Linux Device Drivers, Second Edition

Debugging by Watching

Sometimes minor problems can be tracked down by watching the behavior of an application in user space. Watching programs can also help in building confidence that a driver is working correctly. For example, we were able to feel confident about scull after looking at how its read implementation reacted to read requests for different amounts of data.

There are various ways to watch a user-space program working. You can run a debugger on it to step through its functions, add print statements, or run the program under strace. Here we’ll discuss just the last technique, which is most interesting when the real goal is examining kernel code.

The strace command is a powerful tool that shows all the system calls issued by a user-space program. Not only does it show the calls, but it can also show the arguments to the calls, as well as return values in symbolic form. When a system call fails, both the symbolic value of the error (e.g., ENOMEM) and the corresponding string (Out of memory) are displayed. strace has many command-line options; the most useful of which are -t to display the time when each call is executed, -T to display the time spent in the call, -e to limit the types of calls traced, and -o to redirect the output to a file. By default, strace prints tracing information on stderr.

strace receives information from the kernel itself. This means that a program can be traced regardless of whether or not it was compiled with debugging support (the ...