set_tid_address (nr 218)

Linux Signature

pid_t set_tid_address(int *tidptr);

Description

Sets the clear_child_tid pointer for the calling thread. When the thread exits, the kernel writes 0 to *tidptr and wakes any futex waiters. Returns the caller’s TID.

Current Implementation

• Ignores the tidptr argument entirely (no clear_child_tid tracking).
• Returns the current process’s PID (used as TID since each process is single-threaded).

This is sufficient for musl’s early startup, which calls set_tid_address to discover its own TID.

Source: osl/src/syscalls/process.rs — sys_set_tid_address

Future Work

• Store tidptr in the thread/process structure.
• On thread exit, write 0 to *tidptr and perform a futex wake (needed for pthread_join).
• Return a per-thread TID rather than PID once multi-threading is supported.