I have a header file x.h which is included by more than one *.c source files.
This header file has some structure variables defined.
I have put multiple inclusion prevention guard at the beginning of the header file as:
#ifndef X_H
#define X_H
...
..
//header file declarations and definitons.
#endif//X_H
On building I get linker errors related to multiple definitions. I understand the problem.
Won't a multiple inclusion prevention guard at the top of header file as I have, prevent multiple inclusions of the header file x.h and thereby avoid multiple definitions of the variables that are there in x.h?
#pragma once does not work on this particular compiler, so what is the solution?
Someone had posted this answer to a similar question. It doesn't seem to work for me. How does this solution work?
If the linker is complaining, it means you have definitions rather than just declarations in your header. Here's an example of things that would be wrong.
#ifndef X_H
#define X_H
int myFunc()
{
return 42; // Wrong! definition in header.
}
int myVar; // Wrong! definition in header.
#endif
You should split this into source and header file like this:
Header:
#ifndef X_H
#define X_H
extern int myFunc();
extern int myVar;
#endif
C Source:
int myFunc()
{
return 42;
}
int myVar;
Header guards are only good for a single compilation unit, i.e., source file. If you happen to include a header file multiple times, perhaps because all headers included from main.c in turn include stdio.h then guards will help.
If you have the definition of a function f in x.h which is included by main.c and util.c, then it is like copying and pasting the definition of f into main.c when creating main.o and doing the same for util.c to create util.o. Then the linker will complain and this happens despite your header guards. Having multiple #include "x.h" statements in main.c is possible of course because of these guards.
Using include guards prevents one compilation unit from including the header twice. E.g. if header B.h includes A.h and B.cpp includes A.h and B.h, everything from A.h would be declared twice in the compilation B.cpp if you weren't using include guards.
Your include guards prevent this from happening, all's fine till now.
But you get multiple definitions at link time, i.e. two compilation units define the same thing, this probably means you got a real definition in your header, use extern for all variables, make sure functions are either inline or are defined in the cpp file.
If the functions aren't large, you can use "inline" before them and the linker won't complain.
Using a multiple inclusion guard prevents compiler errors, but you're getting a linker error. Do you have data definitions in the header file that don't use extern?
Maybe X_H is already defined somewhere else? I just ran into this issue, where Xlib defines X_H in /usr/include/X11/X.h.
To check, you can call gcc -dM -E (if you are using gcc), e.g. in the buildsystem I’m using that works with CC=gcc CFLAGS="-dM -E" make. If the output file contains #define X_H even though you remove it from your file (use Y_H for example), then it is already defined outside your source code.
Related
I have a header file x.h which is included by more than one *.c source files.
This header file has some structure variables defined.
I have put multiple inclusion prevention guard at the beginning of the header file as:
#ifndef X_H
#define X_H
...
..
//header file declarations and definitons.
#endif//X_H
On building I get linker errors related to multiple definitions. I understand the problem.
Won't a multiple inclusion prevention guard at the top of header file as I have, prevent multiple inclusions of the header file x.h and thereby avoid multiple definitions of the variables that are there in x.h?
#pragma once does not work on this particular compiler, so what is the solution?
Someone had posted this answer to a similar question. It doesn't seem to work for me. How does this solution work?
If the linker is complaining, it means you have definitions rather than just declarations in your header. Here's an example of things that would be wrong.
#ifndef X_H
#define X_H
int myFunc()
{
return 42; // Wrong! definition in header.
}
int myVar; // Wrong! definition in header.
#endif
You should split this into source and header file like this:
Header:
#ifndef X_H
#define X_H
extern int myFunc();
extern int myVar;
#endif
C Source:
int myFunc()
{
return 42;
}
int myVar;
Header guards are only good for a single compilation unit, i.e., source file. If you happen to include a header file multiple times, perhaps because all headers included from main.c in turn include stdio.h then guards will help.
If you have the definition of a function f in x.h which is included by main.c and util.c, then it is like copying and pasting the definition of f into main.c when creating main.o and doing the same for util.c to create util.o. Then the linker will complain and this happens despite your header guards. Having multiple #include "x.h" statements in main.c is possible of course because of these guards.
Using include guards prevents one compilation unit from including the header twice. E.g. if header B.h includes A.h and B.cpp includes A.h and B.h, everything from A.h would be declared twice in the compilation B.cpp if you weren't using include guards.
Your include guards prevent this from happening, all's fine till now.
But you get multiple definitions at link time, i.e. two compilation units define the same thing, this probably means you got a real definition in your header, use extern for all variables, make sure functions are either inline or are defined in the cpp file.
If the functions aren't large, you can use "inline" before them and the linker won't complain.
Using a multiple inclusion guard prevents compiler errors, but you're getting a linker error. Do you have data definitions in the header file that don't use extern?
Maybe X_H is already defined somewhere else? I just ran into this issue, where Xlib defines X_H in /usr/include/X11/X.h.
To check, you can call gcc -dM -E (if you are using gcc), e.g. in the buildsystem I’m using that works with CC=gcc CFLAGS="-dM -E" make. If the output file contains #define X_H even though you remove it from your file (use Y_H for example), then it is already defined outside your source code.
I have three files in my project.
a.c
b.c
test.h
test.h declares a
namespace test_namespace {
int i;
void f1();
};
test.h is also surrounded by
#ifndef __x
#define __x
...
#endif
now, a.c includes test.h and b.c also includes test.h .
a.c has the main() function, and b.c has the implementation of test_namespace::f1()
However, on compiling this, I get a linking error -
"test_namespace::i is already defined in <b.c's object file mapping in /tmp>"
If I've taken care to include conditional compilation preprocessor directives in test.h, why is it being included in both files a.c and b.c ?
Also is noteworthy that if I compile b.c separately as a shared library and then use it as a shared library while linking a.c's object file, i don't get this error.
Can someone please explain the above error to me, specially in the face of the conditional compilation directives ?
You cannot declare variables inside a headerfile.
The symbol test_namespace::i becomes exported by both a.c and b.c. The linker find's both and doesn't know which one to use.
What you want to do in test.h is:
namespace test_namespace {
extern int i;
void f1();
}
and then declare test_namespace::i in eather a.c or b.c:
namespace test_namespace {
int i;
}
Conditional inclusion is used to prevent headers from being included twice for the same source file not for the whole project. Suppose you have headersa.h and b.h, and b.h #includes a.h. Then if c.c needs things from both headers, it #include both of them. Since the C preprocessor using literal text substitution, when it includes b.h, there will now be two #include "a.h" directives in your file, wreaking the havoc of multiple declarations. (Edit: clarify why you run into problems in this case.)
Include guards are there to protect multiple header inclusions in the build of a compilation unit. They aren't necessary for cases when you have two separate code files and one header, like your example.
(So think more when test.c uses a.h and b.h, in those cases where b.h needs to #include a.h.)
But that's a note about what the include guard convention does and how it isn't buying you anything in this case. The specific technical issue you hit (as others have pointed out) is that you're basically defining the same variable in two different object files, and when the linker goes to pull everything together it doesn't know if you want the variable from a.o or b.o.
( Note: While compilers can generally be set to override things and build C++ code using features like namespace even if the extension is .c - you probably should be using something else, like .cpp: C++ code file extension? .cc vs .cpp )
You're defining test_namespace::i in the header. What you probably
want is extern int i; in the header, and a definition in one of the
source files.
What's happening is that when you say
int i;
this actually does two things:
1) Declares the symbol i
2) Reserves some space and a symbol for i in the object file
The trick is that (1) should only be done once per file (actually you can repeat it in this case) -- which is why you have the conditional include, which you have done correctly -- but (2) should only be done once per program
The solution is to have the header do
// Only declare -- don't create a symbol
extern int i;
And in the *.c file do
int i;
The header guards (#ifndef .. #define .. #endif) are working as they should. Both a.c and b.c include test.h, so they both get a copy of that header. (When you compile a program, what #include does is to literally copy-paste the contents of the header inside the source file.)
Since they both have a copy of the header, they both define the variable test_namespace::i. When the linker tries to link the code generated from a.c with the code generated from b.c it finds that they both define that variable. It doesn't know what to do, so it doesn't complete and outputs an error.
I'm experiencing a very strange (and annoying) linker error on my project.
Say for example I have the file KernelExports.h:
#pragma once
extern "C"
{
DWORD* KeTimeStampBundle;
DWORD KeGetCurrentProcessType();
//etc...
}
I then #include this in my stdafx.h and then add #include "stdafx.h" all all my *.cpp files. The problem is that now whenever I build I get a stream of linker errors: LNK2005: KeTimeStampBundle already defined in stdafx.obj. This should not be happening since the header file is only included in one file and is protected with include guards. The errors stop once I comment out the whole extern "C" block so I know that is what is causing the issue.
What's even stranger is when I add all these source files to a new project it builds without any problems. I don't see what the issue is here, can anyone enlighten me?
My IDE is Visual Studio 2008.
DWORD* KeTimeStampBundle;
This does not declare a function. It declares and defines an object of type DWORD* named KeTimeStampBundle. If you include this header file in multiple source files, you will have multiple definitions of this object (one from each source file in which the header is included).
Include guards and #pragma once ensure that something is only defined once in a given translation unit (source file). They have no effect on how things are defined across multiple translation units.
#pragma once doesn't keep your file from being included multiple times in the project, it keeps it from being included multiple times in the same file. So a.cpp and b.cpp can include the file once, and that defines KeTimeStampBundle twice.
To fix this, put the definition of KeTimeStampBundle and KeGetCurrentProcessType, etc. into a .cpp file and put extern DWORD* KeTimeStampBundle, etc in the header.
Note that this only applies to definitions, not declarations. extern ... and function prototypes are declarations, so they can be done multiple times, but definitions can only occur once in the entire project, accross all .cpp files.
how does header including in c++ work? I have the classes already implemented in .h file and when there is #include in two files, there's this error:
files.h:14:7: error: redefinition of ‘class abstract_file’
files.h:14:20: error: previous definition of ‘class abstract_file’`
multiple times for each class and enum.
Can anyone explain this?
Using include in C++ simply takes the file included and splats the contents into where it is included. To do this without worrying about multiple includes of the same file, you want to use header guards. Use this basic format for all header files:
#ifndef FILENAME_H
#define FILENAME_H
class foo (or whatever else is in the file!) {
...
};
#endif
You can only include a definition one time but headers can be included multiple times. To fix that, add:
#pragma once
to the top of each header file.
While #pragma once is relatively common, if you are using an older compiler it may not be supported. In that case, you need to fall back on manual include guards:
#ifndef MY_HEADER_H
#define MY_HEADER_H
...
#endif
(note that you need to replace MY_HEADER_H with a unique string for each header file)
header files usually define a unique symbol so that they are only included once.
e.g.
#ifndef _myheader_h
#define _myheader_h
// rest of header goes here
#endif
Some compilers support
#pragma once
See Pragma Once on wikipedia.
While many people have solved your error for you, it seems that nobody has answered your initial question:
how does header including in c++ work?
When the preprocessor finds an #include directive it replaces it by the entire content of the specified file.
You can read more on preprocessor directives at cplusplus.com.
Update: To illustrate this, you could try the following if you have gcc handy:
echo '#include <iostream>' > test.cxx
gcc -E test.cxx
You'll see the contrents of iostream whizz past your eyes as the preprocessed source code is sent to standard output.
What you can do is to guard your header from multiple inclusions:
#ifndef MY_HEADER__
#define MY_HEADER__
/* your stuff goes here! */
#endif
You can also use:
#pragma once
but it is not standard although it is supported by many compilers.
Use include guards in header files:
#ifndef FILES_H
#define FILES_H
struct foo {
int member;
};
#endif // FILES_H
This makes sure that your header will only get included once.
Another method is to use #pragma once in your header files, but it's not standard C. It's supported by Visual C, GCC and Clang though, so most likely it's ok to use.
In an .h file, I am declaring a global variable as:
#pragma data_seg(".shared")
#ifndef DEF_VARX
#define DEF_VARX
int VARX=0;
#endif /*DEF_VARX*/
#pragma data_seg()
#pragma comment(linker, "/SECTION:.shared,RWS")
However if I include this file in multiple cpp files, when I try to compile, I get " error LNK2005: "int VARX" (?VARX##3HA) already defined in Dll.obj" error. If I include in only one cpp file, no problem is encountered.
Isn't #IFNDEF.... check enough for preventing this? Am I missin something?
The reason of this behavior is, that you compile the line
int VARX=0;
into each .obj file. Thats OK for compiling, but upon linking the symbol becomes multiply defined, which is illegal. Using
extern int VARX;
in the header file, and
int VARX=0;
in one (and only one) source file resolves this problem.
I think you're supposed to forward declare the variable in the .h and later define it in its shared section in a .cpp, something like:
// in a header file
#pragma once
extern int VARX;
// in a .cpp
#pragma data_seg(".shared")
int VARX=0;
#pragma data_seg()
#pragma comment(linker, "/SECTION:.shared,RWS")
The problem is that is that you prevent multiple inclusion of the file for a given translation unit. (for a given say cpp file)
But if several of your cpp include this VARX.H, then you'll have more than one definition for this variable.
Instead, you should only declare the variable in the .H file, but initialize it to 0 in only one location.
Yes, you're missing the extern keyword.
In your header file, use:
extern int VARX;
In a source file, actually declare space for the variable:
int VARX = 0;
ifdef prevents it for a separe object file. When the header is include in several source (cpp) files, VARX will be dedfined in all of them. Consider declaring it as extern in header file, and initialize in one cpp file.
The problem is you must be including the file in multiple compilation units. Let's say you have a.cpp and b.cpp. Both include your header file. So the compiler will compile (and pre-process) separately, so for both files, DEF_VARX is not yet defined. When you go to link the to object files together, the linker notices that there is a name collision.
As others have suggested, the solution would be to declare it 'extern', then place the actual value in a cpp file, so it only is compiled once, and linked to everything without name collisions.