Ubuntu Manpages

set_tid_address

set pointer to thread ID

#include <linux/unistd.h>

long set_tid_address(int *tidptr);

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.

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.
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.

set_tid_address() always returns the PID of the calling process.

set_tid_address() always succeeds.

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

This system call is Linux-specific.

clone(2), futex(2)

This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.