set_tid_address - set pointer to thread ID

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.

set_tid_address() always succeeds.

Linux 2.5.48. Details as given here are valid since Linux 2.5.49.

Standard C library (libc, -lc)

set_tid_address - set pointer to thread ID

set_tid_address() always returns the caller's thread ID.

clone(2), futex(2), gettid(2) Linux man-pages 6.9.1 2024-05-02 set_tid_address(2)

Linux.

#include<sys/syscall.h> /* Definition of SYS_* constants */ #include<unistd.h>pid_tsyscall(SYS_set_tid_address,int*tidptr);Note: glibc provides no wrapper for set_tid_address(), necessitating the use of syscall(2).