Initializing a struct array inside a struct

Question

I've looked at other questions but I can't seem to find a clear answer for this. How do I declare a struct array inside of a struct? I attempted to do it in main() but I don't know if I'm doing it right and I keep getting this warning: "initialization makes integer from a pointer without cast"

#define MAXCARDS 20     

struct card {
    int priority;       
};

struct battleQ {
    struct card cards[MAXCARDS];
    int head;
    int tail;
    int size;
};

int main (int argc, char *argv[]) {
    struct battleQ bq;
    bq.cards = {
                      malloc(MAXCARDS * sizeof (struct card)), //Trouble with this part
                      0,
                      0,
                      0
                };

        //...

    return 1;
}

Edit after suggestions: Okay now I'm having problems. I keep getting this error:

3 [main] TurnBasedSystem 47792 open_stackdumpfile: Dumping stack trace to TurnBasedSystem.exe.stackdump

I had to change the code a bit and make everything pointers. I tested it and it gives me that error as soon as I try to assign one of its attributes like: bq->head = 0

The entire thing is just supposed to add a card to a queue. Revised code is below:

#include <stdio.h>
#include <stdbool.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

#define MAXCARDS 20

struct card {
    int priority;       
};

// A queue defined by a circular array
struct battleQ {
    struct card *cards[MAXCARDS];
    int head;
    int tail;
    int size;
};

bool battleQEnqueue (struct battleQ *bq, struct card *c);
bool battleQisFull(struct battleQ *bq);

// Method for enqueuing a card to the queue
bool battleQEnqueue (struct battleQ *bq, struct card *c) {
    bool success = false;
    if (battleQisFull(&bq)) {
        printf("Error: Battle queue is full\n");
    } else {
        success = true;
        bq->cards[bq->tail] = c;
        bq->size = bq->size + 1;
        bq->tail = (bq->tail + 1) % MAXCARDS;
    }
    return success;
}

int main (int argc, char *argv[]) {
    int i;
    struct battleQ *bq;
    memset(&bq, 0, sizeof(bq));  // Did I do this properly?
    bq->tail = 0;               // Gives error at this point
    bq->head = 0;               
    bq->size = 0;

    // This is where I create a card and add it to the queue but the main problem
    // is still the initialization above
    for (i = 0; i < 5; i++) {
        struct card *c = malloc(sizeof(c));
        c->priority = i + 10;
        printf("%d,", c->priority);
        battleQEnqueue(&bq, &c);
    }

    return 1;
}

by the way, the trouble is probably caused by malloc returning a void* — Can Poyrazoğlu
– Can Poyrazoğlu, Commented Jun 23, 2013 at 8:12

Rohan · Accepted Answer · 2013-06-23 08:20:16Z

3

bq.cards is array of structs, you don't have to malloc it. You can initialize the entire array as:

memset(bq.cards, 0, sizeof(bq.cards));

If you want to initialize bq do

    memset(&bq, 0, sizeof(bq));

edited Jun 23, 2013 at 8:20

answered Jun 23, 2013 at 8:09

Rohan

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6 Comments

Lekensteyn Over a year ago

This should be: memset(&bq, 0, sizeof bq) in OPs case. (without all the bq.cards = { ... }.

Rohan Over a year ago

@Lekensteyn, I'm not sure OP wants to initiazlise bd or bq.cards from the lvalue of statement it looks like bq.cards.

Lekensteyn Over a year ago

@Rohan Looks like bq to me since he sets the other members to 0 as well.

Rohan Over a year ago

@Ron, As bq is pointer now, you have to allocate memory bq=malloc(sizeof(*bq)) and memset(bq, 0, sizeof(*bq)) or rather use calloc().

Ron Over a year ago

@Rohan Thanks that works. But when I use the same thing to intialize cards[], it gives me that same error. I did: bq->cards = malloc(sizeof(*(bq->cards))); It says incompatible types when assigning struct card*[20] from type *void.

|

alk · Accepted Answer · 2013-06-23 11:16:07Z

1

You might like to initialise the whole structure this way:

...

int main(int argc, char *argv[])
{
  struct battleQ bq =
  {
    {
      { 0 } /* int priority; */
    } /* (initialising the first element/member initialises all element/member) */
  }; 

  //...

  return 1;
}

edited Jun 23, 2013 at 11:16

answered Jun 23, 2013 at 8:17

alk

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Comments

cup · Accepted Answer · 2013-06-23 08:17:23Z

0

The initialization is in the declaration

struct battleQ bq = {
              {{0}, {0},... // 20 times
              },
                  0,
                  0,
                  0
            };

It might be better to have cards as the last element, then you can use a trick called the chumminess of C. http://c-faq.com/struct/structhack.html, where you can have a variable sized battleQ.

I may be wrong on this but what I've found on most compilers is that if you set the first element to zero, everything else will be zero. I remember reading something in the standard about it but I can't remember were or whether it was C or C++.

struct battleQ bq = {{{0}}};

answered Jun 23, 2013 at 8:17

cup

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2 Comments

Daniel Fischer Over a year ago

You don't need to specify an initializer for each array element if they all shall be 0, struct battleQ bq = { {{0}}, 0, 0, 0 }; is enough. You don't even need that much, struct battleQ bq = {0}; does the same. And instead of the struct hack one should use the standard way, a flexible array member.

cup Over a year ago

Yes, I realized that after I wrote it and was going to correct it but I was asked to do something else by a higher authority. Ha ha. Never mind.

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