malloc之后的空闲内存分配

Question

I was reading the "c primer plus" by Stephen Prata. There is sample program for linked list. The program uses malloc to allocate the memory space for a structure array, the code for the sample program is as below.

#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, * 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.\n");
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)
{
    free(current);
    current=current->next;
}
printf("Bye!\n");

return 0;
}

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

My confusion is from the memory free code. The current is freed by free(current); why the following line can get effect? current=current->next; since current is freed, this line should have no way to access the current memeber "next".

Looking forward to your help on this.

Thanks so much.

Answer 1

When you do this

while(current!=NULL)
{
    free(current);
    current=current->next;
}

You make current pointer dangling and you try to access it current=current->next; which will result in undefined behavior.

I would suggest you to free as below. Also Your current pointer will be pointing to NULL since you have looped till end of the list before to the free while loop.

current=head;
while(current!=NULL)
{
    struct film * temp = current;
    current=current->next;
    free(temp);
}

Answer 2

The free(current); does not clean any of the memory which is at the @ of current, just give it back to the memory pool, so it can be reuse, no cleanning of the memory is done, so it will continue to contain the same data better practive would be to add current=NULL; just after