CODEWITHSUNDEEP
c
user786
user786

Reputation: 4364

Returning a local variable from a function and using it in the calling function

Is the following program correct? I return the local variable x from function fun and use it in the calling function. Can anyone explain if there is anything wrong with returning a local variable even if it contains the valid address in main?

#include <stdio.h>

int fun(int j) {
    int x = j;

    return x;
}

int main() {
    printf("%d\n", fun(1));

    // OR
    int k = 9;
    printf("%d\n", fun(k));

    // OR
    printf("%d\n", fun(&k));
    
    return 0;
}

If my code is wrong then why is it?

Upvotes: 2

Views: 71

Answers (1)

chqrlie
chqrlie

Reputation: 144800

It is OK in C for a function to return the value of a local object. The compiler generates code to copy this value to the object into which the function return value is stored or used in an expression. The local object can be a scalar or a structure alike.

Conversely, returning a pointer to a local object is not OK because the object pointed to will have gone out of scope by the time the pointer is dereferenced by the caller.

#include <stdio.h>

int good(int j) {
    int x = j;
    return x;     // OK
}

int *bad(int j) {
    int x = j;
    return &x;    // NOT OK
}

int main() {
    printf("%d\n", good(1));  // prints 1

    int k = 9;
    printf("%d\n", good(k));  // prints 9

    printf("%d\n", *bad(1));  // undefined behavior
    
    return 0;
}

The posted code in incorrect here: printf("%d\n", fun(&k)); because fun expects an int argument, not a pointer to int.

Upvotes: 3

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