I’m trying to understand the code below, and I’m having some trouble understanding why there’s no seg fault when I run it. I suppose I need to use malloc to allocate the list at the beginning of the main function? Can someone explain how the memory for the list is allocated?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct DListElmt_ {
void *data;
struct DListElmt_ *prev;
struct DListElmt_ *next;
} DListElmt;
typedef struct DList_ {
int size;
int (*match)(const void *key1, const void *key2);
void (*destroy)(void *data);
DListElmt *head;
DListElmt *tail;
} DList;
void dlist_init(DList *list, void (*destroy)(void *data));
void dlist_destroy(DList *list);
int dlist_ins_next(DList *list, DListElmt *element, const void *data);
int dlist_ins_prev(DList *list, DListElmt *element, const void *data);
int dlist_remove(DList *list, DListElmt *element, void **data);
#define dlist_size(list) ((list)->size)
#define dlist_head(list) ((list)->head)
#define dlist_tail(list) ((list)->tail)
#define dlist_is_head(element) ((element)->prev == NULL ? 1 : 0)
#define dlist_is_tail(element) ((element)->next == NULL ? 1 : 0)
#define dlist_data(element) ((element)->data)
#define dlist_next(element) ((element)->next)
#define dlist_prev(element) ((element)->prev)
void dlist_init(DList *list, void (*destroy)(void *data)) {
list->size = 0;
list->destroy = destroy;
list->head = NULL;
list->tail = NULL;
return;
}
void dlist_destroy(DList *list) {
void *data;
while (dlist_size(list) > 0) {
if (dlist_remove(list, dlist_tail(list), (void **)&data) == 0 && list->destroy != NULL) {
list->destroy(data);
}
}
memset(list, 0, sizeof(DList));
return;
}
int dlist_ins_next(DList *list, DListElmt *element, const void *data) {
DListElmt *new_element;
if (element == NULL && dlist_size(list) != 0)
return -1;
if ((new_element = (DListElmt *)malloc(sizeof(DListElmt))) == NULL)
return -1;
new_element->data = (void *)data;
if (dlist_size(list) == 0) {
list->head = new_element;
list->head->prev = NULL;
list->head->next = NULL;
list->tail = new_element;
}
else {
new_element->next = element->next;
new_element->prev = element;
if (element->next == NULL)
list->tail = new_element;
else
element->next->prev = new_element;
element->next = new_element;
}
list->size++;
return 0;
}
int dlist_ins_prev(DList *list, DListElmt *element, const void *data) {
DListElmt *new_element;
if (element == NULL && dlist_size(list) != 0)
return -1;
if ((new_element = (DListElmt *)malloc(sizeof(DListElmt))) == NULL)
return -1;
new_element->data = (void *)data;
if (dlist_size(list) == 0) {
list->head = new_element;
list->head->prev = NULL;
list->head->next = NULL;
list->tail = new_element;
}
else {
new_element->next = element;
new_element->prev = element->prev;
if (element->prev == NULL)
list->head = new_element;
else
element->prev->next = new_element;
element->prev = new_element;
}
list->size++;
return 0;
}
int dlist_remove(DList *list, DListElmt *element, void **data) {
if (element == NULL || dlist_size(list) == 0)
return -1;
*data = element->data;
if (element == list->head) {
list->head = element->next;
if (list->head == NULL)
list->tail = NULL;
else
element->next->prev = NULL;
}
else {
element->prev->next = element->next;
if (element->next == NULL)
list->tail = element->prev;
else
element->next->prev = element->prev;
}
free(element);
list->size--;
return 0;
}
void print_list(const DList *list) {
DListElmt *element;
int *data,
fprintf(stdout, "List size is %d\n", dlist_size(list));
i = 0;
element = dlist_head(list);
while (1) {
data = dlist_data(element);
fprintf(stdout, "list[%03d]=%03d\n", i, *data);
i++;
if (dlist_is_tail(element))
break;
else
element = dlist_next(element);
}
return;
}
int main(int argc, char **argv) {
DList list;
DListElmt *element;
int *data, i;
dlist_init(&list, free);
element = dlist_head(&list);
for (i = 10; i > 0; i--) {
if ((data = (int *)malloc(sizeof(int))) == NULL)
return 1;
*data = i;
if (dlist_ins_prev(&list, dlist_head(&list), data) != 0)
return 1;
}
print_list(&list);
element = dlist_head(&list);
for (i = 0; i < 8; i++)
element = dlist_next(element);
data = dlist_data(element);
fprintf(stdout, "Removing an element after the one containing %03d\n", *data);
if (dlist_remove(&list, element, (void **)&data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Inserting 011 at the tail of the list\n");
*data = 11;
if (dlist_ins_next(&list, dlist_tail(&list), data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Removing an element at the tail of the list\n");
element = dlist_tail(&list);
if (dlist_remove(&list, element, (void **)&data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Inserting 012 just before the tail of the list\n");
*data = 12;
if (dlist_ins_prev(&list, dlist_tail(&list), data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Iterating and removing the fourth element\n");
element = dlist_head(&list);
element = dlist_next(element);
element = dlist_prev(element);
element = dlist_next(element);
element = dlist_prev(element);
element = dlist_next(element);
element = dlist_next(element);
element = dlist_next(element);
if (dlist_remove(&list, element, (void **)&data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Inserting 013 before the first element\n");
*data = 13;
if (dlist_ins_prev(&list, dlist_head(&list), data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Removing an element at the head of the list\n");
if (dlist_remove(&list, dlist_head(&list), (void **)&data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Inserting 014 just after the head of the list\n");
*data = 14;
if (dlist_ins_next(&list, dlist_head(&list), data) != 0)
return 1;
print_list(&list);
fprintf(stdout, "Inserting 015 two elements after the head of the list\n");
if ((data = (int *)malloc(sizeof(int))) == NULL)
return 1;
*data = 15;
element = dlist_head(&list);
element = dlist_next(element);
if (dlist_ins_next(&list, element, data) != 0)
return 1;
print_list(&list);
i = dlist_is_head(dlist_head(&list));
fprintf(stdout, "Testing dlist_is_head...Value=%d (1=OK)\n", i);
i = dlist_is_head(dlist_tail(&list));
fprintf(stdout, "Testing dlist_is_head...Value=%d (0=OK)\n", i);
i = dlist_is_tail(dlist_tail(&list));
fprintf(stdout, "Testing dlist_is_tail...Value=%d (1=OK)\n", i);
i = dlist_is_tail(dlist_head(&list));
fprintf(stdout, "Testing dlist_is_tail...Value=%d (0=OK)\n", i);
fprintf(stdout, "Destroying the list\n");
dlist_destroy(&list);
return 0;
}
Thanks for the help.
list (which is the list header) is allocated on the stack. Its elements are malloced.