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| .TH SET_TID_ADDRESS 2 2020-12-21 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| set_tid_address \- set pointer to thread ID |
| .SH SYNOPSIS |
| .nf |
| .B #include <linux/unistd.h> |
| .PP |
| .BI "pid_t set_tid_address(int *" tidptr ); |
| .fi |
| .PP |
| .IR Note : |
| There is no glibc wrapper for this system call; see NOTES. |
| .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. |
| .PP |
| The system call |
| .BR set_tid_address () |
| sets the |
| .I clear_child_tid |
| value for the calling thread to |
| .IR tidptr . |
| .PP |
| 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: |
| .PP |
| futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0); |
| .PP |
| 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 VERSIONS |
| This call is present since Linux 2.5.48. |
| Details as given here are valid since Linux 2.5.49. |
| .SH CONFORMING TO |
| This system call is Linux-specific. |
| .SH NOTES |
| Glibc does not provide a wrapper for this system call; call it using |
| .BR syscall (2). |
| .SH SEE ALSO |
| .BR clone (2), |
| .BR futex (2), |
| .BR gettid (2) |
