我应该得到这个错误：可变大小的对象可能未初始化吗？

Question

I'm making a hash table with an array of 9,997 buckets. I am just currently trying to initialize the entire hash table array with the collisionCount to 0 and the pointers inside the bucket to create the linked list to NULL. The supplied "data.txt" file provides the bucket count on the first line in the text file, so that is where I am getting the bucket count from.

Anyhow, here is some code :

Hashing .h :

#ifndef _Hashing_h
#define _Hashing_h

#include <stdio.h>

typedef struct bucket *bucketP;
typedef struct key *keyP;

/*
 * Creates the initial empty bucket array
 * from parameters 
 */
bucketP createBucketArray(FILE *);

#endif

Hashing.c :

#include <stdio.h>
#include <stdlib.h>

#include "Hashing.h"

struct bucket
{
    int collisionCount;
    keyP *firstKey;
};

struct key
{
    int thisKey;
    keyP *nextKey;
};

int main()
{
    FILE *fptr;
    int bucketCount, keyCount;
    fptr = fopen("data.txt", "r");
    fscanf(fptr, "%d %d", &bucketCount, &keyCount);
    bucketP thisArray = createBucketArray(fptr);
    fclose(fptr);

    return 0;
}

bucketP createBucketArray(FILE *fp)
{
    int thisBucketCount;
    int i = 0;
    fscanf(fp, "%d", &thisBucketCount);
    bucketP thisBucketArray[thisBucketCount] = (bucketP) malloc(sizeof(struct bucket));
    for (i = 0 ; i < thisBucketCount ; i++)
    {
        thisBucketArray[i]->collisionCount = 0;
        thisBucketArray[i]->firstKey = 0x00;
    }
    return thisBucketArray;
}

The error reads as follows :

error : variable-sized object may not be initialized
...thisBucketArray[thisBucketCount] = (bucketP) malloc(sizeof(struct bucket));
   ^

Since this was the first time I got the error message, I researched it and found that the above error message is usually thrown when you try and declare something too large for the stack. However, the value of 'thisBucketCount' is only 9,997. That creates an array of 9,997 pretty simple structs (only two parameters in struct bucket , one being an int and another being a pointer.

Am I really creating an array that is too large for my stack? I feel like I would have to make a WAY BIGGER array for that.

Thanks in advance for any advice, thanks thanks thanks!

Answer 1

Yes. The error is expected: VLAs (variable length arrays) can't be initialized.

But real problem seems to be that you want thisBucketCount number of buckets which is not what this code does:

bucketP thisBucketArray[thisBucketCount]=(bucketP)malloc(sizeof(struct bucket));

You probably want to allocate thisBucketCount number of bucket objects. So you need do:

 bucketP thisBucketArray = malloc(thisBucketCount * sizeof(struct bucket));

 if (thisBucketArray == 0) 
 { 
 /*handle error */
 }

 for (i = 0 ; i < thisBucketCount ; i++)
 {
    thisBucketArray[i].collisionCount = 0;
    thisBucketArray[i].firstKey = 0x00;
 }

Notice that I have removed the cast. casting the result of malloc() is error-prone and dangerous.

You should also check if malloc() failed by checking if it returns NULL .