I am trying to test out my system and wish to emulate a condition, where the child process gets hung. For doing this, I am trying to attach the child process to GDB and putting a break on it. But things don't seem to be going as expected.
Also, in the same vein, how do I know that a spawned child process is not progressing, but is hung?
Use can use SIGSTOP to hang a child process - but that is observably different from the child process going into an infinite loop, or a bad conditional wait - still it may be close enough for testing.
To check a child process has not hung, you have it send heart-beats to the parent (you'll need some kind of communications channel for this - maybe stdin/stdout at a minimum). Then the child has hung if it fails to send a couple of heart-beats messages.
A child process will inherit any pipes created before the fork. You can use this to both "hang" your child and to let it know when to continue. You can have your child process try a blocking read on the pipe, and it will block (i.e. hang) until the parent writes something.
You could also use signals like Douglass mentions. You can let the OS do basic stop/cont or you can implement signal handlers to do something more complex (like entering an infinite loop).
Examples for both of these can be found in the Unix Programming FAQ along with a ton of additional information on process control, signal handling, pipes, etc...
You can try looking in /proc to determine if you are hung. You can read /proc/<child-pid>/stat to get a lot of low-level process information including the current state, the amount of user/kernel time the process has been scheduled, the current stack and instruction pointers, etc... Using a combination of this you can try to determine if the process is hung or not. Check out the proc(5) man page for more info on /proc/<pid>/stat.