Question

I have this C code :

#include<stdio.h>

typedef struct {
    int foo;
} MyStruct;

MyStruct init_mystruct(void);

int main(void) {

    MyStruct mystruct   = init_mystruct();

    if( mystruct == NULL ) {
        /* error handler */
    }

    return(0);
}

MyStruct init_mystruct(void) {

    MyStruct mystruct;
    int is_ok   = 1;

    /*
     * do something ...
     */

    /* everything is OK */
    if( is_ok )
        return mystruct;
    /* something went wrong */
    else
        return NULL;
}

It has a structure and a function to initialize that structure. What I’m trying to do is to return NULL if there was a failure in that function.

The gcc error message :

code.c: In function ‘main’:
code.c:13: error: invalid operands to binary == (have ‘MyStruct’ and ‘void *’)
code.c: In function ‘init_mystruct’:
code.c:34: error: incompatible types when returning type ‘void *’ but ‘MyStruct’ was expected

It looks that returning NULL instead of a structure is not valid, so how do I express the failure of structures initialization in this case (no structure pointer)?

Answer 1

 if( mystruct == NULL )

mystruct is not a pointer, so you cannot compare it with NULL.

You have three options:

1. Add a status field to MyStruct to indicate whether the struct has been initialized correctly.
2. Allocate the struct on the heap and return it by pointer.
3. Pass the structure as a pointer argument and return a status code (thanks @Potatoswatter).