using global variables inside main() in c

Question

I'm trying to use global variables, but I'm getting an error when I'm using it inside main function. Why can't I use "extern" inside main? Codeblocks is showing error in this area only, so I figured out that this is the problem in my code,It is compilation error. I'm using extern because I don't have to send these arrays to the other function when calling from main()

So what I modify I dorder to make my code work?

I'm trying to print array of integers,intialized with rand().

#include<stdio.h>

int check_error(int);
void initialize_array1(int);
void initialize_array2(int);
void print_array1(int);
void print_array2(int);
int find_max(int , int);

void main(void)
{
    int input1,input2;
    printf("Enter the size of the first input: ");
    scanf("%d",&input1);
    while( (check_error(input1)) == 0)
    {
        printf("Enter the size of the input again: ");
        scanf("%d",&input1);
    }

    printf("Enter the size of the second input: ");
    scanf("%d",&input2);
    while( (check_error(input2)) == 0)
    {
        printf("Enter the size of the input again: ");
        scanf("%d",&input2);
    }
    extern int array1[input1], array2[input2];

    print_array1(input1);
    print_array2(input2);
    find_max(input1,input2);

    printf("\n%d\n",find_max(input1,input2));
}

int check_error(int input)
{
    if(input>0 && input<100)
    {
        return 1;
    }
    else
        return 0;
}

void initialize_array1(int input1)
{
    int i;
    for(i=0;i<input1;i++)
    {
        array1[i] = 0;
    }
}


void initialize_array1(int input2)
{
    int i;
    for(i=0;i<input2;i++)
    {
        array2[i] = 0;
    }
}

void print_array1(int input1)
{
    int i;
    printf(" Input array 1");
    for(i=0;i<input1;i++)
    {
        printf("%d ",array1[i]);
    }
    printf("\n");
}

void print_array2(int input2)
{
    int i;
    printf(" Input array 2");
    for(i=0;i<input2;i++)
    {
        printf("%d ",array2[i]);
    }
    printf("\n");
}

int find_max(int input1, int input2)
{
    int max = 0,i;
    for(i=0;i<input1;i++)
    {
        if(array1[i]>max)
        {
            max = array1[i];
        }
    }
    for(i=0;i<input2;i++)
    {
        if(array2[i]>max)
        {
            max = array2[i];
        }
    }
    return max;
}

You forgot to tell us what the error is. Does it compile? Does it link? Also, what are you expecting `extern int array1[input1], array2[input2];` to do? — David Schwartz, Jul 02 '13 at 18:46

score 2 · Answer 1 · edited May 23 '17 at 10:32

This code is incorrect for two reasons:

You declare a variable-length array as external. This is not allowed.
Even if the arrays were fixed-length, you would need to define these arrays some place else.

To fix this code, define array1 and array2 with the max size that you expect the users to enter (add validation to make sure that the input size does not exceed max).

Here is a link to a answer on a related subject: What are extern variables in C?

meaning-matters · Answer 2 · 2013-07-02T19:20:20.193

1

Using extern you're declaring that this variable is defined somewhere else. BTW, you normally only define (e.g. int i;) variables inside a function block.

Something else: Declaring a variable's existence is not enough, it must also be defined somewhere (i.e. space must be set aside for it).

To have a global variable, you at least must define it outside a function.

I think that @dasblinkenlight added a perfect link to his +1 answer!

edited Jul 02 '13 at 19:20

answered Jul 02 '13 at 18:54

meaning-matters

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You report 'You can only _define_ variables inside a function block." Although unusual, _declaring_ variables inside a function block appears to be valid code. e.g. `int foo(void) { extern int bar; return bar; } int bar;` Please advise. – chux - Reinstate Monica Jul 02 '13 at 19:18
@chux Thanks for the valid remark! Updated my answer. Thanks for not decrementing immediately. – meaning-matters Jul 02 '13 at 19:21
1

Look at my history - no downvotes. And what you stated certainly would not deserve one. Further, using extern inside a function opens up an information hiding method of which I was unaware. – chux - Reinstate Monica Jul 02 '13 at 19:40
@chux Great! What do you mean with "information hiding method"? – meaning-matters Jul 02 '13 at 19:56
Information hiding is the control of what one module/class/set of routines sees. It should see all it needs, no more. It should make available only what it should and no more. Well I could go on, but sounds like a good question - maybe its answered all ready. – chux - Reinstate Monica Jul 02 '13 at 20:03
@chux I meant, in this context with the `extern` declaration inside a function. I know what information hiding in general terms is. – meaning-matters Jul 03 '13 at 04:54

using global variables inside main() in c

2 Answers2