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### 1.3 Preprocessing and conditional compilation

Many Fortran compilers including GNU Fortran allow passing the source code through a C preprocessor (CPP; sometimes also called the Fortran preprocessor, FPP) to allow for conditional compilation. In the case of GNU Fortran, this is the GNU C Preprocessor in the traditional mode. On systems with case-preserving file names, the preprocessor is automatically invoked if the filename extension is `.F`, `.FOR`, `.FTN`, `.fpp`, `.FPP`, `.F90`, `.F95`, `.F03` or `.F08`. To manually invoke the preprocessor on any file, use `-cpp`, to disable preprocessing on files where the preprocessor is run automatically, use `-nocpp`.

If a preprocessed file includes another file with the Fortran `INCLUDE` statement, the included file is not preprocessed. To preprocess included files, use the equivalent preprocessor statement `#include`.

If GNU Fortran invokes the preprocessor, `__GFORTRAN__` is defined and `__GNUC__`, `__GNUC_MINOR__` and `__GNUC_PATCHLEVEL__` can be used to determine the version of the compiler. See Overview for details.

While CPP is the de-facto standard for preprocessing Fortran code, Part 3 of the Fortran 95 standard (ISO/IEC 1539-3:1998) defines Conditional Compilation, which is not widely used and not directly supported by the GNU Fortran compiler. You can use the program coco to preprocess such files (http://www.daniellnagle.com/coco.html).