Compiling

The main function of a Fortran compiler is to read a set of source files and write the corresponding set of object modules to the object file.

Most compilers have a number of switches or options which can be set to control how the compiler works and what additional output it produces. Some of the more useful ones, found on many systems, are described below.

• Almost all compilers can produce a listing file: a text file containing a copy of the source code, with the lines numbered, and with error messages and other useful information attached. A list of all the symbolic names and labels used in the program unit is often provided: this should be checked for unexpected entries as they may be the result of spelling mistakes.
• An even more useful addition to the listing is a cross-reference table: this lists every place that each symbolic name has been used. Good compilers indicate which names have only been used once as these often indicate a programming mistake.
• Another widely available option is the detection of syntax which does not conform to the Fortran Standard: this helps to ensure program portability.
• Often it is possible to choose the optimization level. During program development a low level of optimization should be selected if this makes the compiler run faster; it may improve the error detection. Highly optimised machine code may execute faster but if the source code lines are rearranged error messages may be less helpful.
• Many systems allow additional code to be included which check for errors at run-time. Errors such as over-running the bounds of an array or a character string, or arithmetic over-flow can usually be trapped. Such errors are not uncommon, so this assistance is very valuable. Some programming manuals suggest that these options should only be selected during program development and switched-off thereafter in the interests of speed. This is rather like wearing seat-belts in the car only while you are learning to drive and ignoring them as soon as you are allowed out on the motorway. Run-time checks do not usually reduce the execution speed noticeably.