When using G++ (e.g. version 4.5 on Linux) can anyone explain what will/can happen if a user writes a header file for a mixed C/C++ system like this:
#ifdef __cplusplus
extern "C" {
int myCPPfunc(some_arg_list....); /* a C++ function */
}
#endif
but here myCPPfunc() is a normal C++ function with a class def inside - i.e. it was wrongly labeled as a C function.
What is the impact of this?
The main impact of this is that you cannot overload it, e.g. this is legal:
int myCPPfunc(int a);
int myCPPfunc(char a);
But this is not:
extern "C"
{
int myCPPfunc(int a);
int myCPPfunc(char a);
}
It is perfectly legitimate to have the implementation of an extern "C" function use arbitrary C++ features. What you can't do is have its interface be something you couldn't do in C, e.g. argument overloading, methods (virtual or otherwise), templates, etc.
Be aware that a lot of the "something you couldn't do in C" cases provoke undefined behavior rather than prompt compile errors.
This tells the C++ compiler that the functions declared in the header file are C functions.
http://www.parashift.com/c++-faq-lite/mixing-c-and-cpp.html#faq-32.2
This is exactly what extern "C" is for - it allows you to write a C++ function that can be called from C.
Essentially, that declaration is telling the C++ compiler that you want the C++ function myCPPfunc() to have an external interface that is linkable (and therefore callable) from C.
The implementation of the function is still C++ and can still use C++ features.
Typically, the declaration of the function in the header file might look more like:
#ifdef __cplusplus
extern "C" {
#endif
int myCPPfunc(some_arg_list....); /* a C++ function */
#ifdef __cplusplus
}
#endif
That lets the same header file be used by either the C++ compiler or the C compiler, and each will see it as declaring a C callable function.
Related
I'm writing a C program (myapp) which needs to use a particular api; the api is written in C++. I've worked with C and C++, but never both at once, and I'm getting confused.
So, the api provides the following directory, which I've placed in a folder called include, at the same level as my makefile:
libmyapi.a
api/api.h
My main source file is src/myapp.c, and it includes the api using #include "api/api.h".
My make command is (plus some flags, which I haven't listed because I don't think they're relevant here):
gcc -Linclude -lmyapi -Iinclude src/myapp.c -o lib/myapp.sp -lrt
The problem I'm having is that the api.h file contains references to namespaces etc. Eg at one point it has:
namespace MyAPI {
namespace API {
typedef SimpleProxyServer SimpleConnection;
}
}
and obviously the C compiler doesn't know what this means.
So, I assumed I'd need to compile using a C++ compiler, but then someone said I didn't, and I could just "wrap" the code in "extern 'C'", but I don't really understand. Having read around online, I'm not any further on.
Do I need to compile in C++ (ie using g++)?
Do I need to "wrap" the code, and what does that mean? Do I just do
#ifdef __cplusplus
extern "C" {
namespace MyAPI {
namespace API {
typedef SimpleProxyServer SimpleConnection;
}
}
}
#endif
or do I just wrap the lines
namespace MyAPI {
namespace API {
and then their corresponding }}?
The header file calls other header files, so potentially I'll need to do this in quite a lot of places.
So far I've got errors and warnings with all the variations I've tried, but I don't know whether I'm doing the wrapping wrong, setting g++ compiler flags wrong, using the wrong compiler, or what! If I know the method to use, I can at least start debugging. Thank you!
You can write a small C++ program that creates a C binding for the API.
Gvien this API:
namespace MyAPI {
namespace API {
typedef SimpleProxyServer SimpleConnection;
}
}
you can create c_api.h
#ifdef __cplusplus
extern "C" {
#endif
struct api_handle_t;
typedef struct api_handle_t* api_handle;
api_handle myapi_api_create();
void myapi_api_some_function_using_api(api_handle h);
void myapi_api_destroy(api_handle h);
#ifdef __cplusplus
}
#endif
and c_api.cpp
#include "c_api.h"
#include <myapi/api/stuff.hpp>
struct api_handle_t
{
MyAPI::API::SimpleConnection c;
};
api_handle myapi_api_create()
{
return new api_handle_t;
}
void myapi_api_some_function_using_api(api_handle h)
{
//implement using h
}
void myapi_api_destroy(api_handle h)
{
delete h;
}
compile that with a C++ compiler and include the c_api.h file in the C project and link to the library you created with the C++ compiler and the original library.
Basically, your C++ library needs to export a pure C API. That is, it must provide an interface that relies solely on typedef, struct, enum, preprocessor directives/macros (and maybe a few things I forgot to mention, it must all be valid C code, though). Without such an interface, you cannot link C code with a C++ library.
The header of this pure C API needs to be compilable both with a C and a C++ compiler, however, when you compile it as C++, you must tell the C++ compiler that it is a C interface. That is why you need to wrap the entire API within
extern "C" {
//C API
}
when compiling as C++. However, that is not C code at all, so you must hide the extern "C" from the C compiler. This is done by adding the preprocessor directives
#ifdef __cplusplus1
extern "C" {
#endif
//C API
#ifdef __cplusplus1
}
#endif
If you cannot change your libraries header, you need to create a wrapper API that offers this pure C API and calls through to the respective C++ code.
How do I call C++ functions from C?
By writing calling functions whose declarations are valid in the common subset of C and C++. And by declaring the functions with C language linkage in C++.
The problem I'm having is that the api.h file contains references to namespaces
Such header is not written in common subset of C and C++, and therefore it cannot be used from C. You need to write a header which is valid C in order to use it in C.
Do I need to compile in C++ (ie using g++)?
If you have function definitions written in C++, then you need to compile those C++ functions with a C++ compiler. If you have C functions calling those C++ functions, then you need to compile those C functions with C compiler.
A minimal example:
// C++
#include <iostream>
extern "C" void function_in_cpp() {
std::cout << "Greetings from C++\n";
}
// C
void function_in_cpp(void);
void function_in_c(void) {
function_in_cpp();
}
You cannot. You can use C functions in your C++ program. But you cannot use C++ stuff from C. When C++ was invented, it allowed for compatibility and reuse of C functions, so it was written as a superset of C, allowing C++ to call all the C library functions.
But the reverse is not true. When C was invented, there was no C++ language defined.
The only way you can call C++ functions is to convert your whole project into a C++ one... you need to compile your C functions with a C++ compiler (or a C compiler if they are plain C) but for a C function to call a C++ function it must be compiled as C++. You should declare it with:
extern "C" {
void my_glue_func(type1 param1, type2 param2, ...)
{
/* ... */
}
} /* extern "C" */
and link the whole thing as a C++ program (calling the c++ linker)
This is because C doesn't know anything about function overloading, class initializacion, instance constructor calls, etc. So if you even can demangle the names of the C++ functions to be able to call them from C (you had better not to try this), they will probably run uninitialized, so your program may (most) probably crash.
If your main() function happens to be a C function, then there's no problem. C++ was designed with this thing in mind, and so, main() is declared implicitly as extern "C". :)
My C++ program needs to use an external C library.
Therefore, I'm using the
extern "C"
{
#include <library_header.h>
}
syntax for every module I need to use.
It worked fine until now.
A module is using the this name for some variables in one of its header file.
The C library itself is compiling fine because, from what I know, this has never been a keyword in C.
But despite my usage of the extern "C" syntax,
I'm getting errors from my C++ program when I include that header file.
If I rename every this in that C library header file with something like _this,
everything seems to work fine.
The question is:
Shouldn't the extern "C" syntax be enough for backward compatibility,
at least at syntax level, for an header file?
Is this an issue with the compiler?
Shouldn't the extern "C" syntax be enough for backward compatibility, at least at syntax level, for an header file? Is this an issue with the compiler?
No. Extern "C" is for linking - specifically the policy used for generated symbol names ("name mangling") and the calling convention (what assembly will be generated to call an API and stack parameter values) - not compilation.
The problem you have is not limited to the this keyword. In our current code base, we are porting some code to C++ and we have constructs like these:
struct Something {
char *value;
char class[20]; // <-- bad bad code!
};
This works fine in C code, but (like you) we are forced to rename to be able to compile as C++.
Strangely enough, many compilers don't forcibly disallow keyword redefinition through the preprocessor:
#include <iostream>
// temporary redefinition to compile code abusing the "this" keyword
#define cppThis this
#define this thisFunction
int this() {
return 1020;
}
int that() {
return this();
}
// put the C++ definition back so you can use it
#undef this
#define this cppThis
struct DumpThat {
int dump() {
std::cout << that();
}
DumpThat() {
this->dump();
}
};
int main ()
{
DumpThat dt;
}
So if you're up against a wall, that could let you compile a file written to C assumptions that you cannot change.
It will not--however--allow you to get a linker name of "this". There might be linkers that let you do some kind of remapping of names to help avoid collisions. A side-effect of that might be they allow you to say thisFunction -> this, and not have a problem with the right hand side of the mapping being a keyword.
In any case...the better answer if you can change it is...change it!
If extern "C" allowed you to use C++ keywords as symbols, the compiler would have to resolve them somehow outside of the extern "C" sections. For example:
extern "C" {
int * this; //global variable
typedef int class;
}
int MyClass::MyFunction() { return *this; } //what does this mean?
//MyClass could have a cast operator
class MyOtherClass; //forward declaration or a typedef'ed int?
Could you be more explicit about "using the this name for some variables in one of its header files"?
Is it really a variable or is it a parameter in a function prototype?
If it is the latter, you don't have a real problem because C (and C++) prototypes identify parameters by position (and type) and the names are optional. You could have a different version of the prototype, eg:
#ifdef __cplusplus
extern "C" {
void aFunc(int);
}
#else
void aFunc(int this);
#endif
Remember there is nothing magic about header files - they just provide code which is lexically included in at the point of #include - as if you copied and pasted them in.
So you can have your own copy of a library header which does tricks like the above, just becoming a maintenance issue to ensure you track what happens in the original header. If this was likely to become an issue, add a script as a build step which runs a diff against the original and ensures the only point of difference is your workaround code.
I just read a question on SO discussing scenarios in which a piece of code is valid in both C and C++ but would produce different behavior in each language.
This begs the question: Could this ever be a problem when including C headers in C++ code?
I know from this question that you should include C headers like this:
extern "C" {
#include <your_os_or_library_header_in_c.h>
}
But all I found so far is that the extern "C" only guarantees that name mangling is turned off.
I couldn't find any information on whether it evaluates all statements as C, so that e.g. sizeof('a') or 10 //* comment */ 2 (which you could find in an inline function) are parsed as C and not C++. (Note that relying on such behavior as someone who writes a C header is obviously a bad idea, but I'm asking it from a purely academic standpoint of "What if?".)
Does the C++ standard say that enclosing a block of code in extern "C" means that all statements in it must be parsed as C?
extern "C" is a C++ construct which affects language linkage in a C++ program. It does not switch the language to C in any way. All source text inside an extern "C" declaration is still parsed as C++ and as C++ only.
Note that extern "C" affects more than just name mangling - it is (theoretically) possible that functions with C and C++ language linkage could have different calling conventions, for example.
extern "C" only changes linking, i.e. mangling and possibly the calling convention. Try compiling and running this program with both a C and a C++ compiler:
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
inline void putsizeofchar()
{
printf("%zd\n", sizeof(char));
}
#ifdef __cplusplus
}
#endif
int main()
{
putsizeofchar();
return 0;
}
Since header inclusion is textual substitution in both languages, the lack of a header doesn't make any difference.
I have a mix of C C++ code. All compiled with g++. Wherever I have C headers I have the contents of the header file included inside
#if defined(__cplusplus)
extern "C" {
#endif
and
#if defined(__cplusplus)
extern "C" {
#endif
But in one C header file I get g++ compilation errors where I have accidentally used a parameter name as template , which obviously is incorrect and in conflict with c++ keyword template.
I know I can go and change this parameter name, but I am thinking why is this extern "C" declaration not working and why is the header file considered as C++ code and not C as I intended to.
g++ version 4.1.1 Linux Red Hat Enterprise.
The extern "C" only tells the compiler (actually, the linker) that C++ name mangling doesn't apply to the functions declared in that scope. It has nothing to do with the syntax or keywords themselves.
Your best solution is to rename the conflicting symbols.
Consider:
extern "C" {
namespace n
{
int& foo(bool b)
{
if (!b)
throw std::invalid_argument("fail!");
static int i = 0;
return ++i;
}
}
}
This function has C language linkage, but uses references, namespaces and exceptions, which I hope demonstrates that extern "C" doesn't magically switch the compiler to compiling C, it just tells the compiler to use C calling conventions and symbol-naming conventions for the functions and variables with C language linkage (which usually just means it disables name-mangling and causes matching declarations in different namespaces to refer to the same entity.)
I'm taking a programming languages course and we're talking about the extern "C" declaration.
How does this declaration work at a deeper level other than "it interfaces C and C++"? How does this affect the bindings that take place in the program as well?
extern "C" is used to ensure that the symbols following are not mangled (decorated).
Example:
Let's say we have the following code in a file called test.cpp:
extern "C" {
int foo() {
return 1;
}
}
int bar() {
return 1;
}
If you run gcc -c test.cpp -o test.o
Take a look at the symbols names:
00000010 T _Z3barv
00000000 T foo
foo() keeps its name.
Let's look at a typical function that can compile in both C and C++:
int Add (int a, int b)
{
return a+b;
}
Now in C the function is called "_Add" internally. Whereas the C++ function is called something completely different internally using a system called name-mangling. Its basically a way to name a function so that the same function with different parameters has a different internal name.
So if Add() is defined in add.c, and you have the prototype in add.h you will get a problem if you try to include add.h in a C++ file. Because the C++ code is looking for a function with a name different to the one in add.c you will get a linker error. To get around that problem you must include add.c by this method:
extern "C"
{
#include "add.h"
}
Now the C++ code will link with _Add instead of the C++ name mangled version.
That's one of the uses of the expression. Bottom line, if you need to compile code that is strictly C in a C++ program (via an include statement or some other means) you need to wrap it with a extern "C" { ... } declaration.
When you flag a block of code with extern "C", you're telling the system to use C style linkage.
This, mainly, affects the way the linker mangles the names. Instead of using C++ style name mangling (which is more complex to support operator overloads), you get the standard C-style naming out of the linker.
It should be noted that extern "C" also modifies the types of functions. It does not only modify things on lower levels:
extern "C" typedef void (*function_ptr_t)();
void foo();
int main() { function_ptr_t fptr = &foo; } // error!
The type of &foo does not equal the type that the typedef designates (although the code is accepted by some, but not all compilers).
extern C affects name mangling by the C++ compiler. Its a way of getting the C++ compiler to not mangle names, or rather to mangle them in the same way that a C compiler would. This is the way it interfaces C and C++.
As an example:
extern "C" void foo(int i);
will allow the function to be implemented in a C module, but allow it to be called from a C++ module.
The trouble comes when trying to get a C module to call a C++ function (obviously C can't use C++ classes) defined in a C++ module. The C compiler doesn't like extern "C".
So you need to use this:
#ifdef __cplusplus
extern "C" {
#endif
void foo(int i);
#ifdef __cplusplus
}
#endif
Now when this appears in a header file, both the C and C++ compilers will be happy with the declaration and it could now be defined in either a C or C++ module, and can be called by both C and C++ code.
In C++ the name/symbol of the functions are actually renamed to something else such that different classes/namespaces can have functions of same signatures. In C, the functions are all globally defined and no such customized renaming process is needed.
To make C++ and C talk with each other, "extern C" instructs the compiler not to use the C convention.
extern "C" denotes that the enclosed code uses C-style linking and name mangling. C++ uses a more complex name mangling format. Here's an example:
http://en.wikipedia.org/wiki/Name_mangling
int example(int alpha, char beta);
in C: _example
in C++: __Z7exampleic
Update: As GManNickG notes in the comments, the pattern of name mangling is compiler dependent.
extern "C", is a keyword to declare a function with C bindings, because C compiler and C++ compiler will translate source into different form in object file:
For example, a code snippet is as follows:
int _cdecl func1(void) {return 0}
int _stdcall func2(int) {return 0}
int _fastcall func3(void) {return 1}
32-bit C compilers will translate the code in the form as follows:
_func1
_func2#4
#func3#4
in the cdecl, func1 will translate as '_name'
in the stdcall, func2 will translate as '_name#X'
in the fastcall, func2 will translate as '#name#X'
'X' means the how many bytes of the parameters in parameter list.
64-bit convention on Windows has no leading underscore
In C++, classes, templates, namespaces and operator overloading are introduced, since it is not allowed two functions with the same name, C++ compiler provide the type information in the symbol name,
for example, a code snippet is as follows:
int func(void) {return 1;}
int func(int) {return 0;}
int func_call(void) {int m=func(), n=func(0);}
C++ compiler will translate the code as follows:
int func_v(void) {return 1;}
int func_i(int) {return 0;}
int func_call(void) {int m=_func_v(), n=_func_i(0);}
'_v' and '_i' are type information of 'void' and 'int'
Here is a quote from msdn
"The extern keyword declares a variable or function and specifies that it has external linkage (its name is visible from files other than the one in which it's defined). When modifying a variable, extern specifies that the variable has static duration (it is allocated when the program begins and deallocated when the program ends). The variable or function may be defined in another source file, or later in the same file. Declarations of variables and functions at file scope are external by default."
http://msdn.microsoft.com/en-us/library/0603949d%28VS.80%29.aspx