using gdb with unix pipe ./lexer a.cl | ./parser

The normal command line arg won't work because there is nowhere to insert the gdb command. You can try ./lexer a.cl  | gdb parser but this isn't the same as sending the output of the lexer to the debugger.

Tricky solution:
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.

mkfifo foo

After you created a fifo, run from bash:

lexer a.cl>foo

The above process should block.    

gdb parser

Then, in gdb prompt, run

run<foo

And you should see an error message:

Starting program: /usr/class/cs143/cool/assignments/PA3/parser dump_with_types(cout,0);

This is much better than "Segmentation Fault" from the command line when you run ./lexer a.cl | ./parser