Question

I want to define an array outside main function so that it is shared by all threads not only main thread. When the user runs the program, his argument should be the size of the array. How can I achieve this in C?

Answer 1

You can’t with a true array. You can do it with a pointer:

int *p;

int main(int argc, char *argv[])
{
    size_t x;
    if (argc < 2)
        return;

    size_t x = strtoul(argv[1], NULL, 10);
    p = malloc(x * sizeof *p);    

    return 0;
}