Section 14.4. Defining Line Numbers

14.4. Defining Line Numbers

The compiler includes line numbers and source filenames in warnings, error messages, and information provided to debugging tools. You can use the #line directive in the source file itself to change the compiler's filename and line numbering information. The #line directive has the following syntax:

     #line line_number ["filename"]

The next line after a #line directive has the number specified by line_number. If the directive also includes the optional string literal "filename", then the compiler uses the contents of that string as the name of the current source file.

The line_number must be a decimal constant greater than zero. An example:

     #line 1200 "primary.c"

The line containing the #line directive may also contain macros. If so, the preprocessor expands them before executing the #line directive. The #line directive must then be formally correct after macro expansion.

Programs can access the current line number and filename settings as values of the standard predefined macros _ _LINE_ _ and _ _FILE_ _:

     printf( "This message was printed by line %d in the file %s.\n", _  _LINE_  _,             _  _FILE_  _ );

The #line directive is typically used by programs that generate C source code as their output. By placing the corresponding input file line numbers in #line directives, such programs can make the C compiler's error messages refer to the pertinent lines in the original source.