How to mix C and C++

Part of C++ FQA Lite. To see the original answers, follow the FAQ links.

These questions are about mixing C and C++, which may be harder than you'd expect from the names of those languages, but easier than, say, mixing C++ and C++. Stay tuned.

[32.1] What do I need to know when mixing C and C++ code?

FAQ: You should check your vendor's documentation. Most frequently the rules are:

And you'll need to read the rest of this section so that your C functions can call your C++ functions and vice versa.

Or you can compile the C code with your C++ compiler - you may need to change the code, but you may also find bugs this way, so it's a good thing to do. Unless you don't have the C code in source form, of course.

FQA: You have little chances to successfully apply the rules unless you understand the underlying technical problem the rules try to address. The problem is in all the things in C++ that can not be translated to C straight-forwardly (mostly exceptions), and the things which can be translated in several ways (initialization of global variables before main and finalization after main, mangling names of overload & template functions, virtual function calls, constructor prototypes, layout of derived classes, RTTI - this list is quite large). Many languages which are easy to mix with C have such features. However, unlike C++ they also come with a formal or a de-facto standard defining the ABI (application binary interface) or a source-level interface for C interoperability. The C++ standard doesn't bother.

Most of these things can only cause problems when a particular C++ function is explicitly called. This kind of things is addressed in the rest of the questions in this section. However, the global initialization & finalization sequences are never explicitly called - hence the requirement about compiling main and linking the program with the C++ compiler. As to the need to use C and C++ compilers from the same vendor - this is true in theory, but in practice C compilers for a given hardware/OS configuration will interoperate smoothly. So will the C++ compilers as long as the C subset of the calling conventions is involved. However, this requirement is almost always a must when mixing C++ and C++ - for example, when third-party libraries with C++ interfaces are involved. Think about it: mixing C and C++ is easier than mixing C++ and C++. Isn't this amazing?

This situation is one excellent reason not to follow the FAQ's advice to compile your C code with a C++ compiler: C code is more portable. There are other reasons to keep C code in C, such as compilation time, better accessibility (there's no name mangling so functions bundled into a shared library are easier to call), etc.

[32.2] How can I include a standard C header file in my C++ code?

FAQ: Like this: #include <cstdio>, and then std::printf("I like std::!\n"). If you don't like std::, get over it. That's the way standard names are accessed.

If you compile old C code with a C++ compiler, the following will also work: #include <stdio.h>, and then printf("No std::!\n"); - all due to the magic of namespaces.

If you want to include a non-standard C header, see the next questions.

FQA: Um, if printf and std::printf both work, what's there to get over? printf is so standard that there seems to be little point in mentioning it over and over again. As to the "magic of namespaces", this particular case doesn't really seem to have anything to do with it. For some reason, the global unmangled extern "C" printf is also made accessible via namespace std by the C++ standard. What's so mysterious or amusing here? Perhaps the FAQ meant "the magic of standards".

If you want to include a non-standard C header, basically you'll have to tweak them the same way your compiler vendor tweaked the standard C headers.

[32.3] How can I include a non-system C header file in my C++ code?

FAQ: Like this:

extern "C" {
#include "foo.h"

If foo.h is your header, you can change it to make inclusion from C++ easier.

FQA: The reason you have to do this is that C function names are not mangled - in C, there's no overloading, so printf is known to the linkers, debuggers, etc. as printf. But in C++ there may be several functions with the same name. So the compiler has to make up a unique name using an encoding of the argument types. For example, the GNU C compiler generates an assembly function called _Z6printfPKc from C++ source code defining int printf(const char*). Different C++ compilers will do the name mangling differently - one of the many reasons making them incompatible with each other.

Theoretically there may be more differences between C and C++ functions, and extern "C" is your way to tell your C++ compiler "these are C functions, deal with all the differences". In practice, the problem is name mangling. Too bad there's no extern "C++ compiled with a different compiler".

[32.4] How can I modify my own C header files so it's easier to #include them in C++ code?

FAQ: Like this:

#ifdef __cplusplus
extern "C" {
void foo();
void bar();
#ifdef __cplusplus

Ew, macros are evil, wash your hands when you are done.

FQA: Together with the usual #ifndef,#define,#endif trinity, we've just used 7 preprocessor directives to define a single interface. And these seven directives contain zero information specific to that interface. And they don't help the compiler to do things compilers of other languages can do, like locating the implementation of the interface.

If you want to wash your hands after each preprocessor directive you touch in C++ and have some time left to do anything else with those hands, you'll have to work in a bathroom.

[32.5] How can I call a non-system C function f(int,char,float) from my C++ code?

FAQ: Prefix its prototype with extern "C" when you declare it. You can declare a whole bunch of C functions by surrounding the declarations with an extern "C" { ... } block.

FQA: Yeah, we've been through this already. It doesn't matter whether a declaration is in a header file or not. Neither C nor C++ syntax is aware of header files or other preprocessor-related things. Header files are just an automated copy-paste mechanism.

[32.6] How can I create a C++ function f(int,char,float) that is callable by my C code?

FAQ: Prefix the declaration and the definition with extern "C". You can't have more than one f C-callable function since C has no overloading.

FQA: Oh, how simple! And what if the function throws an exception? You didn't think you were going to escape that easily, did you?

I've just tried this with the GNU C and C++ compilers. When a C++ function calls a C function which calls a C++ function which throws an exception, you can't even catch it at the first C++ function, not to mention disposing the resources allocated by the C function.

So, make sure you catch all possible exceptions in your C-callable C++ functions. By the way, C++ exceptions can be of any built-in or user-defined type, and you can't catch an arbitrary exception and check what kind of exception it is at run time, and operator new can throw exceptions. Enjoy.

[32.7] Why is the linker giving errors for C/C++ functions being called from C++/C functions?

FAQ: You probably forgot extern "C", so the linker looks for a mangled C++ name instead of an unmangled C name.

FQA: Quiz: does a typical C++ linker try to check whether the unmangled C name is defined, and if in fact it is, ask you something like "did you forget extern "C"?" Hint: if it actually did this simple thing, how frequently would this question be asked?

You see, one of the advantages of using C++ is that you get to work with mature, industrial-strength tool chains.

[32.8] How can I pass an object of a C++ class to/from a C function?

FAQ: You can use class Fred in C++ (#ifdef __cplusplus), and a typedef struct Fred Fred; otherwise. Then you can define extern "C" functions which accept Fred* (the FAQ contains two screens of code illustrating this point, including both ANSI and K&R C function prototypes).

Note that this way, C++ code will be able to tell whether two pointers to class objects point to the same object, and C code won't. That's because when a pointer to a base class object is compared to a pointer to a derived class object, the compiler may need to do some pointer arithmetics before the comparison. In C++, this is done implicitly when the expression p == q is compiled.

Note that if you convert pointers to objects of classes to void* and compare them, neither C nor C++ compilers will be able to do the right pointer adjustments.

FQA: Please don't follow this advice! This FAQ keeps telling how evil the preprocessor is, and then it proudly presents this really nasty scheme. Defining type names to mean different things based on a preprocessor flag is as close to "evil preprocessor abuse" as it gets. Especially with all these pointer equality subtleties involved (these are ridiculous by themselves - seriously, if you can shoot yourself in the foot by simply comparing two pointers to objects, how "object-oriented" is the language?).

Here's a pretty straight-forward solution: in the header file which is supposed to be used from C, declare a struct FredObj or something (just use a different name than Fred, so that people can at least figure out what each name means! Sheesh!). In the C++ implementation file, define the structure to hold a single member - a Fred object. This doesn't lead to any run-time overhead. The extra syntax needed for dereferencing is worth the benefits - you can compare pointers and have fun in safety.

And if you really need to return objects of classes derived from Fred - just define a structure with a single member of type Fred* and never mind the tiny run-time overhead. If you are using class hierarchies to implement functionality so simple that this tiny run-time overhead is comparable to the actual work done by the classes, throw these class hierarchies away and stop messing up the lives of your innocent users.

Why do these people have to make everything cryptic and dangerous?

[32.9] Can my C function directly access data in an object of a C++ class?

FAQ: Yes, if the class has no virtual functions or non-public members, and so do all objects it contains by value. The FAQ outlines the way inheritance and virtual functions are implemented at a level allowing you to do the pointer arithmetics in order to access member data from C in these clearly illegal cases.

FQA: "Can" may mean many things: "can do it with a particular version of C & C++ compilers", "can do it with all compilers which are actually out there", "can do it with any standard-conforming compilers", and even "should normally do it because provisions were made to make it easy".

The short answer is that you should only do it with the so-called POD types (which basically means "structures defined using C syntax" for people who are not professional language lawyers). The only reasonable cases when breaking the rules is not an entirely moronic act are (1) when you play around with the language to see what's inside, (2) when you have to retrieve data from classes with definitions only available in binary form (you may want to check if your actions are legal first) and (3) you are implementing a debugger or the like, in which case you're writing legitimately non-portable code.

In case (2), you could also ask "Can my C++ function directly access private data in an object of a C++ class". Most often it can if you add a #define private public preprocessor directive at the top of your .cpp file. This works quite portably and does not depend on the layouts of C++ classes in your particular compiler.

People who want their C code to directly access data of a C++ class object for "speed" or something probably don't have enough real problems. The artificial problems they create for themselves will teach them a good lesson pretty soon.

[32.10] Why do I feel like I'm "further from the machine" in C++ as opposed to C?

FAQ: You are! C++ is a high-level language. In C, you can see where every clock cycle is spent; on the other hand, in C++ you can work at higher levels of abstraction and write more compact programs. Of course you can still write bad code - the idea is not to prevent bad programmers from doing it, but make it possible for the reasonable ones to write superior code!

FQA: What is this question doing here? Presumably people try to mix C and C++, and have an extern "C" function implemented in C++ throw an exception, or the initialization stuff before main never gets called, or they compare pointers to C++ class objects from C and it doesn't work, and they don't know why or how to even start figuring it out. The real question probably is "why do I feel like I'm underneath the machine, not just close to it as opposed to C"?

C++ is not a higher-level language than C. The damage caused by low-level errors is still not limited. You still have to think about pointers and object life cycles and integer endianness and many other things. But on top of that, there's a huge amount of things done implicitly, like global initialization and destruction, stack unwinding, base/derived classes pointer adjustment, and many more things - and all of them combine with the low-level errors into a single deep, wide tar pit with the programmer in the middle.

A good high-level language allows you to forget about many small details of program execution. A good low-level language allows you to control the many small details of program execution. C++ is not much of a high-level language, but it's not a very good low-level language either.

As to the remark about "seeing every cycle spent in C programs", I really believe that the FAQ author knows that you can't see that, since that's a pretty basic fact. You can't "see every cycle" spent in assembly programs in most cases - you have to know the exact target processor variant and the system configuration and a zillion other things. The FAQ is probably just being poetical.

But there's more to this remark than factual inaccuracy - it concentrates on a moderate problem, failing to mention an arguably more severe one. Consider the C++ code p = obj.getVec().begin();. The run-time of this code is unclear because it depends on whether obj is a value or a reference (the latter may be slower); in C it would be more clear. But there's another issue: is this code correct at all? If getVec() returns a reference to std::vector object, maybe it is correct, but if it returns it by value, it is certainly wrong. The compiler won't even warn you, and you won't notice the problem in the code without checking the definition of getVec. Not only is it hard to figure out how much time a C++ program runs, it is hard to even tell what it does, which is not supposed to be typical of high-level languages.

Copyright © 2007-2009 Yossi Kreinin
revised 17 October 2009