set_tid_address — set pointer to thread ID
#include <linux/unistd.h>
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 system call.
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.
This call is present since Linux 2.5.48. Details as given here are valid since Linux 2.5.49.
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