#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x) 
#define AT __FILE__ ":" TOSTRING(__LINE__)

printf( AT " message"); // Expect output like: "snipplr.c:5 message"

#define STRINGIFY(x) #x
#define TOSTRING(x) STRINGIFY(x) 
#define AT __FILE__ ":" TOSTRING(__LINE__)

printf( AT " fake error");

http://www.decompile.com/cpp/faq/file_and_line_error_string.htm

Article explains why STRINGIFY and TOSTRING are both needed.

This technique is useful for embedded apps to print debug strings to a debug console. ASSERT or TRACE statements can incorporate these macros. The line numbers are converted to strings at compile-time rather than run-time.

print __FILE__ and __LINE__ using the # stringizing operator

file, line

C++