多次包含头文件
#include <stdio.h>
#include <stdio.h>
int main ()
{
printf ("hello world");
return 0;
}
当我编译它时，编译器不会因包含 stdio.h 两次而给出任何警告/错误。为什么会这样呢？ scanf、printf 等函数现在不是声明和定义了两次吗？
提前致谢
#include <stdio.h>
#include <stdio.h>
int main ()
{
printf ("hello world");
return 0;
}
when I compile this, the compiler doesn't give any warning/error for including stdio.h twice. Why is it so? Aren't the functions scanf, printf etc. declared and defined twice now?
Thanks, in advance
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通常，头文件的编写方式类似于以下示例，以防止此问题：
然后，如果包含多次，则第二个实例会跳过内容。
Typically, header files are written similar to the below example to prevent this problem:
Then, if included more than once, then 2nd instance skips the content.
除了使用包含防护之外，正如 Mark Tolonen 的回答所指出的，只要声明兼容，多次声明一个函数就没有问题。这完全没问题：
事实上，由于每个定义也是一个声明，因此每当您“前向声明”在同一文件中声明的函数时，您就将该函数声明了两次。
不可以的是创建一个函数的多个外部定义；但编写良好的头文件不应创建外部定义 - 仅应创建声明。
printf和scanf函数的（单个）定义应位于目标文件中，该文件在构建程序时与程序链接。In addition to the use of include guards, as pointed out by Mark Tolonen's answer, there is no problem with declaring a function more than once, as long as the declarations are compatible. This is perfectly fine:
In fact, since every definition is also a declaration, whenever you "forward-declare" a function declared in the same file, you are declaring the function twice.
What is not OK is to create multiple external definitions of a function; but well-written header files should not create external definitions - only declarations. The (single) definition of the
printfandscanffunctions should be in an object file, which is linked with your program when it is built.不，头文件通常定义一个标志，然后仅在该标志未定义时才使用
#ifndef包含自身。打开一个看看。
No, the header files usually define a flag and then use
#ifndefto include themselves only if the flag was undefined.Open one up and see.
顺便说一句，“#ifndef”技巧适用于其他人使用的标头（如标准标头）。
如果您需要 #ifndef 作为“私有”程序，那么您就做错了。也就是说，您可以而且应该在项目中组织标题，这样它们就不会被多次包含。
这样做有用的原因之一是可以防止您认为已删除的标题再次弹出。
由于您无法控制公共标头的使用方式，因此此技巧对于公共标头来说是合理的。对于私有标头来说，这个技巧是完全没有必要的。
As an aside, doing the "#ifndef" trick is appropriate for headers used by other people (like the standard headers).
If you need the #ifndef for a "private" program, then you are doing it "wrong". That is, you can and should organize headers within a project so they are not included more than once.
One reason that this is helpful is that keeps headers you think you have deleted from popping up again.
Since you can't control how public headers are used, this trick is reasonable for public headers. This trick is completely unnecessary for private headers.