C printf debug

printf("__LINE__=%d __FILE__=%s\n", __LINE__, __FILE__);

Standard Predefined Macros

http://gcc.gnu.org/onlinedocs/gcc-3.2.3/cpp/Standard-Predefined-Macros.html

__FILE__

This macro expands to the name of the current input file, in the form of a C string constant. This is the path by which the preprocessor opened the file, not the short name specified in #include or as the input file name argument. For example, "/usr/local/include/myheader.h" is a possible expansion of this macro.

__LINE__

This macro expands to the current input line number, in the form of a decimal integer constant. While we call it a predefined macro, it's a pretty strange macro, since its "definition" changes with each new line of source code.

__FILE__ and __LINE__ are useful in generating an error message to report an inconsistency detected by the program; the message can state the source line at which the inconsistency was detected. For example,

     fprintf (stderr, "Internal error: "
                    "negative string length "
                    "%d at %s, line %d.",
            length, __FILE__, __LINE__);