Allocating 2D array with pointer to fixed-size array

Question

Is it valid in C to dynamically allocate 2d arrays this way?

//fixed size
int (*arr2d)[9][9]=malloc(sizeof(int[9][9]));

//variable size
int x=5,y=3;
int (*arr2d)[x][y]=malloc(sizeof(int[x][y]));

//assignment
(*arr2d)[0][1]=2;

EDIT: By "valid" I mean, are there any pitfalls as opposed to:

int i,x=5,y=10;
int **arr2d=malloc(sizeof(int*[x]));
for(i=0;i < x;i++)
    arr2d[i]=malloc(sizeof(int[y]));

arr2d[0][0]=5;

John Bode · Accepted Answer · 2014-09-17 02:08:13Z

3

The only real issue is if you request more memory than the malloc call can satisfy (you may have enough memory available, but not in a single, contiguous block).

To save your sanity, do something like this instead:

T (*arr)[cols] = malloc( sizeof *arr * rows );

This will allow you to index it as arr[i][j] instead of (*arr)[i][j].

answered Sep 17, 2014 at 2:08

John Bode

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