Question

I’m trying to implement a stack structure in C, for storing char arrays into.

I have the following code:

typedef struct {
  size_t size;
  char **data;
} loods1;

loods1 *init(void) {
  loods1 *loods = malloc(sizeof(loods1));
  loods->data = malloc(sizeof(char *) * STACK_MAX);
  for (int i = 0; i < STACK_MAX; i++) {
    *(loods->data + i) = malloc(LABEL_LENGTH_MAX * sizeof(char));
  }   
  loods->size = 0;
  if (loods == NULL) {
    perror("malloc failed\n");
    return NULL;
  }
  return loods;
}

int empty(loods1 *loods) {
  return (loods->size == 0);
}
void push(loods1 *loods, char *name) {
  if (loods->size == STACK_MAX) {
    perror("Stack is full\n");
    exit(0);
  }
  else {
    *((loods->data) + loods->size++) = name;
  }
}
char *pop(loods1 *loods) {
  if (loods->size == 0) {
    printf("size == 0\n");
    return NULL;
  }
  else {
    printf("%s \n", *(loods->data + 1));
    return *(loods->data + (--loods->size));
  }
}
int delete(loods1 *loods) {
  for (int i = 0; i < STACK_MAX; i++) {
    free(*(loods->data + i));
  }   
  free(loods->data);
  free(loods);   
}

There are 2 problems: first off, every time I add a new element to the stack, it overwrites all existing elements (if ‘3’ and ’11’ are added and I want to add ’15’, the new stack will look like ’15’, ’15’, ’15’). And when I want to pop the stack, the popped value is empty. Not null, but an empty string or something?

I have no idea what I’m doing wrong, but there seems to be a mistake somewhere, obviously.

Sammy

Answer 1

In push function, if you are passing char*, it will divert your pointer to where the char * is, and when you do p++, it will go from the char* you passed.

Try change push definition to:

void push(loods1 *loods, const char *name) {
  if (loods->size == STACK_MAX) {
    perror("Stack is full\n");
    exit(0);
  }
  else {
    strcpy((loods->data)[loods->size++], name);
  }
}

From here you might as well need some other changes to your calling program.

Also when you free it, free the single loods does not free all memory you allocated.