c - 解引用指针错误

Question

下面是 Lib.c 和 Lib.h 文件，它们应该是“电影院”代码的一部分，但我无法获得一些协同工作的指示。

我已经尝试了我认为的每一种组合，但无法弄清楚为什么指针不想合作。

StackLib.h

#include <stdio.h>
#ifndef _STACKLIB_H
#define _STACKLIB_H

struct Node {
  char SeatRow;
  int SeatNumber;
  struct node *next_ptr;
}*StackTop;

void PushSoldSeat(struct Node **StackTop, char RowToAdd, int SeatToAdd);
void PopTickets(struct Node *StackTop);

#endif

.

#include "StackLib.h"
#include <stdio.h>
#include <stdlib.h>

void PushSoldSeat(struct Node **StackTop, char RowToAdd, int SeatToAdd) {
  struct Node *NewNode = malloc(sizeof(struct Node));
  NewNode->SeatRow = RowToAdd;
  NewNode->SeatNumber = SeatToAdd;

  if (*StackTop == NULL) {
    *StackTop = NewNode;
    NewNode->next_ptr = NULL;
  } else {
    NewNode->next_ptr = *StackTop; //line 20
    *StackTop = NewNode;
  }
}

void PopTickets(struct Node *StackTop) {
  struct Node *TempPtr;
  if (StackTop == NULL) {
    printf("Pop not executed - stack is empty\n\n");
    return;
  }
  while (StackTop != NULL) {
    StackTop = TempPtr;
    printf("\n______________\n");
    printf("\n|             |\n");
    printf("\n|   Seat %c%d   |\n", StackTop->SeatRow, StackTop->SeatNumber);
    printf("\n|             |\n");
    printf("\n______________\n");
    TempPtr = StackTop->next_ptr; //line 41
    free(StackTop);
  }
  return;
}

当我 gcc 时，终端打印出:

StackLib.c: In function ‘PushSoldSeat’:
StackLib.c:20: error: dereferencing pointer to incomplete type
StackLib.c: In function ‘PopTickets’:
StackLib.c:41: error: incompatible types in assignment

Answer 1

代码存在一些问题，首先是错误

struct node *next_ptr;

正如@chux所指出的，它应该是struct Node *next_ptr;。

其次，在PopTickets()中打印struct Node信息时，这个

while (StackTop != NULL) { /* some code */ }

应该是

struct Node *TempPtr = StackTop;
while (TempPtr != NULL) { /* some code */ }

因为早期的代码会创建段错误。假设只有一个节点，那么这个

TempPtr = StackTop->next_ptr;

使TempPtr为NULL，但StackTop保持不变，并再次循环迭代(不应该)，并且这个StackTop = TempPtr; 使 StackTop 为 NULL 并且当它访问 StackTop->SeatRow 时，即 NULL->SeatRow > 它崩溃了。

这里是示例代码:

trevor.c

#include "StackLib.h"
#include <stdio.h>
#include <stdlib.h>
void PushSoldSeat(struct Node **StackTop, char RowToAdd, int SeatToAdd) {
        struct Node *NewNode = malloc(sizeof(struct Node));
        NewNode->SeatRow = RowToAdd;
        NewNode->SeatNumber = SeatToAdd;

        if (*StackTop == NULL) {
                *StackTop = NewNode;
                NewNode->next_ptr = NULL;
        }
        else {
                NewNode->next_ptr = *StackTop; //line 20
                *StackTop = NewNode;
        }
}

void PopTickets(struct Node *StackTop) {
        struct Node *TempPtr = StackTop;
        if(StackTop == NULL) {
                printf("Pop not executed - stack is empty\n\n");
                return;
        }
        while (TempPtr != NULL) {
                StackTop = TempPtr;
                printf("\n______________\n");
                printf("\n|             |\n");
                printf("\n|   Seat %c%d   |\n", StackTop->SeatRow, StackTop->SeatNumber);
                printf("\n|             |\n");
                printf("\n______________\n");
                TempPtr = StackTop->next_ptr; //line 41
                free(StackTop);
        }
}
int main(void) {
        struct Node *head = NULL;
        PushSoldSeat(&head,'a',5);
        PushSoldSeat(&head,'b',10);
        PushSoldSeat(&head,'c',15);
        PushSoldSeat(&head,'d',20);
        PopTickets(head);
        return 0;
}

StackLib.h

#include <stdio.h>
#include <stdlib.h>
#ifndef _STACKLIB_H
#define _STACKLIB_H

struct Node {
  char SeatRow;
  int SeatNumber;
  struct Node *next_ptr; /* avoid spell mistake */
}StackTop; /* *stackTop is not required */

void PushSoldSeat(struct Node **StackTop, char RowToAdd, int SeatToAdd);
void PopTickets(struct Node *StackTop);

#endif

现在编译您的代码如下

gcc -Wall -Wstrict-prototypes -Werror trevor.c