How to declare the size of an array at runtime in C?

Question

I basically want to the C of equivalent of this (well, just the part with the array, I don't need the class and string parsing and all that):

public class Example
{
    static int[] foo;
    public static void main(String[] args)
    {
     int size = Integer.parseInt(args[0]);
     foo = new int[size]; // This part
    }
}

Pardon my C ignorance. I've been corrupted by java ;)

Answer 1

/* We include the following to get the prototypes for:
 * malloc -- allocates memory on the freestore
 * free   -- releases memory allocated via above
 * atoi   -- convert a C-style string to an integer
 * strtoul -- is strongly suggested though as a replacement
*/
#include <stdlib.h>
static int *foo;
int main(int argc, char *argv[]) {
    size_t size = atoi(argv[ 1 ]); /*argv[ 0 ] is the executable's name */
    foo = malloc(size * sizeof *foo); /* create an array of size `size` */
    if (foo) {  /* allocation succeeded */
      /* do something with foo */
      free(foo); /* release the memory */
    }
    return 0;
}

Caveat:Off the cuff stuff, without any error checking.

Answer 2

In C, you can do that with this, if you ignore the error checking:

#include <stdlib.h>
static int *foo;

int main(int argc, char **argv)
{
     int size = atoi(argv[1]);
     foo = malloc(size * sizeof(*foo));
     ...
}

If you don't want a global variable and you are using C99, you could do:

int main(int argc, char **argv)
{
    int size = atoi(argv[1]);
    int foo[size];
    ...
}

This uses a VLA - variable length array.

Answer 3

int count = getHowManyINeed();
int *foo = malloc(count * sizeof(int));

Answer 4

Note that the memory pointed by foo (which is what we call an array in C) will be filled with uninitialized values.

Answer 5

If you need to initialize the data, you can use calloc:

int* arr = calloc (nb_elems, sizeof(int));
/* Do something with your array, then don't forget to release the memory */
free (arr);

This way, the allocated memory will be initialized with zeroes, which can be useful. Note that you can use any data type instead of int.