Using composite buffers for scatter/gather operations

The Using fixed-length I/O buffers recipe in Chapter 2, I/O Operations, introduces simple I/O buffers, but only slightly touches upon scatter/gather operations and composite buffers. In this recipe, we are going to consider this topic in more detail.

A composite buffer is basically a complex buffer that consists of two or more simple buffers (contiguous blocks of memory) distributed over the process' address space. Such buffers become especially handy in two situations.

The first situation is when the application needs a buffer either to store the message before sending it to the remote application or to receive the message sent by the remote application. The problem is that the size of the message is so big that allocating a single contiguous buffer that is sufficient to store it may fail due to the process' address space fragmentation. In this case, allocating multiple smaller buffers, whose sizes when summed would be enough to store...