CODEWITHSUNDEEP
c++cvisual-c++externdllexport
Stefano
Stefano

Reputation: 4031

VisualC++ referencing C functions from C++

I am trying to use a ONCRCP-Windows port from a C++ application but I have some problem referencing function within the library. In the library there is a C file which contains the following C function definition:

//xdr_stdi.c
….
void
xdrstdio_create(xdrs, file, op)
    register XDR *xdrs;
    FILE *file;
    enum xdr_op op;
{

    xdrs->x_op = op;
    xdrs->x_ops = &xdrstdio_ops;
    xdrs->x_private = (caddr_t)file;
    xdrs->x_handy = 0;
    xdrs->x_base = 0;
}

A declaration of the same function is present in a header file as follow:

// xdr.h
DllExport void   xdrstdio_create(); /* XDR using stdio library */

If I call this function from a main.c file this works correctly but If I call this from a main.cpp it complains saying:

“error C2660: 'xdrstdio_create': function does not take 3 arguments”

This actually makes sense to me and I am honestly surprised that in C this does not complain… I however tried to include the header file wrapping the #include in an extern C{ #include... } but this did not help. Strangely enough replacing the definition in the header by

DllExport void   xdrstdio_create(XDR *xdrs, FILE *file, enum xdr_op op);

Makes the library do not compile with error error 

C2143: syntax error: missing ')' before '*'

I did not spend time tracking this second issue down... i would rather understand why the first case does work fine in C but not in C++

Upvotes: 0

Views: 77

Answers (2)

Some programmer dude
Some programmer dude

Reputation: 409196

C allows you to not specify any arguments in a declaration (function prototype), and the compiler will parse it as the function takes an unknown number of unspecified arguments. In C++ a function declaration without arguments means that the function doesn't take any arguments, it's the same as using the argument void. You need to specify the arguments in the declaration.

Which leads to your error: It's impossible to answer why you get the errors when you add the arguments as the question doesn't show more context, but an educated guess is that you miss some header file. Do you include the header file where XDR is declared? Do you include <stdio.h> (for FILE)?

Upvotes: 3

Martin Bonner supports Monica
Martin Bonner supports Monica

Reputation: 28997

Oh dear. You have hit one of the subtle differences between C and C++.

In C++

void   xdrstdio_create();

declares a function taking zero arguments. In C, it declares a function but you haven't told the compiler how many arguments it takes! It is up to the programmer to pass the right number of arguments of the right type - and if you get it wrong, ... good luck!

This is an ancient style of function declaration which predates the first C standard (nearly 30 years ago now), and is only maintained for backwards compatibility.

In C++, you have to declare the function as:

DllImport void   xdrstdio_create(XDR *xdrs, FILE *file, enum xdr_op op);

... which means you need to have a definition of the typedefs XDR and FILE (the latter will be in <stdio.h> - don't know where you will find XDR.

A better solution might be to find a library which uses a C less than 25 years old (there are very unlikely to be any platforms which have a C++ compiler, but do not have at least a C89 compiler.)

Upvotes: 3

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