Kernel Korner - Easy I/O with IO Channels

IO Channels, a feature of the Glib library, make portable I/O simple and efficient. In this article, Robert shows how.

where gio is the applicable IO Channel, condition is a bitmask of the triggered events and data is the last argument given to g_io_add_watch().

If the return value of the callback is FALSE, the watch is automatically removed.

In our example program in Listing 1, we create two watches, one for each of our IO Channels.

Reading from an IO Channel

The Glib library provides three basic interfaces for reading from an IO Channel.

The first, g_io_channel_read_chars(), is used to read a specific number of characters from an IO Channel into a pre-allocated buffer:

GIOStatus g_io_read_chars (GIOChannel *channel,
                           gchar *buf,
                           gsize count,
                           gsize *bytes_read,
                           GError **error);

This function reads up to count bytes from the IO Channel channel into the buffer buf. Upon successful return, bytes_read will point to the number of bytes actually read. On failure, error will point to a GError structure.

The return value is an integer with one of four values: G_IO_STATUS_ERROR (an error occurred), G_IO_STATUS_NORMAL (success), G_IO_STATUS_EOF (end-of-file was reached) or G_IO_STATUS_AGAIN (resource temporarily unavailable, try again).

The second interface, g_io_channel_read_line(), is used to read an entire line from a given IO Channel. It will not return until a newline-delimited line is read:

g_io_channel_read_line (GIOChannel *channel,
                        gchar **str_return,
                        gsize *length,
                        gsize *terminator_pos,
                        GError **error);

Upon successful return, str_return will contain a pointer to a newly allocated block of memory of length bytes. terminator_pos is the offset into str_return of the terminating character. The data returned by this function must be freed via a call to g_free().

The final interface, g_io_channel_read_to_end(), reads all remaining data from the file into the given buffer:

GIOStatus g_io_channel_read_to_end (GIOChannel *channel,
                                    gchar **str_return,
                                    gsize *length,
                                    GError **error);

Upon successful return, str_return will contain a pointer to a newly allocated block of memory of length bytes, which must be freed via g_free().

This function should not be used on IO Channels that map to file descriptors that do not necessarily return end-of-file when exhausted of data. For example, we could not use this function in our example program, because pipes do not return end-of-file until the other side has closed its end of the connection. Thus, our example would block indefinitely if we use g_io_channel_read_to_end().

Instead, in our example, we use g_io_channel_read_line() to read an entire line from the pipe.

Writing to an IO Channel

Glib provides a single interface for writing to an IO Channel:

g_io_channel_write_chars (GIOChannel *channel,
                          const gchar *buf,
                          gssize count,
                          gsize *bytes_written,
                          GError **error);

A successful call to g_io_channel_write_chars() will write up to count bytes from the buffer pointed at by buf into the file represented by the IO Channel channel. If count is negative one, buf will be treated as a NULL-delimited string. On return, bytes_written contains the actual number of bytes written.

Like C's Standard I/O Library, IO Channels perform buffered I/O to optimize performance. Thus, a write request may not actually be submitted to the kernel after each call to g_io_channel_write_chars(). Instead, glib may wait until a sufficiently large buffer is full and then submit the write request, in one large swoop.

The g_io_channel_flush() function is used to force a submission of any pending write requests to the kernel:

g_io_channel_flush (GIOChannel *channel,
                    GError **error);

Buffering can be turned off altogether, if so desired:

g_io_channel_set_encoding (gio, NULL, NULL);
g_io_channel_set_buffered (gio, FALSE);

The second function disables buffering. The first sets the IO Channel's encoding to NULL, which is required for disabling buffering.

In our example program, we use g_io_channel_write_chars() to write a small string to the pipe. We do not mess with the buffering of the IO Channel.