question on key work "extern"(C prog)

Hello,all.
I have just writed a example for practice the key word "extern" which is used in aspect of function.
I typed it in a file that named main.c and the codes are shown as in following.

error info:

Code:

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/tmp/......
undefined reference 'ret'

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main.c:

Code:

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#include <stdio.h>

extern int ret(int);

int main(void)
{
      int  i,j;
      printf("i:");
      scanf("%d",&i);
      j=ret(i);
      printf("\n%d\n",j);
      return 0;
}

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test.c

Code:

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int ret(int a)
{
    return a*a;
}

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You need to compile both files together.

Besides the extern keyword here has no effect. External linkage is the default for functions. It only has an effect with variables.

thanks, CordneBee.How supernatural!It is so fun.