why i the pointer is not a structure or union?

Question

#include <stdio.h>

struct m_tag {
    short m_tag_id;
    short m_tag_len;
    int   m_tag_cookie;
};
struct packet_tags {
    struct m_tag *slh_first;
}tags;

#define SFIRST(head) ((head).slh_first)
int main(void) {
    printf("%p\n", SFIRST(&tags));
    return 0;
}

In function 'main': error: request for member 'slh_first' in something not a structure or union

what is the problem with this code?

Because if you look at a sign that says "lake this way", the sign is not the lake? — StoryTeller - Unslander Monica, Mar 19 '19 at 06:20
Have you tried to access `slh_first` **without** using the macro? How would that look? — Support Ukraine, Mar 19 '19 at 06:22
Your options are: `stuctPointer->structMember` or `structObject.structMember` but not `stuctPointer.structMember` — Support Ukraine, Mar 19 '19 at 06:26
Btw, are you writing C or C++? You can only compile this small program with a single compiler at a time, after all. Please don't add tags just to attract views, that's an abuse of the system, and will be very much frowned upon by both the C and C++ community here. — StoryTeller - Unslander Monica, Mar 19 '19 at 06:29
Such macros can be useful in C, but should be avoided in C++, prefer inline (templated) functions instead. — Remy Lebeau, Mar 19 '19 at 06:38

Remy Lebeau · Answer 1 · 2019-03-19T06:31:40.487

You are trying to access the slh_first member using operator. which requires an instance of packet_tags on the left side. But you are not passing an instance of packet_tags to SFIRST(), you are passing a pointer to a packet_tags instance. So, in order to access the member, you need to dereference the pointer to reach the instance. Calling operator. on a pointer is what the compiler is complaining about.

Change SFIRST() to this instead:

#define SFIRST(head) ((head)->slh_first)

Or, less preferred but just as valid:

#define SFIRST(head) ((*(head)).slh_first)

score 1 · Answer 2 · edited Mar 19 '19 at 06:32

1

Change your code to below code:

#include <stdio.h>

struct m_tag {
    short m_tag_id;
    short m_tag_len;
    int   m_tag_cookie;
};
struct packet_tags {
    struct m_tag *slh_first;
}tags;

#define SFIRST(head) ((head)->slh_first)
int main(void) {
    printf("%p\n", SFIRST(&tags));
    return 0;
}

edited Mar 19 '19 at 06:32

Remy Lebeau

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answered Mar 19 '19 at 06:30

Pawan Mishra

19
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5

"Change your code to this" is not a useful SO answer. Why is the change required? What was the OP doing wrong? Look at the existing answer to see an example of how one should be answering questions here. Btw, it already suggested the fix you do here. – StoryTeller - Unslander Monica Mar 19 '19 at 06:32
Also this is an nth duplicate. – Antti Haapala -- Слава Україні Mar 19 '19 at 07:05

why i the pointer is not a structure or union?

2 Answers2