malloc() and free() and linked list

[Kurt]

Hi all,

I know there's been a lot of articles and posts because of this malloc
and free combination and I've read quite a lot of them. Still I can't
get my program to work:

My Structure:
typedef struct rule {
char ip[IP_SIZE];
struct rule *next;
} rule_list;

My Function to add a rule to the list of rules:
void add_rule(rule_list **head, char* ip)
{
rule_list *tmp;

if ((tmp = malloc(sizeof(*tmp))) == NULL)
{
syslog(LOG_ERR, "malloc for inserting a new rule crashed! Out of
memory?");
exit(EXIT_FAILURE);
}

/* This is what doesn't work because the
compiler says incompatible types in assignment */
//tmp->ip = malloc(strlen(ip));

/* This would work (without the statement above, but without
allocating space for this member of the struct I can't free it later
on */
strcpy(tmp->ip, ip);

tmp->next = *head;
*head = tmp;
}


My Function to free the whole list including all members:
void freelist(rule_list *head)
{
rule_list *tmp;

printf("freeing the list...");
fflush(stdout);

while (head != NULL) {
printf("head is not null freeing its members...");
fflush(stdout);

/* This will crash on me any time I run it, since I haven't
allocated memory for it with malloc */
if (head->ip != NULL) free(head->ip);

tmp = head->next;
free(head);
head = tmp;
}
printf("list freed!...");
fflush(stdout);

}

I only made it so far:
1. I don't free() the member ip of the struct when freeing the whole
list
--> This will eat up more and more memory while the program is running

2. I try to free() the memory and it won't work --> crash!

What's still wrong here?
Thx a lot,
Kurt

 

[Lawrence Kirby]

Hi all,

I know there's been a lot of articles and posts because of this malloc
and free combination and I've read quite a lot of them. Still I can't
get my program to work:

My Structure:
typedef struct rule {
char ip[IP_SIZE];
struct rule *next;
} rule_list;

My Function to add a rule to the list of rules:
void add_rule(rule_list **head, char* ip)
{
rule_list *tmp;

if ((tmp = malloc(sizeof(*tmp))) == NULL)

Excellent, I seems you have been reading up on this.
You don't need the inner parentheses there i.e. sizeof *tmp is fine, but
you can put them in if you like

{
syslog(LOG_ERR, "malloc for inserting a new rule crashed! Out of
memory?");

Saying "crashed" isn't quite right. Crashed suggests something like a seg
fault whereas what happened here is that malloc() simply failed to perform
the requested operation in a well defined way saying that it can't or
won't do that.

exit(EXIT_FAILURE);
}
}
/* This is what doesn't work because the
compiler says incompatible types in assignment */
//tmp->ip = malloc(strlen(ip));

You can't assign to an array in C. The array you want is being created by
malloc(), make the ip structure member a pointer so it can point to that
array.

There is another problem here in that strlen(ip) doesn't account for the
terminating null character that every string requires. Make that
strlen(ip)+1. Don't forget to test the return value of this malloc() for
null.

Lawrence