Have to create a temp file to serve as the output of the lexer command and have linux block on this command. Then create a separate process to read from the temp file from inside the debugger.
And you should see an error message:
Starting program: /usr/class/cs143/cool/assignments/PA3/parser
This is much better than "Segmentation Fault" from the command line when you run ./lexer a.cl | ./parser