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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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set_tid_address(2) System Calls Manual set_tid_address(2)
set_tid_address - set pointer to thread ID
Standard C library (libc, -lc)
#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_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.
Linux.
Linux 2.5.48.
Details as given here are valid since Linux 2.5.49.
clone(2), futex(2), gettid(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 set_tid_address(2)
Pages that refer to this page: clone(2), PR_GET_TID_ADDRESS(2const), syscalls(2), futex(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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