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And they've all had incompatible changes over the years. There's C89 code that won't compile under C99, a good amount of Perl4 code won't compile under Perl5, let alone 6, Fortran 66 code that won't compile under more modern versions of fortran, etc. This myth that this is a python thing is complete hogwash if you look at the history of programming languages.

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I think I meant to write Perl 5, completely forgot about the Perl 4 vs. Perl 5 excitement in the early 90s. I am not sure yet whether Python 2 to Python 3 is more like Perl 4 to Perl 5 or more like Perl 5 to Perl 6, but for Python's sake I hope the former.

But a C99 program can link against a C89 library, right?

Maybe, maybe not.

Could be that the header file is not compatible with C99 either. This seems to me rather likely. Also it might be that the ABI changed in a way that the C89. This depends on platform and tool chain.

There is a large amount of code which is compatible. It is easy though to get an example which isn't.

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