Question

How to use extern variable and extern function in C

Answer 1

test1.h

extern int extern_n;

test1.c

#include "test1.h"

extern_n = 100;

int add_one(void)
{
    return ++extern_n;
}

test2.c

#include <stdio.h>

#include "test1.h"

void my_print(void)
{
    printf("extern variable: %d\n", extern_n);
}

main.h

extern void my_print(void);
extern int add_one(void);

main.c

#include <stdio.h>

#include "test1.h"
#include "main.h"

int main(int argc, char **argv) 
{ 
    my_print();
    
    extern_n = add_one();
    my_print();
    
    return 0; 

}


/*
  
run:
   
extern variable: 100
extern variable: 101

*/

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