CODEWITHSUNDEEP
cstack
saltandpepper
saltandpepper

Reputation: 1047

Implementing a stack of "strings" in C

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

Upvotes: 2

Views: 4302

Answers (2)

ziq
ziq

Reputation: 1018

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.

Upvotes: 2

Karoly Horvath
Karoly Horvath

Reputation: 96306

I flicked through the code and it seems to be ok, I think the problem is in your client.

Your only store pointers in the stack, you're probably pushing the same pointer on the stack but rewrite the string it points to.

Note that if you really only want to store pointers your 3rd malloc is wasting space and also creates a memory leak.

Upvotes: 1

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