空闲内存用于 C 中的结构但失败（代码来自 C Primer Plus 第 6 版）

Question

我运行来自清单 17.2，P781，C Primer Plus 第 6 版的代码。

/* films2.c -- using a linked list of structures */    
#include <stdio.h>    
#include <stdlib.h>   /* has the malloc prototype */    
#include <string.h>   /* has the strcpy prototype */    
#define TSIZE 45      /* size of array to hold title */                                                                                                                  

struct film {    
    char title[TSIZE];    
    int rating;    
    struct film *next;  /* points to next struct in list */    
};    

char *s_gets(char *st, int n);    


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

    /* Gather and store information */    
    puts("Enter first movie title:");    
    while (s_gets(input, TSIZE) != NULL && input[0] != '[=10=]') {    
        current = (struct film *) malloc(sizeof(struct film));    
        if (head == NULL)   /* first structure */    
            head = current;    
        else   /* subseqent structures */    
            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;    
    } 

    /* show list of movies */    
    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;
    }

    /* Program done, so free allocated memory */
    current = head;
    while (current != NULL) {
        free(current);
        printf("Movie: %s, Rating: %d\n",                                                                                                                                
                current->title, current->rating);
        current = current->next;
    }
    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');  // look for newline
        if (find)    // if the address is not NULL,
            *find = '[=10=]';    // place a null character there
        else
            while (getchar() != '\n')
                continue;      // dispose of rest of line
    }
    return ret_val;
}

我的问题是free(current)之后，current->title和current->rating还在，current->next不是NULL。我不知道为什么。

Answer 1

至少这个循环是不正确的

current = head;
while (current != NULL) {
    free(current);
    printf("Movie: %s, Rating: %d\n",                                                                                                                                
            current->title, current->rating);
    current = current->next;
}

应该有

current = head;
while (current != NULL) {
    printf("Movie: %s, Rating: %d\n",                                                                                                                                
            current->title, current->rating);
    struct film *tmp = current;
    current = current->next;
    free(tmp);
}

否则您正在尝试使用无效的已删除指针。

Answer 2

变量仍然可用，但分配的 space 不是，因此您将有未定义的行为。它现在可以工作，但稍后可能会崩溃。一个好的做法是在释放指针后将指针设置为 NULL。

Answer 3

其他答案中提到的代码不正确。

在随后的一页中，作者确实展示了一个正确的链表释放函数。他说

The code saves the address of the next node because the call to free(), in principle, may make the contents of the current node (the one pointed to by *plist) no longer available.

虽然在技术上是正确的，但这段话使作者失去了 C 语言讲师的资格。这是更正后的段落。

The code saves the address of the next node because the call to free() makes the contents of the current node (the one pointed to by *plist) no longer available, and accessing it is undefined behaviour.

显然作者从来没有提到过这个词"undefined behaviour"。这在任何严肃的 C 教科书中都是不可接受的。

空闲内存用于 C 中的结构但失败（代码来自 C Primer Plus 第 6 版）

free memory used for struct in C but fails (code from C Primer Plus 6th Edition)

c

malloc

free

struct