Why use double indirection? or Why use pointers to pointers?

Question

When should a double indirection be used in C? Can anyone explain with a example?

What I know is that a double indirection is a pointer to a pointer. Why would I ne

Answer 1

For instance if you want random access to noncontiguous data.

p -> [p0, p1, p2, ...]  
p0 -> data1
p1 -> data2

-- in C

T ** p = (T **) malloc(sizeof(T*) * n);
p[0] = (T*) malloc(sizeof(T));
p[1] = (T*) malloc(sizeof(T));

You store a pointer p that points to an array of pointers. Each pointer points to a piece of data.

If sizeof(T) is big it may not be possible to allocate a contiguous block (ie using malloc) of sizeof(T) * n bytes.