C API / const char *text / std.string.toStringz pointer is always NULL on C side

Steven Schveighoffer schveiguy at yahoo.com
Wed May 16 15:12:26 UTC 2018

On 5/16/18 11:07 AM, Robert M. Münch wrote:
> I have an extern(C) function in a DLL with this signature:
> result* myfunc(double x, double y, const char *text, stuff *myStuff, 
> bool flag);
> I call it like this:
> result = myfunc(0, 0, std.string.toStringz("1"), stuff, true);

toStringz isn't necessary, string literals always are null terminated, 
and implicitly cast to const char *.

> The problem is, that on the DLL side *text is always NULL. I doesn't 
> matter what I put as third argument. And even more strange, I have an 
> other function with a const char* as first argument. There everything is 
> working...
> Any idea what cause this could be? This is really strange...

It sounds very much like a mismatch of parameters on the prototype. 
Anything extern(C) has no mangling, and so you can specify the wrong 
parameters and it still compiles.

Double check the parameter types, and if possible, reduce to a simple 
compilable example that can be tested by others.


More information about the Digitalmars-d-learn mailing list