Information Transfer

Information can be transferred to and from an external procedure by any of three methods.

• An argument list: as shown in the two examples above. This is the preferred method of interfacing as it is the most flexible and modular. It is described in detail in the remainder of this section.
• Common blocks: these are lists of variables or arrays which are stored in areas of areas of memory shared between two or more program units. They are useful in special circumstances when procedures have to be coupled closely together, but are otherwise less satisfactory. Common blocks are covered in detail in section 12.
• External files: interfacing via external files is neither convenient nor efficient but it is mentioned here to point out that external files are global. Once a file has been opened in any program unit it can be accessed anywhere in the program provided that the appropriate I/O unit number is available. A unit number can be passed into a procedure as an integer argument.