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Calling JS from C using EM_ASM in Emscripten

December 24, 2014, 2:03 pm

A very simple for calling JS from C and returning a value. Create the following C program, in the file calljs.c, it makes a single call to js_receive which will be our Javascript function:

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#include <stdio.h>
#include <emscripten.h>
 
int main() {
  int ret = EM_ASM_INT({
    return js_receive($0,$1,$2,$3);
  }, 1,"two",3,4);
 
  printf("returned: %d\n",ret);
}
 
compile it with: <pre>emcc calljs.c -o calljs.js</pre>
 
It will then call the JS code which can access the variables and string. Here's an example complete html file:
 
[sourcecode language="html"]
 <!DOCTYPE html>
<html>
<body>
 
<script>
 
function js_receive(a,b,c,d) {
 
  var bstr = Pointer_stringify(b);
  document.write(a + " " + bstr + " " + c + " " + d);
 
  return 42;
}
 
</script>
 
<script src="calljs.js"></script>
 
</body>
</html>
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