Allows to get the integer value associated to the provided member name, inside the provided struct.
|_struct ||The struct where the member value is being required.|
|member_name ||The member name to get its value.|
- The integer value requested. The function will always return -1 if some parameter provided is NULL. There is no way to check that a -1 returned is due to an error or due to be the value which is actually stored inside the struct at the provided member. Check that all parameters provided are not NULL to get a properly function.