Question

I’m trying to create a contiguous block of memory in one function call that has the first part of the memory as pointer arrays to the other blocks. Basically, I’m trying to do:

int **CreateInt2D(size_t rows, size_t cols)
{
    int **p, **p1, **end;
    p = (int **)SafeMalloc(rows * sizeof(int *));
    cols *= sizeof(int);
    for (end = p + rows, p1 = p; p1 < end; ++p1)
        *p1 = (int *)SafeMalloc(cols);
    return(p);
}

void *SafeMalloc(size_t size)
{
    void *vp;

    if ((vp = malloc(size)) == NULL) {
        fputs("Out of mem", stderr);
        exit(EXIT_FAILURE);
    }
    return(vp);
}

But with one block. This is as far as I’ve gotten:

int *Create2D(size_t rows, size_t cols) {
 int **memBlock;
 int **arrayPtr;
 int loopCount;
    memBlock = (int **)malloc(rows * sizeof(int *) + rows * cols * sizeof(int));
    if (arrayPtr == NULL) {
        printf("Failed to allocate space, exiting..."); 
        exit(EXIT_FAILURE);
    }
    for (loopCount = 1; loopCount <= (int)rows; loopCount++) {
        arrayPtr = memBlock + (loopCount * sizeof(int *));
        //I don't think this part is right.  do I need something like arrayPtr[loopCount] = ....
    }
 return(memBlock);
}

Answer 1

Something like this

 int **Create2D(size_t rows, size_t cols) 
 {
    size_t cb = (rows * sizeof(int *)) + (rows * cols * sizeof(int));
    int * pmem = (int *)SafeMalloc(cb);

    int ** prows = (int **)pmem; 
    int * pcol = (int *)&prows[rows]; // point pcol after the last row pointer

    for (int ii = 0; ii < rows; ++ii)
    {
       prows[ii] = pcol;
       pcol += cols;
    }

    return prows;
}