Nested calls can lead to uninitialized arguments in C?

Question

Is len correctly initialized and safe in set_array call?

void object_copy (Object* self, Object* obj) {
    int len;
    object_set_array (self, object_get_array (obj, &len), len);
}

If not, what would you recommend?

Answer 1

You are passing len by value into the object_set_array() call before it is initialized. So that is wrong and a good compiler will warn about that. The variable will contain a random value, so the code will not crash, but it won't do what you probably want.

Answer 2

No. The third parameter of object_set_array() might be evaluated and pushed onto the stack before the second parameter (object_get_array()) is evaluated.

Probably you want

void object_copy (Object* self, Object* obj) {
    int len;
    /* I'm just guessing as to the return type */
    Object *pObj = object_get_array(obj, &len); 
    object_set_array (self, pObj, len);
}

Answer 3

No - there's no defined order of evaluation for arguments to a function call.

Even though the call to object_get_array() has to occur before the call to object_set_array(), the value that the compiler calculates and passes to object_set_array() for the len parameter can be done before the compiler makes the call to object_get_array().

I'm not sure what the 2nd parameter to the object_set_array() call is intended to be - maybe you wanted to use the comma operator:

void object_foo (Object* self, Object* obj) {
    int len;

    object_set_array (self, (object_get_array (obj, &len), len));
}

Which would work, but is confusing and something I wouldn't recommend.

I think what you want is:

void object_foo (Object* self, Object* obj) {
    int len;

    object_get_array (obj, &len);  // get the length from obj

    object_set_array (self, len)); // set the len for self
}

Answer 4

This will also work:

ARRAY object_get_array (Object* self, int* len);
void object_set_array (Object* self, ARRAY array, const int* len);

// ...

void object_copy (Object* self, Object* obj) {
    int len;
    object_set_array (self, object_get_array (obj, &len), &len);
}

because the call to object_get_array() will always be evaluated before the call to object_set_array(), at which point len will have been set to the correct value.