.\" Copyright (C) 2004 Andries Brouwer (aeb@cwi.nl) .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH set_tid_address 2 2024-05-02 "Linux man-pages 6.9.1" .SH NAME set_tid_address \- set pointer to thread ID .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .BR "#include " " /* Definition of " SYS_* " constants */" .B #include .P .BI "pid_t syscall(SYS_set_tid_address, int *" tidptr ); .fi .P .IR Note : glibc provides no wrapper for .BR set_tid_address (), necessitating the use of .BR syscall (2). .SH DESCRIPTION For each thread, the kernel maintains two attributes (addresses) called .I set_child_tid and .IR clear_child_tid . These two attributes contain the value NULL by default. .TP .I set_child_tid If a thread is started using .BR clone (2) with the .B CLONE_CHILD_SETTID flag, .I set_child_tid is set to the value passed in the .I ctid argument of that system call. .IP When .I set_child_tid is set, the very first thing the new thread does is to write its thread ID at this address. .TP .I clear_child_tid If a thread is started using .BR clone (2) with the .B CLONE_CHILD_CLEARTID flag, .I clear_child_tid is set to the value passed in the .I ctid argument of that system call. .P The system call .BR set_tid_address () sets the .I clear_child_tid value for the calling thread to .IR tidptr . .P When a thread whose .I 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 .I clear_child_tid and the kernel performs the following operation: .P .in +4n .EX futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0); .EE .in .P 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. .SH RETURN VALUE .BR set_tid_address () always returns the caller's thread ID. .SH ERRORS .BR set_tid_address () always succeeds. .SH STANDARDS Linux. .SH HISTORY Linux 2.5.48. .P Details as given here are valid since Linux 2.5.49. .SH SEE ALSO .BR clone (2), .BR futex (2), .BR gettid (2)