Question

I am doing an assignment where I need to use pthreads or semaphores to synchronize some processes which access some shared resource. Since all of our examples in class use a global variable as the shared resource I planned on doing the same thing, but I wanted to base the value of the shared resource on a command line argument. I know how to use command line arguments within my main method, but how do I define the size of a global array (the shared resource) based on a command line argument?

Update:

Wallyk’s answer seems like it will work, but I’m still fuzzy on some of the finer details. See the example and comments…

#include <stdio.h>

void print_array(void);

int *array;
int count;

int main(int argc, char **argv){
    int count = atoi(argv[1]);
    array = malloc(count *sizeof(array[0]));
    int i;
    for(i = 0; i < count; i++){ /*is there anyway I can get the size of my array without using an additional variable like count?*/
        array[i] = i;
    }
    print_array();
    return 0;
}

void print_array(){
    int i;
    for(i = 0; i < count; i++){
        printf("current count is %d\n", array[i]);
    }
}

Answer 1

You can’t do a static dynamic declaration like:

int globalarray[n];

Where n is a variable set at runtime. This doesn’t work because the array is initialized before the program begins running.

A good alternative is to use a pointer to dynamic memory:

int *globalarray;

int main (int argc, char **argv)
{
...
   int elements = atoi (argv [j]);  // parse out the program argument array size
   globalarray = malloc (elements * sizeof (globalarray[0]));
}