In C, what does a variable declaration with two asterisks (**) mean?

It declares a pointer to a char pointer.

The usage of such a pointer would be to do such things like:

void setCharPointerToX(char ** character) {   *character = "x"; //using the dereference operator (*) to get the value that character points to (in this case a char pointer}char *y;setCharPointerToX(&y); //using the address-of (&) operator hereprintf("%s", y); //x

Here's another example:

char *original = "awesomeness";char **pointer_to_original = &original;(*pointer_to_original) = "is awesome";printf("%s", original); //is awesome

Use of ** with arrays:

char** array = malloc(sizeof(*array) * 2); //2 elements(*array) = "Hey"; //equivalent to array[0]*(array + 1) = "There";  //array[1]printf("%s", array[1]); //outputs There

The [] operator on arrays does essentially pointer arithmetic on the front pointer, so, the way array[1] would be evaluated is as follows:

array[1] == *(array + 1);

This is one of the reasons why array indices start from 0, because:

array[0] == *(array + 0) == *(array);

C and C++ allows the use of pointers that point to pointers (say that five times fast). Take a look at the following code:

char a;char *b;char **c;a = 'Z';b = &a; // read as "address of a"c = &b; // read as "address of b"

The variable a holds a character. The variable b points to a location in memory that contains a character. The variable c points to a location in memory that contains a pointer that points to a location in memory that contains a character.

Suppose that the variable a stores its data at address 1000 (BEWARE: example memory locations are totally made up). Suppose that the variable b stores its data at address 2000, and that the variable c stores its data at address 3000. Given all of this, we have the following memory layout:

MEMORY LOCATION 1000 (variable a): 'Z'MEMORY LOCATION 2000 (variable b): 1000 <--- points to memory location 1000MEMORY LOCATION 3000 (variable c): 2000 <--- points to memory location 2000

It means that aPointer points to a char pointer.

So

aPointer: pointer to char pointer*aPointer :pointer to char**aPointer: char

An example of its usage is creating a dynamic array of c strings

char **aPointer = (char**) malloc(num_strings);

aPointer gives you a char, which can be used to represent a zero-terminated string.

*aPointer = (char*)malloc( string_len + 1); //aPointer[0]*(aPointer + 1) = (char*)malloc( string_len + 1); //aPointer[1]