Predefined Macros
Many compilers predefine a number of useful macros, including __DATE__, __TIME__, __LINE__, and __FILE__. Each of these names is surrounded by two underscore characters to reduce the likelihood that the names will conflict with names you've used in your program.
When the precompiler sees one of these macros, it makes the appropriate substitutes. For __DATE__, the current date is substituted. For __TIME__, the current time is substituted. __LINE__ and __FILE__ are replaced with the source code line number and filename, respectively. You should note that this substitution is made when the source is precompiled, not when the program is run. If you asked the program to print __DATE__, you would not get the current date; you would ...
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