| 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. |
| 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. |
NAME
set_tid_address - set pointer to thread ID
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/syscall.h>\n /* Definition of \nSYS_*\n constants */
\n#include <unistd.h>pid_t syscall(SYS_set_tid_address, int *\ntidptr\n);Note: glibc provides no wrapper for set_tid_address(), necessitating the use of syscall(2).
DESCRIPTION
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.
RETURN VALUE
set_tid_address() always returns the caller's thread ID.
ERRORS
set_tid_address() always succeeds.
STANDARDS
Linux.
HISTORY
Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.
SEE ALSO
clone(2), futex(2), gettid(2)