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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. set_child_tid If a process is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to child_tidptr, the fifth 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 child_tidptr, the fifth argument 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.
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.
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