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Other Compilers

An individual Fortran source file can be compiled to an object (*.o) file instead of to the final program executable. This allows several portions of a program to be compiled at different times and linked together whenever a new version of the program is needed. However, it introduces the issue of object compatibility across the various object files (and libraries, or *.a files) that are linked together to produce any particular executable file.

Object compatibility is an issue when combining, in one program, Fortran code compiled by more than one compiler (or more than one configuration of a compiler). If the compilers disagree on how to transform the names of procedures, there will normally be errors when linking such programs. Worse, if the compilers agree on naming, but disagree on issues like how to pass parameters, return arguments, and lay out COMMON areas, the earliest detected errors might be the incorrect results produced by the program (and that assumes these errors are detected, which is not always the case).

Normally, g77 generates code that is object-compatible with code generated by a version of f2c configured (with, for example, f2c.h definitions) to be generally compatible with g77 as built by gcc. (Normally, f2c will, by default, conform to the appropriate configuration, but it is possible that older or perhaps even newer versions of f2c, or versions having certain configuration changes to f2c internals, will produce object files that are incompatible with g77.)

For example, a Fortran string subroutine argument will become two arguments on the C side: a char * and an int length.

Much of this compatibility results from the fact that g77 uses the same run-time library, libf2c, used by f2c, though g77 gives its version the name libg2c so as to avoid conflicts when linking, installing them in the same directories, and so on.

Other compilers might or might not generate code that is object-compatible with libg2c and current g77, and some might offer such compatibility only when explicitly selected via a command-line option to the compiler.

Note: This portion of the documentation definitely needs a lot of work!