The relevant C code can be found in the willUseGo.c source file, which will be presented in two parts. The first part of willUseGo.c is as follows:

#include <stdio.h>
#include "usedByC.h"

int main(int argc, char **argv) {
   GoInt x = 12;
   GoInt y = 23;

   printf("About to call a Go function!
");
   PrintMessage();

If you already know C, you should understand why you need to include usedByC.h. This is the way that the C code knows about the available functions of a library.

The second part of the C program follows next:

   GoInt p = Multiply(x,y);
   printf("Product: %d
",(int)p);
   printf("It worked!
");
   return 0;
}

The GoInt p variable is needed for getting an integer value from a Go function, which is converted to a C integer using the (int) p notation.

Compiling and executing willUseGo.c on a macOS High Sierra machine will create the following output:

$ gcc -o willUseGo willUseGo.c ./usedByC.o
$ ./willUseGo
About to call a Go function!
A Go function!
Product: 276
It worked!