set_tid_address (2) Linux Manual Page
set_tid_address – set pointer to thread ID
Synopsis
#include <linux/unistd.h>Note: There is no glibc wrapper for this system call; see NOTES.
pid_t set_tid_address(int *tidptr);
Description
For each thread, 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 thread 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 thread does is to write its thread ID at this address.
- When set_child_tid is set, the very first thing the new thread does is to write its thread ID at this address.
- clear_child_tid
- If a thread 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 thread to tidptr.
When a thread whose clear_child_tid is not NULL terminates, then, if the thread is sharing memory with other threads, then 0 is written at the address specified in clear_child_tid and the kernel performs the following operation:
The effect of this operation is to wake a single thread that is performing a futex wait on the memory location. Errors from the futex wake operation are ignored.