function log...

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How do I get a log of all the functions invoked in a huge C program ?
Does GDB (or ddd) have any such option ?
I mean the simplest possible scenario (please do not go for non-
debuggable object files or other such stuff)

This is required to compare the behaviour of two options which modify
two  global variables and the
program runs differently for the two cases but this is not expected.
So we want to get a diff of the "functions invoked logs" in the two
cases.