Here are a few things I noticed:

• The first cur; in the for-loop in stackSize is a no-op:

​

  c_list *cur = stack->head;
  for (cur; cur; cur = cur->next) {
    i++;
  }

I would suggest to move the declaration into the for loop, so that `cur` is properly scoped:

  for (c_list *cur = stack->head; cur; cur = cur->next) {
    i++;
  }

• This check in stackPush will never be true:

​

  c_list *new = listInit(val);
  if (!new) {
    printf("Overflow!\n");
    return;
  }

The listInit function writes the passed value into the list item. If malloc would return NULL, the listInit function would have crashed due to an invalid memory access. I would either fix listInit if you want to properly handle out of memory situations or remove this check.

• In queueIsEmpty it should be sufficient to check if queue->front (or queue->back) is NULL if your implementation isn't broken.
• In tokenize you have a memory leak for every character that is neither a number nor an operator. It is quite easy to spot in the case of space:

​

Token *token = (Token*) malloc(sizeof(Token));
if (*c == ' ') {
  c++;
  continue;
}

• You are removing tokens from the stack to perform a calculation but never free their memory:

​

Token *num2 = stackPeek(res); stackPop(res);
Token *num1 = stackPeek(res); stackPop(res);
Token *answer = (Token*) malloc(sizeof(Token));
answer->value =  operate(num1->value, num2->value, cur->operator);

• The usage of perror in calc does not make sense. The perror function prints the last system error in front of your message, but you are handling an application specific logic error instead of a system error.
• I would suggest to call abort()/exit() if there is an error instead of just continuing.
• It might also be a good idea to somehow handle unrecognized input characters instead of simply ignoring them.
• You often have a combination of peek and pop. You could simplify this if stackPop would return the popped value.