Comparison of memory models of C and Python

mem_models.md

C & Python Memory Models

Terms

• Binding : The operation/concept of associating a memory location to a variable name
• Indirection : The process of resolving a pointer. The modifications on an indirected pointer will be reflected in the memory location the pointer is pointing to. The pointer itself is not modified.
• Actual : Variable in the caller function with original value
  • Variable a in both programs is the actual
• Formal : Variable in the callee function to which the value is copied
  • Variable b inside functions of both programs is the formal

C Memory Model

• Variable declaration is one-time BINDING to memory location
  • This include argument variables (formals) in functions
• Variable assignment MODIFIES bound memory location
• Function call COPIES value on actual variable to formal variable
• Address operator (&) gets address of referred variable
• "Call by value" and "Call by reference" are merely interpretations of above semantics
  • In call by value, actual gets copied to formal. Actual remains unchanged even when formal is modified.
  • In call by reference, address of actual is copied to formal. Formal is indirected and modified, resulting in modification of memory actual is bound to.

void modify1(int b) {
    b = 6;
}

void modify2(int* b) {
    *b = 6;
}

int main() {
    int a;
    a = 5;

    modify1(a);
    printf("Modify1: %d", a);

    modify2(&a);
    printf("Modify2: %d", a);

    return 0;
}

Python Memory Model

• No variable declaration
• Variable assignment RE-BINDS variable to the new object location
• Function call BINDS the formal variable to the same memory location as actual variable
• Methods modify object in memory, not the variable which is bound to the memory
• Memory objects with no variable bindings get garbage collected eventually
• There is no clear meaning for "call by value" or "call by reference"
  • Assignments to formals will result in rebinding of the formals to new objects in memory. This leaves actual objects unaltered.
  • Method calls to formals will result in modification of actuals also, since both are bound to same memory object.

def modify1(b : []):
    b = [5, 6, 7]

def modify2(b : []):
    b.append(4)

a = [1, 2, 3]

modify1(a)
print("Modify1: ", a)

modify2(a)
print("Modify2: ", a)