171

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'
Leif Gruenwoldt
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akif
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15 Answers15

283

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.

pingw33n
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  • I'm not including at all, I know does its for me. – akif Sep 03 '09 at 11:26
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    For me your code compiles ok with only `` in MSVC2008. `` inclusion makes it generate identical compile errors as you provided. – pingw33n Sep 03 '09 at 11:41
  • Is being included in stdafx.h? – Colin Desmond Oct 04 '09 at 19:49
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    This solution fixed the issue for me on VS 2010 with SDK 7.1. Thanks pingw33n! – adamfisk Sep 24 '11 at 07:20
  • I had `#include #include #include ` in order and was getting winsock2,h file not found. Included `#define _WINSOCKAPI_ `above all 3 includes still the same error – Ava Mar 08 '12 at 19:06
  • 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. – Lucky Luke Aug 15 '12 at 15:54
  • 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. – icecream Jun 20 '13 at 07:22
  • 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. – Nitzan Tomer Aug 08 '13 at 14:45
  • 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. – AndroC Oct 05 '16 at 11:03
  • @pingw33n Thank you! helped to compile some C++ code with VS2014 – sqr163 Jul 07 '17 at 06:19
91

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.

Pavel Machyniak
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    `WIN32_LEAN_AND_MEAN` was the solution for me tanks a lot – Jonatan Cloutier Jun 29 '12 at 21:52
  • 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. – Paweł Stankowski Jan 09 '17 at 06:20
  • If you use `#define _WINSOCKAPI_`, you may also need `#define _WINSOCK_DEPRECATED_NO_WARNINGS`, depending on your circumstances. – Lorien Brune Sep 29 '17 at 21:20
18

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>
Daniel Paull
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7

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.

Yaur
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7

I checked the recursive includes, I spotted the header files which include (recursively) some #include "windows.h" and #include "Winsock.h" and write a #include "Winsock2.h". in this files, i added #include "Winsock2.h" as the first include.

Just a matter of patience, look at includes one by one and establish this order, first #include "Winsock2.h" then #include "windows.h"

canton7
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kiriloff
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  • In my case including "windows.h" as the LAST of all includes removed the compiler error. – Elmue Nov 16 '20 at 15:49
7

I found this link windows.h and winsock2.h which has an alternative that worked great for me:

#define _WINSOCKAPI_    // stops windows.h including winsock.h
#include <windows.h>
#include <winsock2.h>

I was having trouble finding where the issue occurred but by adding that #define I was able to build without figuring it out.

Benjamin Herreid
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6

In VS 2015, the following will work:

#define _WINSOCKAPI_

While the following won't:

#define WIN32_LEAN_AND_MEAN
g00glen00b
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MariuszW
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4

I wouldn't use just FILENAME_H but

#ifndef FILENAME_H_AF06570D_B36E_4B82_8F97_C456AF4A38FD
#define FILENAME_H_AF06570D_B36E_4B82_8F97_C456AF4A38FD

//code stuff
#endif // FILENAME_H_AF06570D_B36E_4B82_8F97_C456AF4A38FD

I have always used a postfix guid. I came across a very poor code base some years ago that had different header files with the same file name and include guard. The files in question had defined a class with the same name. If only namespaces were used. Some projects compiled some didn't. Using unique guards was a part of the solution in differentiating headers and their contents.

On Windows with Visual Studio use guidgen.exe, on Linux uuidgen -t.

Sam
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4

I've run into the same issue and here is what I have discovered so far:

From this output fragment - 

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'

-It appears that both ws2def.h and winsock.h have been included in your solution.

If you look at the file ws2def.h it starts with the following comment -

/*++

Copyright (c) Microsoft Corporation. All rights reserved.

Module Name:

    ws2def.h

Abstract:

    This file contains the core definitions for the Winsock2
    specification that can be used by both user-mode and 
    kernel mode modules.

    This file is included in WINSOCK2.H. User mode applications
    should include WINSOCK2.H rather than including this file
    directly. This file can not be included by a module that also
    includes WINSOCK.H.

Environment:

    user mode or kernel mode

--*/

Pay attention to the last line - "This file can not be included by a module that also includes WINSOCK.H"

Still trying to rectify the problem without making changes to the code.

Let me know if this makes sense.

Shadow The GPT Wizard
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Shailesh Tainwala
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2

#pragma once is based on the full path of the filename. So what you likely have is there are two identical copies of either MyClass.h or Winsock2.h in different directories.

soru
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1

You should use header guard.

put those line at the top of the header file

#ifndef PATH_FILENAME_H
#define PATH_FILENAME_H

and at the bottom

#endif
ntcong
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In my project (I use VS 2008 SP1) works next solution:

Header file:

//myclass.h
#pragma once
#define _WINSOCKAPI_
#include <windows.h>

Cpp class:

//myclass.cpp
#include "Util.h"
#include "winsock2class.h"
#pragma comment(lib, "Ws2_32.lib")

where #include "winsock2class.h" mean class which implemented winsock2.h :

//winsock2class.h
#include <winsock2.h>
#include <windows.h>
#pragma comment(lib, "Ws2_32.lib")
Yahor M
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#pragma once is flakey, even on MS compilers, and is not supported by many other compilers. As many other people have mentioned, using include guards is the way to go. Don't use #pragma once at all - it'll make your life much easier.

Thomi
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    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? – Steve Jessop Sep 03 '09 at 14:34
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    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. – Thomi Sep 03 '09 at 15:12
0

#include guards are the standard way of doing this. #pragma once is not, meaning that not all compilers support it.

Dima
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I actually ran into an issue where I had to define winsock2.h as the first include, it seems it has other issues with includes from other packages. Hope this is helpful to someone who runs into same issue, not just windows.h but all includes.

Jeff
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