#### Stream-Oriented File Descriptors

Asio includes classes added to permit synchronous and asynchronous read and write operations to be performed on POSIX file descriptors, such as pipes, standard input and output, and various devices.

These classes also provide limited support for regular files. This support assumes that the underlying read and write operations provided by the operating system never fail with `EAGAIN` or `EWOULDBLOCK`. (This assumption normally holds for buffered file I/O.) Synchronous and asynchronous read and write operations on file descriptors will succeed but the I/O will always be performed immediately. Wait operations, and operations involving `asio::null_buffers`, are not portably supported.

For example, to perform read and write operations on standard input and output, the following objects may be created:

    posix::stream_descriptor in(my_io_context, ::dup(STDIN_FILENO));
    posix::stream_descriptor out(my_io_context, ::dup(STDOUT_FILENO));

These are then used as synchronous or asynchronous read and write streams. This means the objects can be used with any of the read(), async_read(), write(), async_write(), read_until() or async_read_until() free functions.

###### See Also

posix::stream_descriptor, Chat example (C++11).

###### Notes

POSIX stream descriptors are only available at compile time if supported by the target operating system. A program may test for the macro `ASIO_HAS_POSIX_STREAM_DESCRIPTOR` to determine whether they are supported.