I have some files written in C that I want to insert into my .NET C++/CLI code.
This C code is huge and has not been written by me, so it would be a hard task to me 'translating' all the code.
How can I insert this code and call the functions I need without any compatibility problems?
I used to think that, if C++/CLI is definitely C++ and C is compatible with C++, there is no problem to insert C code into C++/CLI code. But I've read about something called extern "C", which made me change my mind.
How can I insert the code into my project, preferably in another file?
Thank you in advance.
C++ can run C code without modifications. The only difference is the way the names of functions and variables are encoded. The process is called name mangling.
Just enclose your C code (headers and source) in a extern "C" block:
extern "C" {
... (C code)
}
All C code must be written in such a block. This tells C++ compiler how to treat function and variable names. Read more about extern "C" here.
What Mario is writing is correct. I am just modifying his example below. Just add all the header file declaration like below and compile you code to use all the function declared in c into your C++ application. Also include c libraries into your make file as well.
#ifdef __cplusplus
extern "C" {
#endif
//declare your header file here
#include "your c header file"
void foo(int i);//your c function
#ifdef __cplusplus
}
#endif
Related
i am currently looking at the HarmonyOS source code, and I can up with a problem. In the image I know that the function doesn't return anything, but what's the LITE_OS_SEC_TEXT part? I havent seen this keyword before, or is it even a keyword like STATIC or typedef. And if so, how do you define a keyword in C?
how do you define a keyword in C?
You don't. It's impossible.
What you can do is to use macros. They are basically just advanced text replacement. The preprocessor is run before the compiler. That is, in principal. On modern compilers they are run at the same time, but the behavior is the same.
If you want to look at the source code after the preprocessor, you can use the -E parameter to the compiler.
Here is an example of how you can make C code more like Pascal. DON'T DO THIS! JUST FOR FUN!
#include <stdio.h>
#define Begin {
#define End ;}
#define Writeln(X) puts(X)
int main(void)
Begin
Writeln("Hello, World!");
Writeln("Hello again")
End
Probably it is a macro. You should look for it's definition in included header files.
You can read about macros here.
Is it possible to use the Boost.DateTime library from C, and if so are there any examples available (preferably covering the build process using gcc-type tools)? I've searched the Boost documentation and the internet in general, and it seems theoretically possible, but have not found any clear answer one way or the other.
C and C++ are highly interoperable; calling C code from C++ is trivial; calling C++ code from C however is more restricted because the interface must have C linkage and contain only types available in C and cannot use C++ specific features such as default arguments or function overloading for example.
To use C++ code in C requires that the interface has a extern "C" linkage specification, which is itself not valid C, so the specification in any header file must be conditional e.g.:
// interop.h
#if !defined INTEROP_H
#define INTEROP_H
#if defined __cplusplus
extern "C"
{
#endif
// C interface here
void somefunction() ;
#if defined __cplusplus
}
#endif
#endif // INTEROP_H
Then the implementation of the interface itself would be C++ code:
// interop.cpp
#include "interop.h"
void somefunction()
{
// C++ code here callable from C through somefunction()
}
But the interface is callable from C code:
// notcpp.c
#include "interop.h"
int main()
{
somefunction() ;
}
This approach is fine where the interface is simple, so for example if you use the Boost Date/Time library for some very specific procedure the results of which can be represented in C, then that might be appropriate if implementing equivalent functionality in C alone would be prohibitive. But creating C wrappers for entire C++ classes is time consuming error prone, inflexible and and ultimately pointless - it would be generally simpler to compile your C code as C++ then the interoperability barrier disappears as you can then introduce C++ specific code directly into your existing codebase (though it can never thereafter be compiled as C). C++ is multi-paradigm and includes the C library within its own C++ library, and most third-party and OS C library headers already include the C++ conditional C linkage wrapper, so you need not rewrite all your code in the OOP style in order to use C++ if you only want to take advantage of the larger library ecosystem.
No. Boost is a C++ library and Boost.DateTime is no exception. If you look at any of the DateTime headers you'll see C++ features everywhere, from classes to templates to namespaces and much more - none of which are supported in C.
You can, however, create a wrapper library in C++ which exposes the functionality you need as pure C functions, then call those from C.
I am trying to parse the syntax of a C file using pycparser. I send the C file through a preprocessor and then send the output of the preprocessor to be parsed by pycparser. The following code is in one of the C files ...
extern "asm"
{
extern void ASM_Function(void);
}
pycparser throws and exception telling me this is not valid C syntax. Looking at the C BNF the keyword extern does not allow a string literal to precede it. I am correct in reading the BNF? Was this extern functionality added in a later version of C or is this syntax compiler specific?
It looks like a compiler extension. Do you know what compiler the code was originally written for?
Most compilers support declaring a C calling convention by wrapping the function declaration with an:
#ifdef __cplusplus
extern "C" {
#endif
// functions that use C calling convention.
// are declared here.
#ifdef __cplusplus
} /* extern "C" */
#endif
The code appears to be declaring an externally defined assembly function called ASM_Function. You may be able to rewrite this if you know what is the calling convention the assembly function is expecting.
extern "C" is a C++ construct to declare functions that will not use name mangling and will use the cdecl calling convention.
EDIT: Corrected my post.
I am trying to find all places where #include is placed inside extern C block. Is is possible to check this using preprocessor? I would like to do add something like this to my header files:
#ifdef EXTERN_C_IS_IN_EFFECT
#error File included from extern "C" block!
#endif
I am also looking for other ways to fail compilation in such case, e.g. use some special attribute. I am using gcc 4.4.7.
I defined following macro and then use it in every header which needs protection against inclusion in extern C block:
#define ASSERT_NO_EXTERN_C void assert_no_extern_c(int); void assert_no_extern_c(double);
Maybe you can define 2 function prototypes with same name and different parameters. You will get warnings in case of extern "C" block. And it is allowed in C++.
That's not possible because the preprocessor runs before any syntactical analysis is done; that is, the preprocessor doesn't even know what extern "C" is and it's action cannot depend on the presence of such a directive either.
However, linkage specifications do nest, so instead of making sure the includer did not specify extern "C", you could place your header within an extern "C++" specification to make sure that it uses C++ linkage.
Hi I am compiling ffmpeg using xcode, which I believe uses clang for compilation. In ffmpeg there is a struct with a member variable named 'class' I believe this is perfectly fine in C but clang is trying to parse it as a keyword. Any idea how to fix? Basically the following in a cpp file will cause the error:
extern C {
typedef struct {
int class;
} SomeStruct;
}
It tries to interpret class as a keyword.
FYI the file that is throwing the error in ffmpeg is libavcodec/mpegvideo.h and I need to include this to have access to the MpegEncContext struct to pull out motion map info.
EDIT
The above code sample was just to demonstrate the error. But perhaps its fixable in another way. In my actual code I have it like this:
#ifdef __cplusplus
extern "C" {
#endif
#include "libavcodec/mpegvideo.h"
#include "libavformat/avformat.h"
#if __cplusplus
} //Extern C
#endif
How would I get that to include the two files as C files and not C++?
Thanks
It's completely fine in C. When you build that as C++, you encounter an error because class is a C++ keyword.
As far as fixing it, you would normally choose an identifier other than class. However, ffmpeg developers may not be so agreeable with that change. Therefore, you may need to either:
restrict the visibility of that header to C translations
or edit your own copy in order to use it in C++ translations
Fortunately, you are also using a C compiler which has good support of C99 features in this case. C Compilers which do not support C99 well are particularly troublesome with ffmpeg sources (because you would then compile the whole program as C++ for the C99 features, and the conflict count would be much higher).
(there are other dirty tricks you could do to try to work around the problem, but i will not mention them)
Basically the following in a cpp file will cause the error
.cpp files are processed as C++ files, not C, and class is a reserved word in C++.
If you don't have a choice to rename anything in those header files, you could just replace the class token by something else
#ifdef __cplusplus
extern "C" {
# define class videoClass
#endif
#include "libavcodec/mpegvideo.h"
#include "libavformat/avformat.h"
#if __cplusplus
# undef class
} //Extern C
#endif
This is quite a dirty hack, but for such badly interfaced code you don't have much choice. The real solution would be to have all the struct members in these files use names that us some sort of prefix or so, as it is done in the network layer code. There all members have some prefixes as ss_ or sa_ and such problems are very unlikely to occur.