set_tid_address - set pointer to thread ID
#include <linux/unistd.h>
pid_t 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).
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.