I have a copy of the MD5 algorithm and I'm taking a look at the source. It's pretty interesting but there are a few things that I'm curious about and I was wondering if anyone a bit more intuitive than I could help me out.

The function declarations in the MD5 files are a bit unfamiliar to me. There is a macro used called PROTO_LIST, which I'm still not sure as to what this thing is doing exactly, but it's littered everywhere throughout the source. The macros is defined as follows:

(NOTE: PROTOTYPES macro is apparently dependent upon the target compiler supporting function argument prototyping according to the header)

For example, md5.h prototypes three functions that utilise this macro in that particular statement. For example, one prototype looks like this:

The signature here isn't too unfamiliar to me with the exception of the position of the PROTO_LIST macro. So here is a function with an unnamed argument of type MD5_CTX*. As expected, at the function's definition in the accompanying source file, the argument is named but this is where things get really confusing for me as I've never seen this type of definition before:

To me, this resembles an initializer list found in C++ with constructors but I certainly don't think that is the case here. So my questions about this are (1) how is this legal code in C and (2) what functionality has the PROTO_LIST macro provided for the function?

This is an old style C function, where the argument types could (optionally) be specified just before the body of the function. This is now obsolete (having been replaced by prototypes where the arguments are defined inside the brackets), however you will find it in legacy code.

The problem with old style C functions is that it is hard to declare an interface, since newer compilers of C will require a prototype and older ones won't understand it. The PROTO_LIST macro is merely a way of optionally providing the argument list for the newer compilers.

So everywhere you see PROTO_LIST, just ignore it; the contents of the brackets are the argument list of the function.

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Okay so if I understand what you said correctly then the source is implementing a legacy method of prototyping that is now deprecated? So this method of prototyping is also subjective to the target compiler as well?

Thanks for the clarification on that matter. I'd honestly never seen function signatures written that way before. That macro definition really threw me off there.

Yes. Although not strictly a legacy 'prototype'; there was no method at all for prototyping in the earlier versions of C (since the declarator had no arguments). The declaration of the argument types in the function definition were only for compiling the body of the function.

The C99 standard uses the term 'an obsolescent feature' rather than the C++ term 'deprecated', though the meaning is similar; the feature might be taken out of future revisions of the standard.

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