Thread: free memory from malloc

Code:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
	char *name;
	name = (char *) malloc(80*sizeof(char));
	if ( name != NULL ) {
		printf("\nEnter your name: ");
		gets(name);
		printf("\nHi %s\n", name);
		free(name);
		if ( name == NULL ) {
			printf("\nGood!");
		}
		else 
			printf("\nNOT Good!");
		        printf("\nHi %s\n", name);
	}
	return 0;
}

My output:

Code:

Enter your name: c_lady

Hi c_lady

NOT Good!
Hi c_lady

How come after calling the free function I am still able to print the name???

I really don't understand this... isn't this strange?

Because no other process managed, in the 0.0000006 seconds between you freeing the memory and accessing it, to reclaim it and use it for their own. The computer doesn't go around zeroing out memory all the time, it just lets it be. You are still holding a pointer to that chunk of memory, even though you don't own it, which is your real error.

I believe that free just makes it so that the space your string was taking is now available for use by another part of your program or another program. It doesn't change where your pointer points to. Whether or not your or another program has changed what is stored at that location is by chance. Technically, your printf after the if statement will return an undefined value.

That's why some think it is good practice to set variables to NULL after freeing the pointer. I once wrote wrapper functions that keep track of malloc and free and take care of stuff like this, but it sometimes makes me lazy with malloc when I write code outside of my library.

Originally Posted by hellork

That's why some think it is good practice to set variables to NULL after freeing the pointer. I once wrote wrapper functions that keep track of malloc and free and take care of stuff like this, but it sometimes makes me lazy with malloc when I write code outside of my library.

Do you mean something like this?

Code:

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
	char *name;
	name = (char *) malloc(80*sizeof(char));
	if ( name != NULL ) {
		printf("\nEnter your name: ");
		gets(name);
		printf("\nHi %s\n", name);
		free(name);
		name==NULL;
		if ( name == NULL ) {
			printf("\nGood!");
		}
		else 
			printf("\nNOT Good!");
		printf("\nHi %s\n", name);
	}
	return 0;
}

There're already existing discussion about setting NULL after free(). c - Setting variable to NULL after free ... - Stack Overflow
If you read the free() doc, you won't even need to ask here why it's not set to NULL.
free() is just a function, how could it set a pointer to NULL!
eg.

Code:

  
 void wrong_set_function(void *p) {
          p = NULL;    // WRONG not changed in caller
   }

Question 4.8