#line in C: Overriding __FILE__ and __LINE__ for Code Generators

The #line directive overrides the values that __LINE__ and __FILE__ report. #line 100 "generated.c" tells the preprocessor that the next line is line 100 of generated.c. This is used by code generators: yacc, lex, protobuf compilers, and template engines emit C code with #line directives so that error messages and debug information reference the original source file, not the generated intermediate. You will rarely write #line by hand. You will frequently encounter it in generated code.

After the #line directive, __FILE__ reports custom_source.y and __LINE__ starts at 100. The next line is 101. The compiler and debugger use these overridden values for error messages and breakpoints. If this code had a compile error on the line after #line 100, the error would reference custom_source.y:100, not the actual generated file. This is how yacc reports errors in your grammar file rather than in y.tab.c.

#line is infrastructure. You will not write it often. But when you see it in generated code, you will understand why it is there and what it does. Next: include guards in depth, the pattern that every C header file must implement.