Compatibility across separate source files

When the definitions for two structures, unions, or enumerations are defined in separate source files, each file can theoretically contain a different definition for an object of that type with the same name. The two declarations must be compatible, or the run time behavior of the program is undefined. Therefore, the compatibility rules are more restrictive and specific than those for compatibility within the same source file. For structure, union, and enumeration types defined in separately compiled files, the composite type is the type in the current source file.

The requirements for compatibility between two structure, union, or enumerated types declared in separate source files are as follows:

• If one is declared with a tag, the other must also be declared with the same tag.
• If both are completed types, their members must correspond exactly in number, be declared with compatible types, and have matching names.

For enumerations, corresponding members must also have the same values.

For structures and unions, the following additional requirements must be met for type compatibility:

• Corresponding members must be declared in the same order (applies to structures only).
• Corresponding bit fields must have the same widths.