Why conditionally compile an obsolete K&R definition versus standard C definition?

Question

I came across this code in the wild and got very confused.

#ifdef __STDC__
    double cos(double x)
#else
    double cos(x)
    double x;
#endif
{
    // ...
}

I fail to see why the latter in the #else would be preferable in any circumstances. Are there any reasons to write function definitions as above as opposed to the very standard and widely accepted way as shown below?

double cos(double x) {
    // ... 
}

@DanielA.White How old? And in that case why not just use the old style? — user16217248, Apr 13 '23 at 23:06
This used to be very common in the 1990s. You got better error checking with the shiny new compilers that supported C89, but the program could still be built with older compilers that didn't. — zwol, Apr 13 '23 at 23:20
@zwol That seems to make sense. I found this code in the linked file in [this](https://stackoverflow.com/q/67124504/16217248) question and at the top there is `Copyright (C) 1993`. — user16217248, Apr 13 '23 at 23:33

score 1 · Accepted Answer · answered Apr 13 '23 at 23:44

1

It may be necessary to use the latter form when compiling code with an ancient compiler or when compiling with a modern compiler but it is necessary to use an ancient version of the C language because some of the code being compiled uses old C features that are incompatible with modern C. Either of these is exceptionally rare these days.

answered Apr 13 '23 at 23:44

Eric Postpischil

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Basically making code portable to within an inch of its life. – user16217248 Apr 14 '23 at 00:19

Why conditionally compile an obsolete K&R definition versus standard C definition?

1 Answers1