Send an error message from a c function by using WSTP library

Question

Consider that Func is a function written in c language by using WSTP library. In some situations, it can return an error to Mathematica.

void Func()
{
   ...
   WSPutSymbol(stdlink, "$Failed");
}

In addition to show a $Failed message, I want to show an error message (a red colored message). How can I do this?

Answer 1

The tutorial I suggested to you has a complete example. See section 1.6. Please read through the entire first chapter.

---

Here's a copy of the code from there:

In your template file, ensure that both argument passing and return are set to Manual, then add the following:

:Evaluate: AddTwo::ovflw = "The sum cannot fit into a C long type."

A modification of the addtwo example:

void addtwo(void) {
   long i, j, sum;
   MLGetLongInteger(stdlink, &i);
   MLGetLongInteger(stdlink, &j);
   sum = i + j;
   if(i>0 && j>0 && sum<0 || i<0 && j<0 && sum>0) {
     MLEvaluate(stdlink, "Message[AddTwo::ovflw]");
     MLNextPacket(stdlink);
     MLNewPacket(stdlink);
     MLPutSymbol(stdlink, "$Failed");
   } else {
     MLPutLongInteger(stdlink, sum);
   }
}