Non-blocking, Async-friendly print functions

This module overrides everything in the Sys module that might block.

Returns the underlying epoll value.

An Fd.t is a wrapper around a Unix file descriptor, with additional information about the kind of file descriptor and logic to ensure that we don't use a file descriptor that has been closed, or close a file descriptor that is in use.

Fd_by_descr is a table of the open Fd.ts, indexed by file descriptor number.

File_descr_watcher_intf.S provides an API for for watching a set of file descriptors to see if they are ready for reading or writing.

An interruptor provides a file descriptor that can be used to cause a file-descr-watcher to detect the file descriptor is ready for reading.

The In_thread module has functions for interaction between the Async world and other (kernel) threads.

Async.Process is for creating child processes of the current process, and communicating with children via their stdin, stdout, and stderr.

Reader is Async's main API for buffered input from a file descriptor.

Read_write is like Dirpair, except "buy/sell" has been changed to "read/write".

Threading model:

shutdown ?force status initiates shutdown, which runs all the at_shutdown functions, waits for them to finish, and then exits with the supplied status.

We override values from Core.Signal that we don't want people to use with Async.

A signal manager keeps track of a set of signals to be managed and the signal handlers for them.

A thread pool is a set of OCaml threads used to do work, where each piece of work is simply a thunk.

The Thread_safe module has functions that are safe to call from threads outside async.

A thread-safe pipe is a thread-safe interface to the write end of a normal Async.Pipe.

Unix_syscalls provides an interface to many of the functions in OCaml's standard Unix module.

Writer is Async's main API for output to a file descriptor.