Thread: structs of structs

Compilation is not dependent on correct memory allocation - that's why we see run time memory errors.

Borland Windows compiler doesn't like a cast'less malloc:

Code:

my_parent = malloc(sizeof(parent));

fails with:

Code:

Error E2034 struct.c 12: Cannot convert 'void *' to 'parent *' in function
main()

stdlib is included.

doesn't like that either:

Code:

Error E2108 struct.c 12: Improper use of typedef 'parent' in function main()
Error E2109 struct.c 12: Not an allowed type in function main()
Error E2034 struct.c 12: Cannot convert 'void *' to 'parent *' in function
main()

http://www.cs.toronto.edu/~heap/270F02/node31.html

Should be the following according to the above link:

Code:

my_parent = (struct parent *)malloc(sizeof(struct parent));

Originally Posted by sand_man

I can see that it needs space allocated but why does it compile fine for me as it is?
Strange isn't it?

Some compilers do actually give warnings about this sort of thing (eg a variable used without being initialised), particularly if you turn on maximum warning levels. However, there is no requirement for a compiler to do that.

It's not that strange that the code compiles without a hiccup, and bombs when run.

The C standard states (with a few exceptions, such as static variables) that the value of anything which has not been explictly initialised is implementation dependent. That means that a compiler is allowed to put any value into a variable that you don't initialise, and also means the compiler is not required to complain in the process. Then couple that with the fact that dereferencing ANY pointer yields undefined behaviour unless that pointer points at a valid object: the reason that is undefined behaviour is that, in some cases, it is technically impossible for a compiler to detect that an invalid pointer is being dereferenced, let alone do anything about it.

Fair enough Salem but I've got a load of win32 code in my app :/

Can I just get something really clear with reference to my previous lump of code:

Code:

struct base
{
        char *string;
};

struct parent
{
        struct base my_base;
};

int main()
{
        struct parent *my_parent;
        char *temp_string;

        temp_string = (char*)malloc(21 * sizeof(char));
        //strcpy(temp_string,"temp string for test");

        my_parent->my_base.string = temp_string; //is this ok
        strcpy(temp_string,"temp string for test");

        printf("String = %s\n",my_parent->my_base.string);

        return 0;
}

Is there any problem pointing two char pointers at the same memory like the code above is doing (as long a dereferencing is handled correctly)?

Thanks again with your responses on this.

Yes it is legal to have 2 pointers point to the same memory location. And it's no problem as long as you are not trying to call free twice on the same memory location.
Btw you still haven't allocated space for my_parent.
Your code should be:

Code:

struct base
{
        char *string;
};

struct parent
{
        struct base my_base;
};

int main()
{
        struct parent *my_parent;
        char *temp_string;

        temp_string = (char*)malloc(21 * sizeof(char));
        my_parent = (struct parent *)malloc(sizeof(struct parent));

        my_parent->my_base.string = temp_string; //is this ok
        strcpy(temp_string,"temp string for test");

        printf("String = %s\n",my_parent->my_base.string);

        free(my_parent->my_base.string); // don't free temp_string it's just an alias for  my_parent->my_base.string       
        free(my_parent);        
        
        return 0;
}

Kurt

Thanks Kurt.

alright :/