关键字 extern 对该变量作“外部变量声明”，表示该变量是一个已经定义的外部变量。有了此声明，就可以从“声明”处起，合法地使用该外部变量
#include "stdio.h"
#include "stdlib.h"
#include "utils.h"
#include "gnu/libc-version.h"
void main_test(void)
{
printf("TEST ENTRY\n");
printf("GNU libc version: %s\n", gnu_get_libc_version());
/* int g_x, g_y;*/
extern int g_x, g_y;
printf("g_x=%d, g_y=%d\n", g_x, g_y);
}
int g_x = 0;
int g_y = 1;
void test(void)
{
printf("test, g_x=%d, g_y=%d\n", g_x, g_y);
}