Using macros in C to define data structures

Question

I am trying to mask the concept of using macros to define data structure operations. The following code is a simple example of using the built-in list library in FreeBSD. In the library, all operations are defined as macros. I have seen this approach in several other libraries.

I see that this has some advantages, for example. the ability to use any data structure as an item in a list. But I do not quite understand how this works. For instance:

• What is stailhead? It seems "fair."
• How to transfer headand entriesin function?
• What type head, how can I declare a pointer to it?

Is there a standard name for this technique that I can use to search on Google or any book that explains this concept? Any links or a good explanation of how this technique works will be greatly appreciated.

Thanks to Niklas B. I ran gcc -Eand got this definition forhead

struct stailhead {
  struct stailq_entry *stqh_first;
  struct stailq_entry **stqh_last; 
} head = { ((void *)0), &(head).stqh_first };

and this is for stailq_entry

struct stailq_entry {
 int value;
 struct { struct stailq_entry *stqe_next; } entries;
};

So, I think it headhas a type struct stailhead.

#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>

struct stailq_entry {
        int value;
        STAILQ_ENTRY(stailq_entry) entries;
};

int main(void)
{
        STAILQ_HEAD(stailhead, stailq_entry) head = STAILQ_HEAD_INITIALIZER(head);
        struct stailq_entry *n1;
        unsigned i;
        STAILQ_INIT(&head);                     /* Initialize the queue. */

        for (i=0;i<10;i++){
                n1 = malloc(sizeof(struct stailq_entry));   /* Insert at the head. */
                n1->value = i;
                STAILQ_INSERT_HEAD(&head, n1, entries);
        }
        n1 = NULL;

        while (!STAILQ_EMPTY(&head)) {
                n1 = STAILQ_LAST(&head, stailq_entry, entries);
                STAILQ_REMOVE(&head, n1, stailq_entry, entries);
                printf ("n2: %d\n", n1->value);
                free(n1);
        }

        return (0);
}

Answer 1

Read this first to get what these macros do. Then go to queue.h. You will receive your treasury!

I found some gold coins for you -

#define STAILQ_HEAD(name, type)                                         \
struct name {                                                           \
        struct type *stqh_first;/* first element */                     \
        struct type **stqh_last;/* addr of last next element */         \
}

Allows you to go a little deeper and answer your questions.

What is a stailhead? It is like "just."

#define STAILQ_HEAD(name, type)                                         \
struct name {                                                           \
        struct type *stqh_first;/* first element */                     \
        struct type **stqh_last;/* addr of last next element */         \
}
 STAILQ_HEAD(stailhead, entry) head =
 STAILQ_HEAD_INITIALIZER(head);
 struct stailhead *headp;            /* Singly-linked tail queue head. */

So stailheadis a structure

?

#define STAILQ_ENTRY(type)                                              \
struct {                                                                \
        struct type *stqe_next; /* next element */                      \
}

So entries head ( ) , , . &structure_variable

, ?

!

.