C Primer Plus（第六版）中文版 中的錯誤1

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define TSIZE 45

struct film {
    char title[TSIZE];
    int rating;
    struct film *next;
};
char * s_gets(char * st, int n);

int main(void)
{
    struct film * head = NULL;
    struct film * prev;
    struct film * current;
    char input[TSIZE];

    puts("Enter first movie title:");
    while(s_gets(input, TSIZE) != NULL && input[0] != '\0')
    {
        current = (struct film *) malloc(sizeof(struct film));
        if(head == NULL)
            head = current;
        else
            prev->next = current;
        current->next = NULL;
        strcpy(current->title, input);
        puts("Enter your rating <0-10>:");
        scanf("%d", &current->rating);
        while(getchar() != '\n')
            continue;
        puts("Enter next movie title (empty line to stop):");
        prev = current;
    }

    if(head == NULL)
        printf("No data entered. ");
    else
        printf("Here is the movie list:\n");
    current = head;
    while(current != NULL)
    {
        printf("Movie: %s Rating: %d\n",
               current->title, current->rating);
        current = current->next;
    }

    current = head;
    while(current != NULL)
    {
        current = head;
        head = current->next;
        free(current);
    }
    printf("Bye!\n");

    return 0;
}

char * s_gets(char * st, int n)
{
    char * ret_val;
    char * find;

    ret_val = fgets(st, n, stdin);
    if(ret_val)
    {
        find = strchr(st, '\n');
        if(find)
            *find = '\0';
        else
            while(getchar() != '\n')
                continue;
    }

    return ret_val;
}

上面的代碼來自《C Primer Plus》（第六版）中文版，第573-574頁。

其中代碼的第51行有錯誤。

不知道這個錯誤是印刷有錯，還是原書有錯。在此指出。

 

改正代碼：

是將第51行的while（current != NULL）改為

while (head != NULL)就可以了。

 

原因如下：

將第51-56行代碼之間添加一些輸出，代碼如下：

    while(current != NULL)
    {
        printf("current1 = %p\n", current);
        printf("head1 = %p\n\n",head);
        current = head;
        printf("current2 = %p\n", current);
        printf("head2 = %p\n\n",head);
        head = current->next;
        printf("current3 = %p\n", current);
        printf("head3 = %p\n\n",head);
        free(current);
        printf("current4 = %p\n", current);
        printf("head4 = %p\n\n",head);
    }

輸出結果如下：

 

從上面的輸出結果可以看出，最后一個head2指針已經是00000000了，是一個指向未知地址的指針。所以再次釋放一個current就會出錯。

 改正后的輸出：