Answer.

Background

The return operator appeared in C to explicitly terminate a function's execution and pass a result back to the calling code. In early programming languages, the ability to return values was not always present, and the return mechanism allowed for explicitly specifying the result of computations. This increased the expressiveness and safety of programs.

Issue

The main task is to correctly terminate the function and, if necessary, return a value corresponding to a defined type. Errors often occur due to returning a value of the wrong type, pointers to non-existent or local variables, or ignoring the returned value on the calling side.

Solution

• return; is used only for functions of type void (which return nothing).
• return expression; is used for functions with a non-void type and terminates the function with the specified return value.
• The return type must exactly match the declared function prototype.
• When returning structures, a copy of the structure is returned. When returning a pointer, only a copy of the address is made.
• It is dangerous to return pointers to local variables (they are destroyed when exiting the function).

Example code:

#include <stdio.h>
struct Point { int x, y; };

struct Point make_point(int x, int y) { // returning a structure (copy)
    struct Point p = {x, y};
    return p;
}

int* dangerous() {
    int num = 42;
    return &num; // dangerous: returning address of a local variable!
}

void do_nothing() {
    return; // correct for void functions
}

int main() {
    struct Point p = make_point(3, 4);
    printf("%d %d\n", p.x, p.y);
    int* ptr = dangerous(); // UB: ptr points to a destroyed area
}

Key features:

• return immediately terminates the function's execution
• the return type must match the declared type
• when returning structures/objects, a copy is made, not a reference is returned

Trick Questions.

Can return be used in functions without a return value (void)?

Answer: Yes, writing "return;" is allowed for void functions, but an expression (return x;) cannot be specified for a void function.

What happens when returning an array from a function?

Answer: In C, you cannot return an array directly. You can only return a pointer (for example, to a static array), but it is more common to return a pointer and size or use a dynamically allocated array.

int* make_arr() {
    static int arr[5] = {1,2,3,4,5};
    return arr; // static array lives after function exit
}

Why is it dangerous to return a pointer to a local variable?

Answer: Once the function exits, the memory for the local variable is freed (stack area). Using the returned pointer leads to undefined behavior.

Common Mistakes and Anti-Patterns

• Returning a pointer to a variable located on the stack
• Mismatching the types of the returned expression and the return type of the function
• Skipping the return path in functions that are declared to return a value

Real-Life Example

Negative Case

A function returns a pointer to a local variable, causing the caller to receive "garbage", unpredictable behavior, and rare floating bugs.

Pros:

• Fast implementation

Cons:

• Random crashes, data corruption if the stack changes after the function exits

Positive Case

Using a returned structure (copied by value) or returning a pointer to static/dynamic memory:

Pros:

• Predictable behavior
• No "dangling" pointers

Cons:

• Sometimes costly (copying large structures), or you need to remember to free memory explicitly