Question

So on a project I’m working on I’m getting the following error:

*** glibc detected *** ./social_network: munmap_chunk(): invalid pointer: 0x09a913b0 ***

A few printf statements revealed the the struct I’m freeing that causes this error has a memory address of….17. So how is this possible? What could have happened to make this appear…?

Here’s some output right before the crash:

temp is: 162075592
temp user is  162075408
After free of temp
inside while loop, first line
before free of temp
temp is: 162075760
temp user is  162075480
After free of temp
inside while loop, first line
before free of temp
temp is: 162075568
temp user is  17

And here’s the code that outputs the output

 21 void destroy() {
 22 
 23     struct user_node *node;
 24 
 25     struct friend_node *temp;
 26     struct friend_node *next_temp;
 27     
 28     //iterate over the list of users
 29     while (*head != NULL) {
 30         printf("before a_friend\n");
 31         temp = (*head)->a_friend;
 32         
 33         //Iterate over friends of users, free friend nodes
 34         while (temp != NULL) {
 35             printf("inside while loop, first line\n");
 36             next_temp = temp->next_friend;
 37             printf("before free of temp\n");
 38             printf("temp is: %d\n", (int) temp);
 39             printf("temp user is  %d\n", (int)temp->user);
 40             free(temp); <==================================SEGFAULT IS HERE!!!!!!!!!!!!!
 41             printf("After free of temp\n");
 42             temp = next_temp;
 43             next_temp = NULL;
 44         }
 45 
 46         node = (*head)->next_user;
 47         printf("before free of: %s\n", (*head)->name);
 48         free(*head);
 49         printf("after free of that person...\n");
 50         *head = node;
 51         printf("*head = node afterwards\n");
 52         node = NULL;
 53     }
 54     printf("before final free...\n");
 55     free(head);
 56 }

EDIT:

struct friend_node

 23 //node used to store a pointer to a user_node and next friend
 24 struct friend_node {
 25     struct friend_node *next_friend;
 26     struct user_node *user;
 27 }; 

struct user_node

 12 //Struct definitions
 13 //node used to store information about a user
 14 struct user_node {
 15     char name[21];
 16     int age;
 17     int gender;
 18     char location[21];
 19     struct friend_node *a_friend;
 20     struct user_node *next_user;
 21 };
 22    

Answer 1

I don’t see anything in your code that would suggest that you are freeing a struct at address 17, as you seem to incorrectly believe.

Both the crash message and your dump clearly show that you are attempting to free pointer temp that points to address 162075568 (0x09a913b0). That pointer points to invalid memory, which is why free crashes. This is also why you might see 17 stored inside that memory. That memory is invalid, so there’s nothing surprising in the fact that it stores random garbage.

Forget about that 17. It has nothing to do with anything. You problem is 162075568 (0x09a913b0). This value is invalid and that is the source of the problem.