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_SET-
TID 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 per-
forming 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.
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