Question

The following snippet declares a 2-D array of 4 X 10 using malloc function

/* Declare a pointer to an array that has 10
ints in each row. */
int (*p)[10];
register int i, j;
/* allocate memory to hold a 4 x 10 array */
p = malloc(40*sizeof(int));

But I do not understand how does p become a 2-D array. Initially p is declared to be an array of pointers that point to int. What happens after the call to malloc ? I am unable understand this.

Answer 1

In C, pointers and arrays are not the same, despite looking very similar. Here p is of type “pointer to array of 10 ints”. You’re using it as a “pointer to array of 4 arrays of 10 ints”, which is a single block of memory (the only pointer is the outermost pointer). It’s basically a dynamically allocated int[4][10].

The trick to reading these definitions is to realize that they’re written the same way you use the item. If you have:

*x[10];

The array subscript is applied first, then the pointer dereference. So it’s an array of pointers if you define int *x[10]. If you use parenthesis to override normal precedence, you can get the pointer dereference to happen first, so you have a pointer to an array.

Confusing? It gets worse. In function arguments, the outermost array of a function parameter is converted into a pointer.

int *p[10]; // array of 10 pointer to int
int (*p)[10]; // pointer to array of 10 ints
void foo(int *p[10] /* pointer to pointer to int */);
void foo(int (*p)[10] /* pointer to array of 10 ints */);

Further, arrays are converted to pointers when you use them.

int x[10]; // array of 10 int
sizeof(x); // 10 * sizeof(int)
int *y = x; // implicitly converts to a pointer to &x[0]!
sizeof(y); // sizeof(int *)

This means that you can allocate memory for an array of arrays, then let that implicitly convert to a pointer to an array, which you in turn use as if it were an array of arrays!

Anyway, this is all very confusing so please do not ever use this in production code – at least, not without a clarifying typedef:

typedef int vector[3];
vector *array_of_vectors; // actually a pointer to a vector, 
                          // but you can use it as an aray to a vector if enough
                          // memory is allocated