set_tid_address - set pointer to thread ID
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
pid_t syscall(SYS_set_tid_address, int *tidptr);Note: glibc provides no wrapper for
set_tid_address(), necessitating the use of
syscall(2).
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_tidIf 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_tidIf 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.