What does the declaration void** mean in the C language?

Question

I'm beginning to learn C and read following code:

public void** list_to_array(List* thiz){
    int size = list_size(thiz);
    void **array = malloc2(sizeof(void *) * size);
    int i=0;
    list_rewind(thiz);
    for(i=0; i<size; i++){
        array[i] = list_next(thiz);
    }
    list_rewind(thiz);
    return array;
}

I don't understand the meaning of void**. Could someone explain it with some examples?

In this instance, it means "pointer to an array of void* pointers" — Variable Length Coder, Jul 12 '12 at 02:07
@VariableLengthCoder, no that would be `void* (*array)[]`. You probably mean "pointer to the first element of an array of `void` pointers". — Jens Gustedt, Jul 12 '12 at 04:41
@mlzboy do consider closing this question by accepting an answer with which you are satisfied. — Ravindra S, Apr 08 '17 at 16:56

score 4 · Answer 1 · edited May 23 '17 at 12:25

4

void** is a pointer to a pointer to void (unspecified type). It means that the variable (memory location) contains an address to a memory location, that contains an address to another memory location, and what is stored there is not specified. In this question's case it is a pointer to an array of void* pointers.

Sidenote: A void pointer can't be dereferenced, but a void** can.

void *a[100];
void **aa = a;

By doing this one should be able to do e.g. aa[17] to get at the 18th element of the array a.

To understand such declarations you can use this tool and might as well check a related question or two.

edited May 23 '17 at 12:25

Community

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answered Jul 12 '12 at 19:59

Ravindra S

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Answer doesn't stand on its own - if and when those links break, this will have zero value. – dwanderson Mar 26 '17 at 18:33
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@dwanderson Hey thanks. I have updated the answer. I must have been bit lazy back in 2012 when I wrote this answer. My bad. :) – Ravindra S Apr 08 '17 at 16:55
Impressive follow up! always appreciate when people respond to years-old questions. Thanks! – dwanderson Apr 10 '17 at 18:47

cybertextron · Answer 2 · 2012-07-12T11:46:25.527

1

void** is a pointer to void*, or a pointer to a void pointer if you prefer! This notation is traditionally used in C to implement a matrix, for example. So, in the matrix case, that would be a pointer to an array of pointers.

edited Jul 12 '12 at 11:46

answered Jul 12 '12 at 01:54

cybertextron

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It's a pointer to a `void` pointer, but what's a `void` pointer to a `void` pointer? – Carl Norum Jul 12 '12 at 03:01
@CarlNorum I'm sorry for any mistyping. – cybertextron Jul 12 '12 at 11:47

score 0 · Answer 3 · edited Jul 12 '12 at 20:09

Normally void * pointers are used to denote a pointer to an unknown data type. In this case your function returns an array of such pointers thus the double star.

In C, a pointer is often used to reference an array. Eg the following assignment is perfectly legal:

char str1[10];
char *str2 = str1;

Now when void is used, it means that instead of char you have a variable of unknown type.

Pointers to an unknown data type are useful for writing generic algorithms. Eg. the qsort function in standard C library is defined as:

void qsort ( void * base, 
             size_t num, 
             size_t size, 
             int ( * comparator ) 
             ( const void *, const void * ) );

The sorting algorithm itself is generic, but has no knowledge of the contents of the data. Thus the user has to provide an implementation of a comparator that can deal with it. The algorithm will call the comparator with two pointers to the elements to be compared. These pointers are of void * type, because there is now information about the type of data being sorted.

Take a look at this thread for more examples http://forums.fedoraforum.org/showthread.php?t=138213

-1 for saying that `char **strarray2 = strarray1;` is "perfectly legal". — Jens Gustedt, Jul 12 '12 at 04:44
Jens is right, `char[][]` is equivalent to `char[]` (it's just convenience syntax that lets the compiler do 2D array pointer arithmetic for you), while a `char**` is an array of pointers to char arrays. — Adam, Jul 12 '12 at 05:09
Thanx for pointing that out, I did not have a C compiler at hand when I answered it to correct me :( — anttix, Jul 13 '12 at 02:16

score 0 · Answer 4 · answered Jul 29 '12 at 07:57

void pointers are used to hold address of any data type. void** means pointer to void pointer. Void pointers are used in a place where we want a function should receive different types of data as function argument. Please check the below example

void func_for_int(void *int_arg)
{
    int *ptr = (int *)int_arg;
    //some code
}

void func_for_char(void *char_arg)
{
    char *ptr = (char *)char_arg;
    //some code
}

int common_func(void * arg, void (*func)(void *arg))
{
    func(arg);
}

int main()
{
    int a = 10;
    char b = 5;

    common_func((void *)&a, func_for_int);
    common_func((void *)&b, func_for_char);

    return 0;
}

What does the declaration void** mean in the C language?

4 Answers4