Using gdb/ddd to debug child processes

If you have tried to debug a child process using ddd, you may have noticed that ddd steps into the parent (and not the child) after the call to fork(). It is possible to debug the child as well, but it requires a special procedure. Since the child is a seperate process, it will require a second debugger window, and we will make use of gdb's ability to "attach" to a process which is already running.

Before you start, you must do the following:

• Make sure your call to fork() assigns a value to some variable so you can read it easily, e.g. "pid = fork()".
• Make sure you place a sleep() statement in the child as the first line of code after the fork(), e.g. "sleep(60)" [make the sleep() long enough for step 4 below ...]. The sleep() statment can be removed once debugging is complete.
• Compile your program with the "-g" option set, e.g. gcc myProg.c -o myProg -g

Now you are ready to start:

1. Start 2 copies of ddd in the background, like "ddd myProg & ddd myProg &". It is important that the two copies being running concurrently.
2. Pick (arbitrarily) one window to be the "parent" and set a breakpoint after the call to fork() (but not in any code the child will be executing ... that is, set the breakpoint somewhere in the parent's code ... if you set the breakpoint in the child's code, DDD will kill the child as soon as it is created!).
3. Run the parent to the breakpoint. Note the value returned by fork(), i.e. the process ID of the child.
4. In the "child" window, type "attach " in the gdb console window where is the child's process ID. Note: the gdb console is found at the bottom of the ddd window; this is where you can type commands directly to gdb.
5. Set a breakpoint in the child after the sleep() statement, and click on "cont" (in the popup "Command Tool" window) to allow the child to continue execution to the breakpoint.