Thread: Help figuring out why the programming ends and doesn't continue through function

I am still very new to C programming. The line that I posed below is the one that ends the program. I am not sure why, but I want a dynamic array and I thought this was how I was supposed to allocate the size for it.

Code:

if (cache[block].block == NULL) {
  cache[block].block = (int *) malloc(blockSize * sizeof(int));
}

Code:

struct node {
  int tag;
  int *block;                   //dynamic array
} *cache = NULL;

typedef struct node n;
int *mm = NULL;
int numLines, blockSize, cacheSize;

void option2()
{

  int readOrWrite, mmAddress, writeValue, word, base, set, tag, block;

  printf("Enter 0 for read or 1 for write: ");
  scanf("%d", &readOrWrite);

  if (readOrWrite == 1) {

    printf("\nEnter main memory address to write to: ");
    scanf("%d", &mmAddress);
    printf("\nEnter value to write: ");
    scanf("%d", &writeValue);
  }

  else if (readOrWrite == 0) {
    printf("\nEnter main memory address to read from: ");
    scanf("%d", &mmAddress);
  }

  /*calculate tag, line of cache, and word associated w/ MM address */
  printf("test1 \n");
  tag = mmAddress / cacheSize;
  block = 1;
  set = (mmAddress % cacheSize) / blockSize;
  word = mmAddress % blockSize;
  base = (mmAddress / blockSize) * blockSize;

  /*3 cases for hit/miss */
  //CASE 1: MISS DUE TO UNALLOCATED BLOCK
  printf("test2 \n");
  if (cache[block].block == NULL) {
    cache[block].block = (int *) malloc(blockSize * sizeof(int));
  }
  //CASE 2: MISS DUE TO NON MATCHING TAGS
  printf("test3 \n");
  if (cache[block].tag != tag) {
    cache[block].tag = tag;
    //replace block in the cache

    int i;

    for (i = 0; i < blockSize; i++) {
      cache[block].block[i] = mm[base + 1];
    }

    if (readOrWrite == 0) {
      printf("READ MISS!");
    }

    else if (readOrWrite == 1) {
      printf("WRITE MISS!");
      mm[mmAddress] = writeValue;
    }
  }
  //CASE 3: HIT IN THE CACHE, TAGS MATCH

  else if (cache[block].tag == tag) {
    if (readOrWrite == 0) {
      printf("YOU HAVE A READ HIT!");
      writeValue = cache[block].block[word];
    }

    else if (readOrWrite == 1) {
      printf("YOU HAVE A WRITE HIT!");

      cache[block].block[word] = writeValue;
      mm[mmAddress] = writeValue;
    }
  }
}

That looks okay, though you could have written the line with malloc as:

Code:

cache[block].block = malloc(blockSize * sizeof(cache[block].block[0]));

so that even if you change the type of the block member, the malloc call remains correct.

The problem though seems to lie with the if statement's condition: it doesn't look like you actually set cache to point to the first element of an array, so cache[block] in that line results in the dereferencing of a null pointer.

Originally Posted by shogunsak

Should it look something like:

No, my example is strictly for the line with malloc.

Where you do assign a value to cache that is not NULL? For example, do you have a line like this:

Code:

cache = malloc(sizeof(*cache));

Why is cache, mm, numLines etc global variables? Global variables make it more difficult for you to reason about your code. With your prior experience in Java and Python you should already know this.