I want to know the information about the function arguments

[冠人 王 via gcc]

When I modify the gcc source code, sometimes I do not know what parameters does 
the function call.
For example, 
bool warn_if_unused_value(const tree exp,location_t locus){    function 
declaration ...
}
I want to know what "exp" and "locus" are by using fprintf such as
fprintf(stderr,"%s\n%s\n",exp,locus) --> I know it does not work since exp and 
locus is not strings
How should I do to know the content of variable "exp" and "locus" ?