set_tid_address - set pointer to thread ID
#include <linux/unistd.h>
long set_tid_address(int *tidptr);
Note: There is no glibc wrapper for this system call; see NOTES.
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.
- 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:
futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
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.
set_tid_address() always returns the caller's thread ID.
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.
Glibc does not provide a wrapper for this system call; call it using
syscall(2).
clone(2),
futex(2),
gettid(2)