C - "Dereferencing pointer to incomplete type" ， typedefs 还可以

Question

我想实现一个哈希表。我得到了上面的错误，但我敢说我已经正确地声明了我所有的结构体/类型。(抱歉，我还在学习C语言)错误发生在test.c的while循环中，我在这里放置了//error。怎么了?

test.c

#include "mentry.h"
#include "mlist.h"
int main() {
  FILE *fPtr = fopen("S.txt", "r");
  MList *ml = ml_create();
  MEntry *m2;
  int i = 1;
  while ((m2 = me_get(fPtr)) != NULL) {
    unsigned long int hash = me_hash(m2, ml->numbuckets); // ERROR HERE
    i++;
  }
  return 0;
}

mlist.c

#include "mentry.h"
#include "mlist.h"
#define NUMBUCKETS 3
#define BUCKETSIZE 5
typedef struct bucket {
  int size; // number of elements in this bucket
  MEntry *entries; // array of MEntries
} Bucket;
typedef struct mlist {
  int numbuckets; // number of buckets
  Bucket **buckets; // an array of Bucket pointers
} MList;
/* ml_create  - creates a new mailing list
              - returns pointer to start of mailing list*/
MList *ml_create(void) {
  MList *ml = malloc(sizeof(MList));
  ml->numbuckets = NUMBUCKETS; // random initial number
                               // will increment once a bucket is full
  // init Buckets
  int b;
  for (b=0; b<ml->numbuckets; b++) {
    Bucket *bk = malloc(sizeof(Bucket));
    bk->size = 0;
    bk->entries = calloc(BUCKETSIZE, sizeof(MEntry));
    ml->buckets[b] = bk;
  }
  return ml;
}

mlist.h

#ifndef _MLIST_H_
#define _MLIST_H_
#include "mentry.h"
typedef struct mlist MList;
/* ml_create  - creates a new mailing list
              - returns pointer to start of mailing list*/
MList *ml_create(void);
#endif /* _MLIST_H_ */

Answer 1

当您试图访问main中的ml->numbuckets时，它所知道的关于m1类型的信息如下:

typedef struct mlist MList;

它知道MList是一个结构类型，但它不知道里面有什么。

你需要把完整的定义放在mlist.h:

#ifndef _MLIST_H_
#define _MLIST_H_
#include "mentry.h"
typedef struct mlist {
  int numbuckets; // number of buckets
  Bucket **buckets; // an array of Bucket pointers
} MList;
/* ml_create  - creates a new mailing list
              - returns pointer to start of mailing list*/
MList *ml_create(void);
#endif /* _MLIST_H_ */

Answer 2

怎么了?

只有当MList的完整定义在该行可见时，才能使用ml->numbuckets。

由于MList的完整定义仅在mlist.c中可见，因此只能在该文件中访问struct的成员。

一种方法是在mlist.h中声明一个函数:

unsigned long int me_hash_list(MEntry* m1, MList* m2);

并在mlist.c中定义为:

unsigned long int me_hash_list(MEntry* m1, MList* m2)
{
   return me_hash(m1, m2->numbuckets);
}

，然后从main调用me_hash_list。

while ((m2 = me_get(fPtr)) != NULL) {
  unsigned long int hash = me_hash_list(m2, ml);
  i++;
}