call is present since Linux 2.5.48. Details as given here are
valid since Linux 2.5.49.