Kernel Korner - Easy I/O with IO Channels

Glib's IO Channels provide several powerful features:

Although designed for powerful and complex GNOME applications, Glib is actually a separate library from GNOME and readily usable from any C application.

A main loop, sometimes called an event loop, allows a single-threaded process to wait for and handle events originating from multiple sources. Most GUI programmers are familiar with main loops: they allow event-driven GUI programming to register callback functions that are invoked in response to events, such as a button press or window close. The Gtk+ main loop is built on top of Glib's main loop.

The Glib main loop is implemented using multiplexing I/O—in Linux, via the poll() system call. Events are associated with file descriptors, which are watched via poll(). In this manner, the application need not check incessantly for new events but can sleep, consuming no processor time, so long as there is no activity.

Glib's main loop associates a callback function with each event. When the event occurs, the main loop will call back into the given function. Callbacks are invoked in the order that their associated event occurs, although priority may be given to events to change this order. Because multiple events may be watched and multiple callback functions registered, even a single-threaded process can juggle numerous events.

The Glib library is GNOME's base library, providing portability wrappers and a set of helper functions to make programming in C less arduous. Although part of GNOME, Glib is very much usable on its own, and many non-GNOME projects do indeed utilize Glib without touching any other parts of GNOME. In fact, Glib is even beneficial to console applications. This article makes no assumptions about the use of other GNOME components; the interfaces covered work equally well in a complex GNOME application and a simple console program.

Compiling an application with the requisite Glib support is made easy by the pkg-config program. You can build a binary gio from the source file gio.c with the following command:

gcc -Wall -O2 \
        `pkg-config --cflags --libs glib-2.0` \
        -o gio \
        gio.c

An IO Channel is represented by the GIOChannel data structure. Its fields are private, and it is accessed using only the official IO Channel interfaces.

Each IO Channel is associated with a single file or other “file-like” object. On Linux, an IO Channel can be associated with any open file descriptor, including sockets and pipes. Once associated, the IO Channel is used to access the file.

Watches are created against a given IO Channel, along with a set of events on which to wait for and a callback function to invoke in response. The watches then integrate with Glib's main loop. When an event occurs—say, a socket has new data available for reading—the watch is triggered and the callback is automatically invoked.

The watch lies at the heart of the power of IO Channels: applications can create multiple watches and integrate them, along with numerous other events, into the Glib main loop, providing event-driven programming to even simple single-threaded applications.

Listing 1 is a complete and working console application that uses IO Channels to communicate across two pipes. It creates two IO Channels, one for the read side of the pipe and another for the write side of the pipe. It then registers watches for these two IO Channels. One watch invokes a callback, gio_in(), when the pipe is available for reading (that is, when a read from the read side of the pipe will not block). The other watch invokes a callback, gio_out(), when the pipe is available for writing (that is, when a write to the write side of the pipe will not block). The gio_out() callback writes a small message into the pipe. The gio_in() callback reads the available data from the pipe and prints it to standard out.