Memory Leaks in C - c

Question

So I'm very new to C, and I'm writing a matrix compression function for a trivial Bitmap Image recognition program. I have the following code, and Valgrind in telling me I have memory leaks at the following marked lines, although I have no idea what's causing it. Any advice would be appreciated.
/* Returns a NULL-terminated list of Row structs, each containing a NULL-terminated list of Elem structs.
* See sparsify.h for descriptions of the Row/Elem structs.
* Each Elem corresponds to an entry in dense_matrix whose value is not 255 (white).
* This function can return NULL if the dense_matrix is entirely white.
*/
Row *dense_to_sparse(unsigned char *dense_matrix, int width, int height) {
Row *result = NULL;
_Bool first_row;
for (int row = height - 1; row >= 0; row--) {
first_row = 0;
for (int elem = width - 1; elem >= 0; elem--) {
unsigned char curr_item = dense_matrix[(row*width) + elem];
if (curr_item!= 255) {
if (!first_row) {
(Memory Leak) Row *curr_row = (Row *) malloc(sizeof(Row));
if (curr_row == NULL) {
allocation_failed();
}
curr_row->next = result;
curr_row->y = row;
curr_row->elems = NULL;
result = curr_row;
//free(curr_row);
first_row = 1;
}
(Memory Leak) Elem *curr_elem = (Elem *) malloc(sizeof(Elem));
if (curr_elem == NULL) {
allocation_failed();
}
curr_elem->value = curr_item;
curr_elem->x = elem;
curr_elem->next = result->elems;
result->elems = curr_elem;
//free(curr_elem);
}
}
}
return result;
}
I believe it may be a problem with freeing curr_row and curr_elem, although when I try to free them at the end of each loop, it gives me a runtime error:
parsify(73897,0x7fff75584310) malloc: * error for object 0x7fbf81403a48: incorrect checksum for freed object - object was probably modified after being freed.

Answer 1

You need to call free on every pointer that you get from malloc. C doesn't automatically free up memory that you allocate, so you need to tell it "I'm done." Free is how you do this.
EDIT: You should also probably call free at the very end of the function, after you know you're done with the memory. If you do it at the end of the loop, you may run into problems with using memory you already freed.
EDIT EDIT: When you free it, note that you have put result in curr_row->next. You're probably accessing this later, post-free, which is a serious problem. You likely want to free all of them at the same time, as clearly you still need the memory (you still have pointers to it).

Answer 2

You cannot free the memory in dense_to_sparse because the whole point of the function is to create and return the newly allocated data structure. Presumably, the code that calls dense_to_sparse wants to use the result.
You will need a separate function to deallocate the memory that you should call once you no longer need it.
void free_sparse_matrix (Row *matrix)
{
Row *row = matrix;
while (row != NULL) {
Row *next_row = row->next;
Elem *elem = row->elems;
while (elem != NULL) {
Elem *next_elem = elem->next;
free (elem);
elem = next_elem;
}
free (row);
row = next_row;
}
}