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

Question

I run the code from Listing 17.2, P781, C Primer Plus 6th Edition.

/* 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] != '\0') {    
        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 = '\0';    // place a null character there
        else
            while (getchar() != '\n')
                continue;      // dispose of rest of line
    }
    return ret_val;
}

My question is that after free(current), current->title and current->rating still is there, and current->next is not NULL. I don't know why.

Answer 1

The variable is still available for use but the space allocated isn't so you will have undefined behavior. It can work now but it can crash later. A good practice is to set your pointer to NULL after freeing it.