C struct变量分配？

Question

I'm new to C so please correct any mistakes I have.

Here's some code that's sort of like the code I have

//typdef stuff for apple, *apple_t here
apple_t get() {
    apple a;
    a.num = 0;

    apple_t ap = &a;
    printf("set num to %d\n", ap->num);
    return ap;
}

// ap above is placed into read(ap)
void read(apple_t ap) {
    printf("num: %d\n", ap->num);
}

Why is it that for the "set" print ap->num == 0, but when I do the printf in read function I get some junk number like -1218550893? What's going on? Is the integer set being freed? What is C doing? And how do you fix this?

Answer 1

You are returning the address of a local variable.

In this case the variable a is a local variable. It is lost after the function ends.

There are two options to fix this:

1. Return it by value. Don't return its address.
2. Allocate memory for it using malloc() . But you must be sure to free() it later.

Answer 2

You are returning a local variable, that is not available after the function has returned.

C supports returning structs, so no need for a pointers at all:

apple_t get() {
    apple_t a;
    a.num = 0;
    return a;
}

The following code will copy the result, not returning a local variable.