C - double free - c

Question

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
struct strqueue {
struct lnode *front;
struct lnode *back;
int length;
};
struct lnode {
char *item;
struct lnode *next;
};
StrQueue create_StrQueue(void) {
struct strqueue *sq = malloc(sizeof(struct strqueue));
sq->length = 0;
sq->front = NULL;
sq->back = NULL;
return sq;
}
void destroy_nodes(struct lnode *l) {
while (l!=NULL) {
struct lnode *c = l;
l=l->next;
free(c);
}
}
void destroy_StrQueue(StrQueue sq) {
destroy_nodes(sq->front);
free(sq);
}
void sq_add_back(StrQueue sq, const char *str) {
struct lnode *n = malloc(sizeof(struct lnode));
n->item = malloc(sizeof(char)*(strlen(str)+1));
strcpy(n->item, str);
n->next = NULL;
if (sq->length == 0) {
sq->front = n;
sq->back = n;
} else {
sq->back->next = n;
sq->back = n;
}
sq->length++;
}
char *sq_remove_front(StrQueue sq) {
if (sq->front == NULL) {
return NULL;
} else {
struct lnode *f = sq->front;
char *temp = sq->front->item;
sq->front = sq->front->next;
sq->length--;
//Delete the line below will not cause an error of not free all memory
free(f->item);
free(f);
return temp;
}
}
int sq_length(StrQueue sq) {
return sq->length;
}
Here I wanna make the strqueue like a linked list but when I use it, it always says that I am attempting to double free something. Which part of my code is wrong? Is there a memory leak or something wrong about the memory allocation?

Answer 1

In
struct lnode *f = sq->front;
char *temp = sq->front->item;
sq->front = sq->front->next;
sq->length--;
//Delete the line below will not cause an error of not free all memory
free(f->item);
free(f);
return temp;
It returns pointer temp to freed memory in free(f->item), reading the string through that pointer is undefined behaviour. And if you free it that is going to be the double free. Basically, the returned pointer is useless.
The fix would be to avoid doing free(f->item) in that function. The caller would need to free the pointer to the string after use.
Singly-linked list is best represented by:
struct lnode *head, **tail;
Initialized as:
head = NULL;
tail = &head;
In this case there is no need for special handling of an empty list on appending. Appending always is:
*tail = n;
tail = &n->next;
Removing from the front is:
struct lnode *n = head;
if(head) {
head = head->next;
if(!head)
tail = &head;
}
return n;

Answer 2

You could try this in char *sq_remove_front(StrQueue sq) :
if (f->item != NULL) {
free(f->item);
f->item = NULL;
}
if (f != NULL) {
free(f);
sq->front = NULL;
}
It would avoid performing free() twice on the pointers.