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Combining C++ and C - how does #ifdef __cplusplus work?
(4 answers)
Closed 8 years ago.
I'm having a problem with eclipse CDT.
Where i am having a C++ project that uses the C FatFs library. I'm trying to implement the fatfs files.
Question: In multiple files i'm adding
#ifdef __cplusplus
extern "C"
{
#endif
// code..
#ifdef __cplusplus
}
#endif
wrapper. But for some reason, in the one .h file _cplusplus is defined, and in the other .h file the __cplusplus is not defined.
Any suggestions?
Can send screenshot for clarification.
Whether __cplusplus is defined or not depends on how the file that includes the header is being compiled. If the file is being compiled as C source (.c) it will not be defined. if the file is being compiled as C++ source (.cpp, .cc, or any other extension associated as a C++ source file) then __cplusplus will be defined.
Double check the file extensions and if necessary the settings in your project to ensure that the files are being compiled correctly.
Look here: Combining C++ and C — how does #ifdef __cplusplus work?
extern "C" doesn't really change the way that the compiler reads the
code. If your code is in a .c file, it will be compiled as C, if it is
in a .cpp file, it will be compiled as C++ (unless you do something
strange to your configuration).
What extern "C" does is affect linkage. C++ functions, when compiled,
have their names mangled -- this is what makes overloading possible.
The function name gets modified based on the types and number of
parameters, so that two functions with the same name will have
different symbol names.
Code inside an extern "C" is still C++ code. There are limitations on
what you can do in an extern "C" block, but they're all about linkage.
Also, you probably want two #ifdef __cpluspluss:
#ifdef __cplusplus
extern "C" {
#endif
// ...
#ifdef __cplusplus
}
#endif
Otherwise, your C code will never see your definitions.
Related
I have a open source C project and if I try to execute examples it is working fine but if i try to call those functions from C++ by using extern (including all headers with function definition) it is throwing error undefined reference to function_name. Can someone help me out how to properly wrap the C code using C++ and calling functions directly from C++ by just including headers of C?
You need extern "C" language linkage to declare C functions in your C++ code.
This (among other things) prevents the name mangling otherwise used by the C++ compiler for functions.
Commonly extern "C" is added using conditional compilation in header files by checking the existence of the __cplusplus macro:
#ifdef __cplusplus
extern "C" {
#endif
// Function declarations
#ifdef __cplusplus
} // End of the extern "C" block
#endif
Why shouldn’t extern "C" be specified for a function that needs to be defined as a C function? What effect would that have on the compiler when compiling the file as a C source?
If there is no effect on the C compiler, can’t we just define a function in a header file as below by removing the #ifdef __cplusplus check?
extern "C" {
int MyFunc();
}
An answer to another question says that the #ifdef is needed, but I don’t understand why:
Regarding #2: __cplusplus will be defined for any compilation unit that is being run through the C++ compiler. Generally, that means .cpp files and any files being included by that .cpp file. The same .h (or .hh or .hpp or what-have-you) could be interpreted as C or C++ at different times, if different compilation units include them. If you want the prototypes in the .h file to refer to C symbol names, then they must have extern "C" when being interpreted as C++, and they should not have extern "C" when being interpreted as C -- hence the #ifdef __cplusplus checking.
The construct extern "C" is a C++ construct and is not recognized by a C compiler. Typically, it will issue a syntax error message.
A common trick is to define a macro, for example EXTERN_C, that would expand to different thing depending on if you compile using C or C++. For example:
In a common header file:
#ifdef __cplusplus
#define EXTERN_C extern "C" {
#define EXTERN_C_END }
#else
#define EXTERN_C
#define EXTERN_C_END
#endif
In other files:
EXTERN_C
int MyFunc(void);
EXTERN_C_END
If you compile a source file as C, it will not recognize extern "C", and would usually result in a compilation error.
If you compile a source file as C++, it will recognize extern "C", and the correct names will be linked.
Therefore, you can only use it reliably to specify C symbol names for files you compile as C++.
If you compile sources as C and C++, or your interfaces are intended for C and C++ clients, you would need to specify this one way or another in order for your clients to get the correct symbols when linking (and so on).
Related: You are allowed to write extern "C++" - for C++ translations.
Just a small question:
Can C++ use C header files in a program?
This might be a weird question, basically I need to use the source code from other program (made in C language) in a C++ one. Is there any difference between both header files in general? Maybe if I change some libraries...
I hope you can help me.
Yes, you can include C headers in C++ code. It's normal to add this:
#ifdef __cplusplus
extern "C"
{
#endif
// C header here
#ifdef __cplusplus
}
#endif
so that the C++ compiler knows that function declarations etc. should be treated as C and not C++.
If you are compiling the C code together, as part of your project, with your C++ code, you should just need to include the header files as per usual, and use the C++ compiler mode to compile the code - however, some C code won't compile "cleanly" with a C++ compiler (e.g. use of malloc will need casting).
If on, the other hand, you have a library or some other code that isn't part of your project, then you do need to make sure the headers are marked as extern "C", otherwise C++ naming convention for the compiled names of functions will apply, which won't match the naming convention used by the C compiler.
There are two options here, either you edit the header file itself, adding
#ifdef __cplusplus
extern "C" {
#endif
... original content of headerfile goes here.
#ifdef __cplusplus
}
#endif
Or, if you haven't got the possibility to edit those headers, you can use this form:
#ifdef __cplusplus
extern "C" {
#endif
#include <c_header.h>
#ifdef __cplusplus
}
#endif
Yes, but you need to tell the C++ compiler that the declarations from the header are C:
extern "C" {
#include "c-header.h"
}
Many C headers have these included already, wrapped in #if defined __cplusplus. That is arguably a bit weird (C++ syntax in a C header) but it's often done for convenience.
I want to write a C code, say "test.c", and call some C++ functions from it.
I have a header file header.h which has functions defined in it
and a C++ file which has definitions of the functions.
I'm unable to figure out compilation commands and how to use the extern command.
Can someone clarify this?
I want to write a C code, say "test.c" call some C++ functions from it.
Try something like this in your header:
#ifdef __cplusplus
extern "C"
{
#endif
void foo (void);
#ifdef __cplusplus
};
#endif
Then implement foo() in your .cpp file. Make sure that your .cpp file also includes the header.
Why shouldn’t extern "C" be specified for a function that needs to be defined as a C function? What effect would that have on the compiler when compiling the file as a C source?
If there is no effect on the C compiler, can’t we just define a function in a header file as below by removing the #ifdef __cplusplus check?
extern "C" {
int MyFunc();
}
An answer to another question says that the #ifdef is needed, but I don’t understand why:
Regarding #2: __cplusplus will be defined for any compilation unit that is being run through the C++ compiler. Generally, that means .cpp files and any files being included by that .cpp file. The same .h (or .hh or .hpp or what-have-you) could be interpreted as C or C++ at different times, if different compilation units include them. If you want the prototypes in the .h file to refer to C symbol names, then they must have extern "C" when being interpreted as C++, and they should not have extern "C" when being interpreted as C -- hence the #ifdef __cplusplus checking.
The construct extern "C" is a C++ construct and is not recognized by a C compiler. Typically, it will issue a syntax error message.
A common trick is to define a macro, for example EXTERN_C, that would expand to different thing depending on if you compile using C or C++. For example:
In a common header file:
#ifdef __cplusplus
#define EXTERN_C extern "C" {
#define EXTERN_C_END }
#else
#define EXTERN_C
#define EXTERN_C_END
#endif
In other files:
EXTERN_C
int MyFunc(void);
EXTERN_C_END
If you compile a source file as C, it will not recognize extern "C", and would usually result in a compilation error.
If you compile a source file as C++, it will recognize extern "C", and the correct names will be linked.
Therefore, you can only use it reliably to specify C symbol names for files you compile as C++.
If you compile sources as C and C++, or your interfaces are intended for C and C++ clients, you would need to specify this one way or another in order for your clients to get the correct symbols when linking (and so on).
Related: You are allowed to write extern "C++" - for C++ translations.