C中的外部语法？

Question

当我涉及一个全局变量时，我在我的c代码中使用“extern”时有点困惑。 我尝试了以下操作，并收到了编译错误：

MAIN.C：

extern unsigned short *videobuffer;
//I also tried this in a separate and it failed with the same compilation error//
extern (unsigned short *)videobuffer;

lib.c：

unsigned short *videobuffer = (unsigned short *)0x6000000;

我收到的错误：

[COMPILE] Compiling main.c
main.c:16: error: expected identifier or '(' before 'unsigned'
make: *** [main.o] Error 1

Answer 1

有效代码通常是人们想要的：

的main.c

#include <stdio.h>
extern unsigned short *videobuffer;

int main() {
    printf("%p\n", videobuffer);
}

lib.c

unsigned short *videobuffer = (unsigned short *)0x6000000;

Answer 2

以下是正确的：

MAIN.C：

extern unsigned short *videobuffer;

lib.c：

unsigned short *videobuffer = (unsigned short *)0x6000000;

extern关键字告诉编译器有一个变量，但它不在这个编译单元中。