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set_tid_address(2) [linux man page]

SET_TID_ADDRESS(2)					     Linux Programmer's Manual						SET_TID_ADDRESS(2)

NAME
set_tid_address - set pointer to thread ID SYNOPSIS
#include <linux/unistd.h> long set_tid_address(int *tidptr); DESCRIPTION
The kernel keeps for each process two values called set_child_tid and clear_child_tid that are NULL by default. set_child_tid If a process is started using clone(2) with the CLONE_CHILD_SETTID flag, set_child_tid is set to child_tidptr, the fifth argument of that system call. When set_child_tid is set, the very first thing the new process does is writing its PID at this address. clear_child_tid If a process is started using clone(2) with the CLONE_CHILD_CLEARTID flag, clear_child_tid is set to child_tidptr, the fifth argument of that system call. The system call set_tid_address() sets the clear_child_tid value for the calling process to tidptr. When clear_child_tid is set, and the process exits, and the process was sharing memory with other processes or threads, then 0 is written at this address, and a futex(child_tidptr, FUTEX_WAKE, 1, NULL, NULL, 0); call is done. (That is, wake a single process waiting on this futex.) Errors are ignored. RETURN VALUE
set_tid_address() always returns the PID of the calling process. 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) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2004-09-10 SET_TID_ADDRESS(2)

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SET_TID_ADDRESS(2)					     Linux Programmer's Manual						SET_TID_ADDRESS(2)

NAME
set_tid_address - set pointer to thread ID SYNOPSIS
#include <linux/unistd.h> long set_tid_address(int *tidptr); Note: There is no glibc wrapper for this system call; see NOTES. 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 argu- ment 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. NOTES
Glibc does not provide a wrapper for this system call; call it using syscall(2). SEE ALSO
clone(2), futex(2), gettid(2) COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/. Linux 2017-09-15 SET_TID_ADDRESS(2)

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