# Module Thread

    module Thread: sig .. end

Lightweight threads for Posix `1003.1c` and Win32.

    type t

The type of thread handles.

## Thread creation and termination

    val create : ('a -> 'b) -> 'a -> t

`Thread.create funct arg` creates a new thread of control, in which the function application `funct arg` is executed concurrently with the other threads of the program. The application of `Thread.create` returns the handle of the newly created thread. The new thread terminates when the application `funct arg` returns, either normally or by raising the `Thread.Exit` exception or by raising any other uncaught exception. In the last case, the uncaught exception is printed on standard error, but not propagated back to the parent thread. Similarly, the result of the application `funct arg` is discarded and not directly accessible to the parent thread.

    val self : unit -> t

Return the handle for the thread currently executing.

    val id : t -> int

Return the identifier of the given thread. A thread identifier is an integer that identifies uniquely the thread. It can be used to build data structures indexed by threads.

    exception Exit

Exception that can be raised by user code to initiate termination of the current thread. Compared to calling the `Thread.exit` function, raising the `Thread.Exit` exception will trigger `Fun.finally` finalizers and catch-all exception handlers. It is the recommended way to terminate threads prematurely.

- Since 4.14.0

    val exit : unit -> unit

Terminate prematurely the currently executing thread.

    val kill : t -> unit

This function was supposed to terminate prematurely the thread whose handle is given. It is not currently implemented due to problems with cleanup handlers on many POSIX 1003.1c implementations. It always raises the `Invalid_argument` exception.

## Suspending threads

    val delay : float -> unit

`delay d` suspends the execution of the calling thread for `d` seconds. The other program threads continue to run during this time.

    val join : t -> unit

`join th` suspends the execution of the calling thread until the thread `th` has terminated.

    val yield : unit -> unit

Re-schedule the calling thread without suspending it. This function can be used to give scheduling hints, telling the scheduler that now is a good time to switch to other threads.

## Waiting for file descriptors or processes

The functions below are leftovers from an earlier, VM-based threading system. The `Unix` module provides equivalent functionality, in a more general and more standard-conformant manner. It is recommended to use `Unix` functions directly.

    val wait_read : Unix.file_descr -> unit

This function does nothing in the current implementation of the threading library and can be removed from all user programs.

    val wait_write : Unix.file_descr -> unit

This function does nothing in the current implementation of the threading library and can be removed from all user programs.

    val wait_timed_read : Unix.file_descr -> float -> bool

See `Thread.wait_timed_write`.

    val wait_timed_write : Unix.file_descr -> float -> bool

Suspend the execution of the calling thread until at least one character or EOF is available for reading (`wait_timed_read`) or one character can be written without blocking (`wait_timed_write`) on the given Unix file descriptor. Wait for at most the amount of time given as second argument (in seconds). Return `true` if the file descriptor is ready for input/output and `false` if the timeout expired. The same functionality can be achieved with `Unix.select`.

    val select : Unix.file_descr list ->
           Unix.file_descr list ->
           Unix.file_descr list ->
           float -> Unix.file_descr list * Unix.file_descr list * Unix.file_descr list

Same function as `Unix.select`. Suspend the execution of the calling thread until input/output becomes possible on the given Unix file descriptors. The arguments and results have the same meaning as for `Unix.select`.

    val wait_pid : int -> int * Unix.process_status

Same function as `Unix.waitpid`. `wait_pid p` suspends the execution of the calling thread until the process specified by the process identifier `p` terminates. Returns the pid of the child caught and its termination status, as per `Unix.wait`.

## Management of signals

Signal handling follows the POSIX thread model: signals generated by a thread are delivered to that thread; signals generated externally are delivered to one of the threads that does not block it. Each thread possesses a set of blocked signals, which can be modified using `Thread.sigmask`. This set is inherited at thread creation time. Per-thread signal masks are supported only by the system thread library under Unix, but not under Win32, nor by the VM thread library.

    val sigmask : Unix.sigprocmask_command -> int list -> int list

`sigmask cmd sigs` changes the set of blocked signals for the calling thread. If `cmd` is `SIG_SETMASK`, blocked signals are set to those in the list `sigs`. If `cmd` is `SIG_BLOCK`, the signals in `sigs` are added to the set of blocked signals. If `cmd` is `SIG_UNBLOCK`, the signals in `sigs` are removed from the set of blocked signals. `sigmask` returns the set of previously blocked signals for the thread.

    val wait_signal : int list -> int

`wait_signal sigs` suspends the execution of the calling thread until the process receives one of the signals specified in the list `sigs`. It then returns the number of the signal received. Signal handlers attached to the signals in `sigs` will not be invoked. The signals `sigs` are expected to be blocked before calling `wait_signal`.

## Uncaught exceptions

    val default_uncaught_exception_handler : exn -> unit

`Thread.default_uncaught_exception_handler` will print the thread's id, exception and backtrace (if available).

    val set_uncaught_exception_handler : (exn -> unit) -> unit

`Thread.set_uncaught_exception_handler fn` registers `fn` as the handler for uncaught exceptions.

If the newly set uncaught exception handler raise an exception, `Thread.default_uncaught_exception_handler` will be called.