set_tid_address(2) - Linux man page

Name

set_tid_address - set pointer to thread ID

Synopsis

#include <linux/unistd.h>
long set_tid_address(int *tidptr);

Description

For each process, 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 process 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 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 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 process to tidptr.

When a process whose clear_child_tid is not NULL terminates, then, if the process is sharing memory with other processes or 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 process 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 PID of the calling process.

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.

Referenced By

prctl(2)