Accessing Nested Structure

Question

I have a nested structure of this type -

typedef struct {
    head     HEAD;  
    tail     TAIL;
    start    START;
    end      END;
}NODE;

I have a pointer to this nested structure -

 NODE* settings = malloc(sizeof(NODE));

and I pass this pointer to nested struct to a function. In the function, I need to access the individual structs within the main struct, to add the bytes within the struct. This is the function prototype -

int checksumVerify( NODE* settings)
{
    unsigned int i;
    unsigned char checksum = 0;

    //I need to access the size of individual structs from the pointer to main struct and then add all the bytes in the inner struct together.
    for (i = 0; i < sizeof(*settings)->HEAD; i++)
    {
        checksum += (char)(settings)->HEAD;
        settings++;
    }
    //Like wise I need to do it for the second struct as well
    for (i = 0; i < sizeof(*settings)->TAIL; i++)
    {
        checksum += (char)(settings)->TAIL);
        settings++;
    }
    return ((int)((checksum == 0) ? 1 : 0));
}

I do not know know the syntax to access the size of individual structs and accessing each entry in each of the individual struct is wrong here. What is the correct syntax for both?

I know it does not compile. I want to know what is the correct syntax — SMG, Oct 08 '18 at 19:29
@JimRhodes It's definitely homework. And the variable names are awful. — nicomp, Oct 08 '18 at 19:35
Swordfish's answer gives you the syntax for `sizeof`. You have multiple bugs inside your `for` loops. You will need a `char*` pointer variable and `settings++` is very bad. You should also read up on structure packing. — Jim Rhodes, Oct 08 '18 at 19:44

Swordfish · Accepted Answer · 2018-10-08T20:33:53.133

0

I know the syntax to access the size of individual structs and accessing each entry in each of the individual struct is wrong here. What is the correct syntax for both?

sizeof(*settings)->TAIL --> sizeof(settings->TAIL)

NODE* settings;
settings->HEAD.some_member_of_head;
settings->TAIL.some_member_of_tail;

Can I just loop with the size of individual struct and not access individual members within the inner struct?

char *p = (char*)&settings->HEAD;
for (size_t i = 0; i < sizeof(settings->HEAD); ++i)
    checksum += p[i];

But as @JimRhodes said: You should inform yourself about padding and packing.

edited Oct 08 '18 at 20:33

answered Oct 08 '18 at 19:31

Swordfish

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Can I just loop with the size of individual struct and not access individual members within the inner struct? – SMG Oct 08 '18 at 19:46
@SMG I put an example in my answer. – Swordfish Oct 08 '18 at 19:54
You are missing an '&'. `char* p = (char*) &settings->HEAD`. – Jim Rhodes Oct 08 '18 at 20:33
@JimRhodes I'll go buy a `&` and stuff it in there somewhere. – Swordfish Oct 08 '18 at 20:34

Accessing Nested Structure

1 Answers1