头文件中声明的extern变量的链接器错误

Question

I have a header file ah which is included in 2 .c files ac and bc

ah

#ifndef A_H_INCLUDE
#define A_H_INCLUDE
extern int g;
void chk(int f);
#endif

ac

# include <stdio.h>
# include "a.h"

void chk(int f)
{
    if(g==1) printf("\n CORRECT\n");
    else printf("\n INcorrect::%d: \n",f);
}   

bc

# include <stdio.h>
# include "a.h"

int main()
{
  int g;
  printf("\n ENTER::");
  scanf("%d",&g);
  chk(56);
  return 0;
}

On compiling the code it is giving me error

gcc a.c b.c a.h -o j
/tmp/ccEerIPj.o: In function `chk':
a.c:(.text+0x7): undefined reference to `g'
collect2: ld returned 1 exit status

I have already checked Variable declaration in a header file and i guess i am doing the same thing.

Any suggestion or pointer to problem in code will be appreciated.

Many thanks

Answer 1

Add int g; to the global scope of your ac file to provide a definition for the variable.

(In case you're wondering, the local variable inside your main() function has no linkage, since it's an automatic variable at block scope. It has nothing to do with your problem.)