set_tid_address - Online Linux Manual Page

Section : 2
Updated : 2022-10-30
Source : Linux man-pages 6.03

NAME

set_tid_address − set pointer to thread ID

LIBRARY

Standard C library

(

libc

−lc

)

SYNOPSIS

#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).

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.

VERSIONS

This call is present since Linux 2.5.48. Details as given here are valid since Linux 2.5.49.

STANDARDS

This system call is Linux-specific.