两种头文件包含函数的区别_in file included from main.c

两种头文件包含函数的区别

In file included from main.c:3:
b/test.h:3: warning: "PI" redefined
    3 | #define PI 3.1415926
      | 
In file included from main.c:2:
a/test.h:3: note: this is the location of the previous definition
    3 | #define PI 3.14
      | 
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在多个目录下定义头文件时如果有相同头文件时候#pragma once会警告并且把后面的覆盖前面的，而#ifndef则不会进入第二个。

main.c

#include <stdio.h>
#include "a/test.h"
#include "b/test.h"

int main(int argc, char const *argv[])
{
    printf("%lf\n",PI);
    return 0;
}

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a/test.h

#pragma once

#define PI 3.14

// #ifndef _TEST_H_
// #define _TEST_H_
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