Why is my pointer giving back value at address directly?

Question

Here is my code:

typedef struct { int** matrix; int rows; int cols; }intMat; intMat create_intMat(int rows,int cols,int matrix[rows][cols]){ intMat A; A.rows = rows; A.cols = cols; int** mat; mat = malloc(A.rows*sizeof(int*)); // Pointer for matrix rows for(int i=0;i<A.rows;i++) mat[i] = malloc(A.cols*sizeof(int)); // Pointer of each matrix row to a matrix column for (int i=0;i<A.rows;i++){ for (int j=0;j<A.cols;j++){ mat[i][j]=matrix[i][j]; } } A.matrix = mat; return A; } int main() { int mat[2][2] = {{1,2},{3,4}}; intMat A; A = create_intMat(2,2,mat); printf("%d\n",A.matrix[1][1]); return 0; }

I'm a beginner to pointers and got the pointer part of the code from another forum. I don't get it, though. int** mat is a pointer to a pointer to an int, so if I call it as is then it should give me back gibberish address, not the int being pointed to. However, the printf statement returns 4, which is the value of the int being pointed to! How come this is happening?

Actually, you are dereferencing it twice through the use of the [] operator. Remember that a[b] is defined to be equivalent to *(a+b), so when you write mat[1][1] you are doing *(*(mat+1)+1), which, as you see, is dereferencing mat twice, getting from int ** to int. — Matteo Italia
– Matteo Italia, Commented Feb 25, 2015 at 23:09
@MatteoItalia -- you should consider posting that as an answer rather than a comment. — Happy Green Kid Naps
– Happy Green Kid Naps, Commented Feb 25, 2015 at 23:13

Mooing Duck · Accepted Answer · 2015-02-25 23:09:11Z

A.matrix is a pointer to a 2 pointers, which each point to 2 ints.
A.matrix[1] gets the second of those pointers - a pointer to 2 ints.
A.matrix[1][1] gets the second of those ints.

R Sahu · Accepted Answer · 2015-02-25 23:09:28Z

When you use:

int i; int* p = &i;

or

int* p = malloc(sizeof(int));

p points to a single integer.

When you use:

int array[10]; int *p = array;

or

int *p = malloc(sizeof(int)*10);

p points to an array of 10 integers.

Similarly, a variable declared with:

int** mat;

can point to one int* or an array of int*s.

In your case, mat points to an array of 2 int*, each of which points to an array of 2 ints.

That makes use of mat[1][1] perfectly valid code.

in your code int *p = array; should it not instead be int *p = &array;? And thanks for the explanation!

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