hello.asm.js

Module["asm"] = (function(global, env, buffer) {
 "use asm";
 var HEAP8 = new global.Int8Array(buffer);
 var STACKTOP = env.STACKTOP | 0;
 var _some_external_JS = env._some_external_JS;
 function _main() {
  var i1 = 0, i2 = 0;
  i1 = STACKTOP;
  STACKTOP = STACKTOP + 16 | 0;
  i2 = i1;
  HEAP8[i2 >> 0] = 1;
  _some_external_JS(i2 | 0);
  STACKTOP = i1;
  return 0;
 }
 return {
  _main: _main
 };
});

hello.c

#include <stdint.h>

typedef struct {
  uint8_t op;
  uint32_t operand1;
  uint32_t operand2;
} opcode_t;

extern void some_external_JS(opcode_t* opcode);

int main() {
  opcode_t op;
  op.op = 1;
  some_external_JS(&op);
}