Manpages - set_tid_address.2

set_tid_address - set pointer to thread ID

  #include <sys/syscall.h> /* Definition of SYS_* constants */
  #include <unistd.h>

  pid_t syscall(SYS_set_tid_address, int *tidptr);

Note: glibc provides no wrapper for *set_tid_address*(), necessitating the use of *syscall*(2).

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 returns the caller’s thread ID.

*set_tid_address*() always succeeds.

This call is present since Linux 2.5.48. Details as given here are valid since Linux 2.5.49.

This system call is Linux-specific.

*clone*(2), *futex*(2), *gettid*(2)

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