8.10 Exercise: Spooling a Slow Device

This exercise uses asynchronous I/O to overlap the handling of I/O from a slow device with other program calculations. Examples include printing or performing a file transfer over a slow modem. Another example is a program that plays an audio file in the background while doing something else. In these examples, a program reads from a disk file and writes to a slow device.

Write a program that uses aio_read and aio_write to transfer data to a slow device. The source of information is a disk file. Model your program after Programs 8.13 and 8.14. Pass the name of the input and output files as command-line arguments.

The main program still initiates the first read. However, now the signal handler initiates an aio_write if the asynchronous read completes. Similarly, when the asynchronous write completes, the signal handler initiates another aio_read .

Begin testing with two named pipes for the input and the output. Then, use a disk file for the output. Redirect the output from the pipe to a file and use diff to check that they are the same. If a workstation with a supported audio device is available, use an audio file on disk as input and "/dev/audio" as the output device.

Keep statistics on the number of bytes transferred and the number of write operations needed. Add a signal handler that outputs this information when the program receives a SIGUSR1 signal. The statistics can be kept in global variables. Block signals when necessary to prevent different signal handlers from accessing these shared variables concurrently.

This program is particularly interesting when the output goes to the audio device. It is possible to tell when the program is computing by the gaps that occur in the audio output. Estimate the percentage of time spent handling I/O as compared with calculation time.