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SET_TID_ADDRESS

Section: Linux Programmer's Manual (2)
Updated: 2014-07-08
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NAME

set_tid_address - set pointer to thread ID  

SYNOPSIS

#include <linux/unistd.h>

long set_tid_address(int *tidptr);
 

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.  

CONFORMING TO

This system call is Linux-specific.  

SEE ALSO

clone(2), futex(2), gettid(2)  

COLOPHON

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Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
VERSIONS
CONFORMING TO
SEE ALSO
COLOPHON
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Time: 14:28:28 GMT, February 25, 2017