How to properly allocate enough memory (malloc) when creating a new struct in C?

Question

Given the struct below, I am creating a function that takes in a person_in_queue and a position_num and allocating a new queue_t struct that is added to the end of a list of queue_t structs as specified by the first argument.

typedef struct queue {
  int position_num;
  char *person_in_queue;

  struct queue *next_in_line;
} queue_t;

I have written my code as such:

queue_t *add_to_queue(queue_t *input_queue, char *person_in_queue, int position_num) {

  input_queue = malloc(sizeof(queue_t));
  assert(input_queue != NULL);

  input_queue->position_num = position_num;
  input_queue->person_in_queue = (char *) malloc((strlen(new_text) + 1) * sizeof(char));
  assert(input_queue->person_in_queue != NULL);

  strcpy(input_queue->person_in_queue, person_in_queue);
  return input_queue;

}

Said code compiles, however, I am told that my code fails given that less memory is allocated than what is expected. At the moment, I'm not sure where I am going wrong here. Do note that I need to use malloc() !

Many thanks!

Answer 1

sizeof is an operator in C, not a funciton, but parenthesis are necessary to evaluate types.

To allocate memory for the struct, use the size of the type.

input_queue = malloc(sizeof (queue_t));

Or use the de-referenced pointer or object size (parenthesis not necessary here).

input_queue = malloc(sizeof *input_queue);

Answer 2

I am told that my code fails given that less memory is allocated than what is expected.

That must refer to the malloc((strlen(new_text) + 1) * sizeof(char)) . Apparently new_text is a global string with no visible connection to person_in_queue , which latter is to be duplicated. Change the call to malloc(strlen(person_in_queue) + 1) .

How to properly allocate enough memory (malloc) when creating a new struct in C?

Apart from the above, the allocation is basically okay, but as Marco Bonelli noted, you're taking input_queue as argument and immediately overwriting it … That doesn't make much sense … It would rather make sense to return the allocated queue_t object if input_queue is initially NULL , otherwise the passed input_queue unchanged. This can be accomplished by changing the first two statements of your function's body to

    queue_t *head_queue = input_queue, **pr = &input_queue;
    while (*pr) pr = &(*pr)->next_in_line;  // find end of list
    *pr =   // link new struct to list
    input_queue = malloc(sizeof(queue_t));
    assert(input_queue != NULL);
    input_queue->next_in_line = NULL;       // don't forget to initialize!

and the return statement to

    return head_queue ? head_queue : input_queue;

- the former also sets the link pointers next_in_line correctly.