Tuesday, April 8, 2014

Allocate a two dimensional array using one call of malloc in C

Problem

Write a function called my2DAlloc which allocates a two dimensional array. Minimize the number of calls to malloc and make sure that the memory is accessible by the notation arr[i][j].

Solution

We will use one call to malloc.
Allocate one block of memory to hold the row vector and the array data. The row vector will reside in rows * sizeof(int*) bytes. The integers in the array will take up another rows * cols * sizeof(int) bytes.
Constructing the array in a single malloc has the added benefit of allowing disposal of the array with a single free call rather than using a special function to free the subsidiary data blocks.
int** My2DAlloc(int rows, int cols) {
    int header = rows * sizeof(int*);
    int data = rows * cols * sizeof(int);
    int** rowptr = (int**)malloc(header + data);
    int* buf = (int*)(rowptr + rows);
    int k;
    for (k = 0; k < rows; ++k) {
        rowptr[k] = buf + k*cols;
    }
    return rowptr;
}

Here is a demo:


References
http://tianrunhe.wordpress.com/2012/04/23/allocate-a-two-dimensional-array-using-one-call-of-malloc-in-c/

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