CentOS 7.0 - man page for set_tid_address (centos section 2)
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SET_TID_ADDRESS(2) Linux Programmer's Manual SET_TID_ADDRESS(2)
set_tid_address - set pointer to thread ID
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
The system call set_tid_address() sets the clear_child_tid value for the calling process
When a process whose clear_child_tid is not NULL terminates, then, if the process is shar-
ing 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
This system call is Linux-specific.
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2012-07-19 SET_TID_ADDRESS(2)
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