无法在C中返回多维数组

Question

I'm trying to write a function in C, which returns the pointer to an array. Each element of this array is a pointer to the initialized structure, and in each structure there's also a pointer to an array.

struct queueRecord;
typedef struct queueRecord* queue;

queue createQueue(int maxSize){
    queue newQueue = malloc(sizeof(struct queueRecord));
    newQueue->array = malloc(sizeof(int)*maxSize);
    newQueue->capacity=maxSize;
    newQueue->front=0;
    newQueue->rear=0;
    return newQueue;
}

This just works fine as well as in the following function:

queue* initQArr(int maxSize){
    queue arr[10];
    for (int i=0; i<10; i++) {
        arr[i]=createQueue(maxSize);
    }
    return arr;
}

when I pass the parameter 10 in it and set a breakpoint at return, everything seems good (excuse me I can't post images, the debug information is like following):

arr(queue[10])
 [0]=(queue)0x100105480
   capacity=(int)10
   size=(int)0
   front=(int)0
   rear=(int)0
   array=(int*)0x1001054a0
 [1]=(queue)0x1001054d0
 ...
 ...
 [9]=(queue)0x100105760

However!When I invoke this function and return to a new array, I got an array with only 5 elements:

queue *digitsQArr = initQArr(length);
int digit = 0;

(debug information as following):

digitQArr = (queue*)0x7fff5fbff6d0
    *digitsArr=(queue)0x100105480
        capacity=(int)10
        size=(int)0
        front=(int)0
        rear=(int)0
        array=(int*)0x1001054a0
    [1]=(queue)0x1001054d0
    ...
    ...
    [4]=(queue)0x1001055d0

Answer 1

In initQArr , you return the address of a local variable, arr :

queue* initQArr(int maxSize){
    queue arr[10];
    for (int i=0; i<10; i++) {
        arr[i]=createQueue(maxSize);
    }
    return arr;
}

The returned pointer will point to the local array arr , which is no longer valid once the function returns. You could follow the example of createQueue and allocate memory on the heap:

queue* initQArr(int maxSize){
    queue *arr = malloc(10 * sizeof(*arr));
    for (int i=0; i<10; i++) {
        arr[i]=createQueue(maxSize);
    }

    return arr;
}

After returning, the pointer arr is still valid, because it points to memory on the heap. You should free such memory after using it, like the queue pointers allocated in createQueue .