Blobonat Blobonat - 19 days ago 5
C Question

Correct usage of free()

I started to improve my basic C knowledge.

For that I tried to work with pointer data types.

I created a list like this:

typedef struct lsEle *listPointer;
typedef struct lsEle {int index; listPointer next; int value;} element;


with the global variable

listPointer header;


In my
remove(int index)
function I now want to remove the element from the list at the index.

void removeAtIndex(int index) {
if (index < 0) {
printf("You have to enter a index >= 0\n");
return;
} else {
listPointer tmp = header;
if (index == 0) {
if (header->next == NULL) {
header = NULL;
} else {
header = header->next;
}
free(tmp);
} else {
int counter = 0;
while (1) {
if (tmp->index == index - 1) {
break;
}
if (tmp->next != NULL) {
tmp = tmp->next;
counter++;
} else {
break;
}
}
if (index - 1 != counter) {
printf("You have to enter a index <= %d\n", counter);
return;
} else {
listPointer tmp_tmp = tmp->next;
tmp->next = tmp_tmp->next;
free(tmp_tmp);
}
}
//Now update all index
while (1) {
if (tmp->next != NULL) {
tmp = tmp->next;
tmp->index = tmp->index - 1;
} else {
break;
}
}
}
}


The code works fine.

My question is now: Do I use free() correctly? My goal was to remove the element "object" from the heap.

What happens when I do not use free? After the function exit from
remove
I don't have access to
tmp_tmp
any more, but does the "removed" object stay in memory?

Answer

Do I use free() correctly?

You use free correctly if you previously allocate memory with malloc, calloc or realloc.

What happens when I do not use free?

Memory is not deallocated and you got memory leak.

After the function exit from remove I don't have access to tmp_tmp any more, but does the "removed" object stay in memory?

You deallocate memory pointed by tmp_tmp with free(tmp_tmp) so object removed from memory.