Is it safe to return a pointer to a compound literal declared within a function or should you use malloc?

Question

Is it safe to return the pointer to a compound literal from a function?

I know the local variables of a function get deallocated after the function returns and that malloc'd memory is an exception to that. However, the compiler isn't giving any warning when I try to return the pointer to a compound literal. Does this mean it is safe to do so?

I compiled using gcc -Wall -o test test.c

The following code compiled without any warnings and outputs 20

#include <stdio.h>

struct test {
    int item;
    struct test *next;
} struct_1 = { .item = 10, .next=NULL };

struct test * returnStruct(){
    struct test * structPointer = (& ((struct test) { .item = 20, .next = NULL }));
    return(structPointer);
}

int main(){
    struct_1.next = returnStruct();
    printf("%d", (struct_1.next)->item);
    return(0);
}

but this code causes a warning

#include <stdio.h>

int * testr(){
    int i = 568;
    return(&i);
}

int main(){
    printf("%d", *testr());
    return(0);
}

warning: address of stack memory associated with local variable 'i' returned [-Wreturn-stack-address]
        return(&i);
                ^
1 warning generated.

Also, if compound literals don't get deallocated after the function returns, does that mean I need to call free on all compound literals in a function?

Answer 1

Is it safe to return the pointer to a compound literal from a function?

No. Compound literals have the same storage duration as the block they were declared in. So in case they were declared inside a function, they have automatic storage duration and you shouldn't return them for the same reason as why you shouldn't return a pointer to any other local variable.

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The following code compiled without any warnings

gcc has broken diagnostics, they cannot be trusted. With -Wall -O3 I get this warning:

warning: function returns address of local variable [-Wreturn-local-addr]
return(structPointer);

Without -O3 I do not. Yet another gcc bug. Seems it has been around forever too.

---

As for how to solve it, perhaps in this case simply return the struct by value. This is usually not a good idea because structs are often large, but it works just like memcpy.

Answer 2

Is it safe to return a pointer to a compound literal declared within a function…

No. The lifetime of a compound literal inside a function is limited to execution of its enclosing block, per C 2018 6.5.2.5 5:

… If the compound literal occurs outside the body of a function, the object has static storage duration; otherwise, it has automatic storage duration associated with the enclosing block.

… so should you use malloc?

Yes, or some other form of dynamic allocation or provision for the object’s lifetime to persist beyond the end of function execution.