c - 定义 = 时预处理器错误

Question

我正在尝试一些笨拙的预处理并想出了这样的东西:

#include <stdio.h>

#define SIX =6

int main(void)
{
  int x=6;
  int y=2;

  if(x=SIX)
    printf("X == 6\n");
  if(y=SIX)
    printf("Y==6\n");

  return 0;
}

gcc 给我错误:

test.c: In function ‘main’:
test.c:10:8: error: expected expression before ‘=’ token
test.c:12:8: error: expected expression before ‘=’ token

这是为什么？

Answer 1

==是一个单独的token，不能一分为二。你应该在你的代码上运行 gcc -E

来自 GCC 手册页:

-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

Input files that don't require preprocessing are ignored.

对于您的代码，gcc -E 给出以下输出

  if(x= =6)
    printf("X == 6\n");

  if(y= =6)
    printf("Y==6\n");

第二个 = 是导致错误消息的原因 expected expression before ‘=’ token