meaning of void*

kpachopoulos · 10-12-2005, 04:07 PM

What does "void*" mean in c code? I have only used java and in general i understand pointers, but i cannot understand this.
First this can be used both as a return value and as a parameter for a function. Can it?
When do we use it? Reading some code, i have only associated it with allocated part/s of memory Can somebody explain? A couple of examples maybe?
Thanks

itsme86 · 10-12-2005, 04:27 PM

You use it when you want a pointer of an unspecified type. For instance, malloc() returns a void * because it has no idea what you're going to assign that pointer to. You might want it assigned to an int *, or a char *, or even a struct my_struct *.

Yes, you can also use it as a parameter to a function. Say you want to create a function that can add ints or floats. You could make it something like this:

Code:

void *sum(void *a, void *b, int is_flt)
{
  if(is_flt)
    return (*(float *)a) + (*(float *)b);
  else
    return (*(int *)a) + (*(int *)b);
}

Then you can call it something like:

Code:

int a = 5, b = 7;

sum(&a, &b, 0);

And likewise for floats except the 3rd argument would be 1 instead of 0.

Hko · 10-12-2005, 04:39 PM

Quote:

Originally posted by nocturna_gr
What does "void*" mean in c code?

It means "pointer to no-specific-type".

Quote:

First this can be used both as a return value and as a parameter for a function. Can it?

That's correct. If it's used as a return value, the function returns a pointer to some block of memory, while not stating of which type it is. It can be used for chars, int's, binary data,..

Quote:

When do we use it? Reading some code, i have only associated it with allocated part/s of memory

Often it's known of which type the pointer we are dealing with and we don't use void*. But consider for example malloc() that talks to the kernel to get (allocate) a block of memory for your program. It's of course impossible for malloc() to know in advance for which type of variable you are going to use it. So malloc() returns a non-type-specific pointer: void* (see man malloc).

Sometimes it can be handy to assign a type (cast) such a block of code. E.g. if you are going to store int's in it because int's are 4 bytes. When you have a pointer int *idx it will advanced 4 bytes when you do idx++, and 12 bytes if you do idx = idx + 3.

To assign a type int to a block of memory returned by malloc():

Code:

int *intblockpointer;
int *idx;

intblockpointer = (int*)malloc(1000 * sizeof(int));

/* Find the 0 */
idx = intblockpointer;
while (*idx <> 0) {
    idx++ /* Actually advances 4 bytes (sizeof(int) == 4) */
}
free(intblockpointer);

Note: this isn't good code, it's just a short example.
It's not good because if it doesn't find a 0-int, the idx pointer goed wild.

deiussum · 10-12-2005, 04:48 PM

Since you say you understand Java, you can also think of a void* as something similar to Java's object class. Everything in Java inherits from the object class, so you can use that as a parameter to functions that you want to be able to take anything.

However in C++ there is no one class that all other classes derive from like this. And in C, you don't even have classes or inheritance. So, as has already been stated, you can use void* to get a pointer to any type you want.