C - how to get rid of memory leaks? - c

Question

How can I get rid of memory leaks from for example this function:
void AddData(Data **head, char *sentence, int number) {
Words *words = NULL;
char delimiters[] = " \n\0";
char *token = strtok(sentence, delimiters);
while (token != NULL) {
if (IsAlphabetical(token) == 1) {
char *string = (char *)malloc((strlen(token) + 1) * sizeof(char));
strcpy(string, token);
AddWords(&words, string);
free(string);
}
token = strtok(NULL, delimiters);
}
Data *temp = *head;
Data *newData = (Data *)malloc(sizeof(Data));
newData->lineNumber = number;
newData->words = words;
newData->pNext = NULL;
if (*head == NULL)
*head = newData;
else {
while (temp->pNext != NULL)
temp = temp->pNext;
temp->pNext = newData;
}
}
My personal opinion is the leaks appear because of newData, temp and words variables.
I have a few similar functions that cause the same problem.
I have also function for deleting Data struct, but when I call it at the end of previous function in such way DeleteData(&temp) program will not execute. I think it's because my whole list is deleted.
void DeleteData(Data **head) {
Data *temp = *head;
while (temp->pNext != NULL) {
Data *next = temp->pNext;
DeleteWords(&temp->words);
free(temp);
temp = next;
}
free(temp); /* to check that */
*head = NULL;
}
How can I fix this?

Answer 1

Here are some problems I identified:
The trailing \0 in char delimiters[] = " \n\0"; is useless.
Testing if (IsAlphabetical(token) == 1) might be too restrictive. In C anything non 0 is true so you might test if (IsAlphabetical(token) != 0) or just if (IsAlphabetical(token)).
Why allocate a copy of the string to pass to AddWords(&words, string); and then free(string) thereafter? If Addwords() does not keep the pointer it gets, there is no need to allocate a copy.
Does AddWords() call strtok()? strtok() is non-reentrant, which means that if AddWords() or any other function called in this loop, such as IsAlphabetical() calls strtok(), the context used in the loop will be corrupted. You should use strtok_r() instead.
in function DeleteData, why do you iterate testing while (temp->pNext != NULL)? The last item in the list does not get freed properly, the DeleteWords(&temp->words); is not called. This may cause a memory leak. You should just write:
void DeleteData(Data **head) {
Data *temp = *head;
while (temp != NULL) {
Data *next = temp->pNext;
DeleteWords(&temp->words);
free(temp);
temp = next;
}
*head = NULL;
}