C++ Redefinition Header Files (winsock2.h)

Question

How do I prevent from including header files twice? The problem is I'm including the in MyClass.h and then I'm including MyClass.h in many files, so it includes multiple times and redefinition error occurs. How to prevent?

I'm using #pragma once instead of include guards, and I guess that's fine.

MyClass.h:

// MyClass.h
#pragma once

#include <winsock2.h>

class MyClass
{

// methods
public:
 MyClass(unsigned short port);
 virtual ~MyClass(void);
};

EDIT: Few of the errors I'm getting

c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(91) : warning C4005: 'AF_IPX' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(460) : see previous definition of 'AF_IPX'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(124) : warning C4005: 'AF_MAX' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(479) : see previous definition of 'AF_MAX'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(163) : warning C4005: 'SO_DONTLINGER' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(402) : see previous definition of 'SO_DONTLINGER'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(206) : error C2011: 'sockaddr' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(485) : see declaration of 'sockaddr'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(384) : error C2143: syntax error : missing '}' before 'constant'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(384) : error C2143: syntax error : missing ';' before 'constant'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(384) : error C2059: syntax error : 'constant'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(437) : error C2143: syntax error : missing ';' before '}'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(437) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(437) : error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(518) : warning C4005: 'IN_CLASSA' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(287) : see previous definition of 'IN_CLASSA'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(524) : warning C4005: 'IN_CLASSB' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(293) : see previous definition of 'IN_CLASSB'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(530) : warning C4005: 'IN_CLASSC' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(299) : see previous definition of 'IN_CLASSC'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(541) : warning C4005: 'INADDR_ANY' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(304) : see previous definition of 'INADDR_ANY'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(543) : warning C4005: 'INADDR_BROADCAST' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(306) : see previous definition of 'INADDR_BROADCAST'
c:\program files\microsoft sdks\windows\v6.0a\include\ws2def.h(577) : error C2011: 'sockaddr_in' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(312) : see declaration of 'sockaddr_in'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(132) : error C2011: 'fd_set' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(68) : see declaration of 'fd_set'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(167) : warning C4005: 'FD_SET' : macro redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(102) : see previous definition of 'FD_SET'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(176) : error C2011: 'timeval' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(111) : see declaration of 'timeval'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(232) : error C2011: 'hostent' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(167) : see declaration of 'hostent'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(245) : error C2011: 'netent' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(180) : see declaration of 'netent'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(252) : error C2011: 'servent' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(187) : see declaration of 'servent'
c:\program files\microsoft sdks\windows\v6.0a\include\winsock2.h(264) : error C2011: 'protoent' : 'struct' type redefinition
        c:\program files\microsoft sdks\windows\v6.0a\include\winsock.h(199) : see declaration of 'protoent'

Solution 1

This problem is caused when including <windows.h> before <winsock2.h>. Try arrange your include list that <windows.h> is included after <winsock2.h> or define _WINSOCKAPI_ first:

#define _WINSOCKAPI_    // stops windows.h including winsock.h
#include <windows.h>
// ...
#include "MyClass.h"    // Which includes <winsock2.h>

See also this.

Solution 2

As others suggested, the problem is when windows.h is included before WinSock2.h. Because windows.h includes winsock.h. You can not use both WinSock2.h and winsock.h.

Solutions:

• Include WinSock2.h before windows.h. In the case of precompiled headers, you should solve it there. In the case of simple project, it is easy. However in big projects (especially when writing portable code, without precompiled headers) it can be very hard, because when your header with WinSock2.h is included, windows.h can be already included from some other header/implementation file.

• Define WIN32_LEAN_AND_MEAN before windows.h or project wide. But it will exclude many other stuff you may need and you should include it by your own.

• Define _WINSOCKAPI_ before windows.h or project wide. But when you include WinSock2.h you get macro redefinition warning.

• Use windows.h instead of WinSock2.h when winsock.h is enough for your project (in most cases it is). This will probably result in longer compilation time but solves any errors/warnings.

Solution 3

Oh - the ugliness of Windows... Order of includes are important here. You need to include winsock2.h before windows.h. Since windows.h is probably included from your precompiled header (stdafx.h), you will need to include winsock2.h from there:

#include <winsock2.h>
#include <windows.h>

Solution 4

By using "header guards":

#ifndef MYCLASS_H
#define MYCLASS_H

// This is unnecessary, see comments.
//#pragma once

// MyClass.h

#include <winsock2.h>

class MyClass
{

// methods
public:
    MyClass(unsigned short port);
    virtual ~MyClass(void);
};

#endif

Solution 5

I ran into this problem when trying to pull a third party package which was apparently including windows.h somewhere in it's mess of headers. Defining _WINSOCKAPI_ at the project level was much easier (not to mention more maintainable) than wading through their soup and fixing the problematic include.

View more solutions

Comments

[Kiran Kumar]

You are already using #pragma once, so it should be included only once.

[Svetlozar Angelov]

Your compiler doesn't support pragma once?

[akif]

I'm using Visual Studio 2008, why is then <winsock2.h> included twice?

[Svetlozar Angelov]

It might be included twice from some of the included headers from MyClass.h

[akif]

then how to remedy the problem?

[Kiran Kumar]

@Manzoor: Is this the error you are getting "fatal error C1189: #error : WINDOWS.H already included. MFC apps must not #include <windows.h>"

[Svetlozar Angelov]

winsock2 and winsock have common structures. You have to include just one of them, not the both

[akif]

#pragma once and include guards are same things aren't they?

[Svetlozar Angelov]

I guess I am wrong (4 upvotes by now), but I think using include guards is the same as pragma once, you put them both?

[akif]

Well, I have #pragma once which afaik is the same header guards

[akif]

@Angelov: Yes, that's what I'm saying they are the same things. The problem is not with my header files, but I think <winsock2.h> itself doesn't have header guards or may be something else.

[Kiran Kumar]

@Manzoor: What is the problem your trying address then?

[akif]

The problem is that I include <winsock2.h> in my header and then in another header I include my previous header file and so comes the redefinition of WinSock2 structures. Why?

[Timo Geusch]

They are not quite the same - the header guards will prevent re-inclusion of the file at the preprocessor level, plus they're obviously a tad more portable than #pragma once.

[akif]

I meant they are built for the same purposes :)

[ntcong]

#pragma once is non-standard, afaik

[ntcong]

#pragma once relies on compiler, it's not the same as header guard, which is more portable

[KitsuneYMG]

By definition #pragma is compiler dependent (non-standard). It may not work on all compilers. I know that visual studio accepts #pargma once. I'm not sure if gcc does. I know that include guards ALWAYS work. I use both #pragma once and include guards for maximum protability. It seems that MSVC has optimized handling of #pragma once and gcc has optimized handling of include guards. The only difference with my standard header is that #praga once is outside the include guards.

[DevSolar]

The '#pragma' command is specified in the ANSI standard to have an arbitrary implementation-defined effect. In the GNU C preprocessor, '#pragma' first attempts to run the game 'rogue'; if that fails, it tries to run the game 'hack'; if that fails, it tries to run GNU Emacs displaying the Tower of Hanoi; if that fails, it reports a fatal error. In any case, preprocessing does not continue. -- Richard M. Stallman, The GNU C Preprocessor, version 1.34

[DevSolar]

Above comment posted to stress that #pragma does "something", while #ifndef / #define / #endif is fully portable, standard compliant, and recommended practice. @ Manzoor Ahmed: Try to boil down your problem to the minimum amount of code required to reproduce it. It's a very useful practice for debugging such problems.

[DevSolar]

@ Svetlozar Angelov: I retained the #pragma once from the original code, not being 100% sure what it does, and simply wrapped the whole thing. Shouldn't hurt either way.

[DevSolar]

Then your problem is not related to multiple inclusions of MyClass.h. Again: Make a copy of your code, then methodically cut away files and source lines not necessary to reproduce the error, until you see the problem. Trust me, works every time.

[akif]

I'm not including <windows.h> at all, I know <winsock2.h> does its for me.

[ntcong]

pragma one is known to be faster than header guard, but it's not standard. So sometimes developer use both of them.

[ransomenote]

For me your code compiles ok with only <winsock2.h> in MSVC2008. <windows.h> inclusion makes it generate identical compile errors as you provided.

[Steve Jessop]

@DevSolar: you're way out of date. GCC has supported #pragma once for years.

[DevSolar]

@onebyone: That Richard Stallman quote was humor. You know? Like when you tell people that relying on undefined behaviour might format their hard drives...

[Thomi]

a symbolic link or NTFS junction will also cause the system to break.

[Steve Jessop]

I know, but the questioner isn't relying on undefined behaviour, and Stallman's quote is not applicable. He's relying on defined behaviour of his compiler (just not defined in the C++ standard). So #pragma doesn't do "something", it does exactly what it says in MS's documentation. Accuse me of humourlessness, I'll accuse you of derailing the conversation, someone can chip in with a Nazi analogy, and we'll have the usenet trifecta ;-)

[DevSolar]

Hrr, hrr... I'm game. :-D You might want to note, though, that the original, unedited post I was referring to did not state the compiler being used...

[Steve Jessop]

Unfortunately, I've seen more than zero botched include guards, either where a typo means the guard doesn't actually work, or where files of the same name in different directories use the same token, or where the token used begins with a double underscore or underscore then capital-letter (and hence is non-portable just like #pragma once). So for inherently non-portable code, like anything using winsock.h, I was deeply untroubled by #pragma once right up to the point you said it was flakey. When does it fail, other than not being supported at all?

[Thomi]

When using #pragma once, the compiler takes the header file node name as the unique Id. This can fail if you have symbolic links or NTFS junctions in your source tree (more common than you might think), or even if you have a file of the same name in another system include directory (this has happened to me before when I has version 1 and version 2 of the same library installed to two different system include paths). Bottom line: for me, I prefer to have more control, and live with the occasional wetware mistake, rather than trust a compiler to do it for me.

[Colin Desmond]

Is <windows.h> being included in stdafx.h?

[adamfisk]

This solution fixed the issue for me on VS 2010 with SDK 7.1. Thanks pingw33n!

[Ava]

I had #include <winsock2.h> #include <ws2tcpip.h> #include <windows.h> in order and was getting winsock2,h file not found. Included #define _WINSOCKAPI_ above all 3 includes still the same error

[Jonatan Cloutier]

WIN32_LEAN_AND_MEAN was the solution for me tanks a lot

[Lucky Luke]

Ran into this problem in the past a few times and usually solved it by including winsock2.h first. Didn't know about WINSOCKAPI, and it made things easier in this project. Thanks pingw33n.

[icecream]

I had trouble eliminating all inclusions of winsock.h even though I located all my own inclusions of windows.h. I finally solved it by adding _WINSOCKAPI_ to the project options under Properties->C/C++->Preprocessor in that way making sure that it was defined before all compilation.

[Nitzan Tomer]

thanks so much!!! I do not include any of those, but some other files I'm including do.. Your post made me change the order in which I include those files and it worked. You just saved me hours upon hours of searching, reading and swearing.

[phyatt]

On Qt, in the .pro file, it looks like this: DEFINES += _WINSOCKAPI_

[Leif Gruenwoldt]

@phyatt: you should turn that into an answer, if you don't I will!

[phyatt]

@LeifGruenwoldt go for it! Glad I could help.

[AndroC]

Im using VS2013. Putting _WINSOCKAPI_ in Project Properties > Configuration Properties > C/C++ > Preprocessor > Preprocessor definitions for both Debug and Release configurations did the trick for me.

[Paweł Stankowski]

About _WINSOCK_ solution: You shouldn't grt macro redefinition warning if both definitions are identical. The common bug is that people add definition to the project without setting any value and expect empty definition. However, if you add -D_WINSOCK_ to cmd line, it will set _WINSOCK_ to 1. To create empty definition, -D_WINSOCK_= must be passed.

[sqr163]

@pingw33n Thank you! helped to compile some C++ code with VS2014

[Lorien Brune]

If you use #define _WINSOCKAPI_, you may also need #define _WINSOCK_DEPRECATED_NO_WARNINGS, depending on your circumstances.

[Elmue]

In my case including "windows.h" as the LAST of all includes removed the compiler error.