I work with a library which defines its internal division operator for a scripting language. Unfortunately it does not zero-check the divisor. Which leads to lot of headaches. I know the signature of the operator.
double ScriptClass::Divide(double&, double&);
Sadly it isn't even a C function. Is there any way I could make my application use my own Divide function instead of ScriptClass::Divide function?
EDIT:
I was aware of dlopen(NULL,..) and replacing "C" functions with user defined ones. Can this be done for class member functions (Without resorting to using mangled names)?
Various linkers and dynamic linker implementations will provide something that looks like a solution to this, as others have mentioned.
However, if you redefine one C++ function using any of those features (GNU ld's --wrap, ld.so's LD_PRELOAD, etc.), you are violating the one-definition rule and are thus invoking undefined behaviour.
While compiling your library, the compiler is allowed to inline the function in question in any way that it sees fit, which means that your redefinition of the function might not be invoked in all cases.
Consider the following code:
class A
{
public:
void foo();
void bar();
};
void A::foo()
{
std::cout << "Old version.\n";
}
void A::bar()
{
foo();
}
GCC 4.5, when invoked with -O3, will actually decide to inline the definition of foo() into bar(). If you somehow made your linker replace this definition of A::foo() with a definition of your own, A::bar() would still output the string "Old version.\n".
So, in a word: don't.
Generally speaking it's up to the programmer, not the underlying divide operator to prevent division by zero. If you're dividing by zero a lot that seems to indicate a possible flaw in the algorithm being used. Consider reworking the algorithm, or if that's not an option, guard calls to divide with a zero check. You could even do that inside a protected_divide type function.
All that being said, assuming that since it looks like a C++ function you have a C++ library compiled with all the same options you're using to build your application so name mangling matches you might be able to redefine the function into a .so and use LD_PRELOAD to force it to load. If you link statically, I think you can create the function into your own .o file and linking that prior to the library itself will cause the linker to pick up your version.
LD_PRELOAD is your friend. As an example, see:
https://web.archive.org/web/20090130063728/http://ibm.com/developerworks/linux/library/l-glibc.html
There's no getting away from the mangled names, I don't think, but you can use ld's --wrap option to cause a particular function to be given a new name based on its old name. You can then write a new version of it, and forward to the old version too if you like.
Quick overview here:
http://linux.die.net/man/1/ld
I've used this in the past to hook into malloc (etc.) without having to recompile the runtime library, though this wasn't on Linux (it was an embedded thing with no runtime loading). I didn't use it to wrap C++ functions, but if you can handle the C++ calling convention somehow, and you can create a function with the original function's mangled name, and get the compiler to accept a call to a function that has some ugly name with funny chars in it... I don't see why it shouldn't be possible to make it work.
Just short Q,
Cant you just wrap the class with your own code?
It'll be some headache at the start but after than you can simplify a lot of functions.
(Or even just wrap the function with a macro)
Related
In my code (either C or C++; let's say it's C++) I have a one-liner inline function foo() which gets called from many places in the code. I'm using a profiling tool which gathers statistics by line in the object code, which it translates into statistics by using the source-code-line information (which we get with -g in clang or GCC). Thus the profiler can't distinguish between calls to foo() from different places.
I would like the stats to be counted separately for the different places foo() get called. For this to happen, I need the compiler to "fully" inline foo() - including forgetting about it when it comes to the source location information.
Now, I know I can achieve this by using a macro - that way, there is no function, and the code is just pasted where I use it. But that wont work for operators, for example; and it may be a problem with templates. So, can I tell the compiler to do what I described?
Notes:
Compiler-specific answers are relevant; I'm mainly interested in GCC and clang.
I'm not compiling a debug build, i.e. optimizations are turned on.
I had an interview recently and one question asked was what is the use of extern "C" in C++ code. I replied that it is to use C functions in C++ code as C doesn't use name-mangling. I was asked why C doesn't use name-mangling and to be honest I couldn't answer.
I understand that when the C++ compiler compiles functions, it gives a special name to the function mainly because we can have overloaded functions of the same name in C++ which must be resolved at compile time. In C, the name of the function will stay the same, or maybe with an _ before it.
My query is: what's wrong with allowing the C++ compiler to mangle C functions also? I would have assumed that it doesn't matter what names the compiler gives to them. We call functions in the same way in C and C++.
It was sort of answered above, but I'll try to put things into context.
First, C came first. As such, what C does is, sort of, the "default". It does not mangle names because it just doesn't. A function name is a function name. A global is a global, and so on.
Then C++ came along. C++ wanted to be able to use the same linker as C, and to be able to link with code written in C. But C++ could not leave the C "mangling" (or, lack there of) as is. Check out the following example:
int function(int a);
int function();
In C++, these are distinct functions, with distinct bodies. If none of them are mangled, both will be called "function" (or "_function"), and the linker will complain about the redefinition of a symbol. C++ solution was to mangle the argument types into the function name. So, one is called _function_int and the other is called _function_void (not actual mangling scheme) and the collision is avoided.
Now we're left with a problem. If int function(int a) was defined in a C module, and we're merely taking its header (i.e. declaration) in C++ code and using it, the compiler will generate an instruction to the linker to import _function_int. When the function was defined, in the C module, it was not called that. It was called _function. This will cause a linker error.
To avoid that error, during the declaration of the function, we tell the compiler it is a function designed to be linked with, or compiled by, a C compiler:
extern "C" int function(int a);
The C++ compiler now knows to import _function rather than _function_int, and all is well.
It's not that they "can't", they aren't, in general.
If you want to call a function in a C library called foo(int x, const char *y), it's no good letting your C++ compiler mangle that into foo_I_cCP() (or whatever, just made up a mangling scheme on the spot here) just because it can.
That name won't resolve, the function is in C and its name does not depend on its list of argument types. So the C++ compiler has to know this, and mark that function as being C to avoid doing the mangling.
Remember that said C function might be in a library whose source code you don't have, all you have is the pre-compiled binary and the header. So your C++ compiler can't do "it's own thing", it can't change what's in the library after all.
what's wrong with allowing the C++ compiler to mangle C functions also?
They wouldn't be C functions any more.
A function is not just a signature and a definition; how a function works is largely determined by factors like the calling convention. The "Application Binary Interface" specified for use on your platform describes how systems talk to each other. The C++ ABI in use by your system specifies a name mangling scheme, so that programs on that system know how to invoke functions in libraries and so forth. (Read the C++ Itanium ABI for a great example. You'll very quickly see why it's necessary.)
The same applies for the C ABI on your system. Some C ABIs do actually have a name mangling scheme (e.g. Visual Studio), so this is less about "turning off name mangling" and more about switching from the C++ ABI to the C ABI, for certain functions. We mark C functions as being C functions, to which the C ABI (rather than the C++ ABI) is pertinent. The declaration must match the definition (be it in the same project or in some third-party library), otherwise the declaration is pointless. Without that, your system simply won't know how to locate/invoke those functions.
As for why platforms don't define C and C++ ABIs to be the same and get rid of this "problem", that's partially historical — the original C ABIs weren't sufficient for C++, which has namespaces, classes and operator overloading, all of which need to somehow be represented in a symbol's name in a computer-friendly manner — but one might also argue that making C programs now abide by the C++ is unfair on the C community, which would have to put up with a massively more complicated ABI just for the sake of some other people who want interoperability.
MSVC in fact does mangle C names, although in a simple fashion. It sometimes appends #4 or another small number. This relates to calling conventions and the need for stack cleanup.
So the premise is just flawed.
It's very common to have programs which are partially written in C and partially written in some other language (often assembly language, but sometimes Pascal, FORTRAN, or something else). It's also common to have programs contain different components written by different people who may not have the source code for everything.
On most platforms, there is a specification--often called an ABI [Application Binary Interface] which describes what a compiler must do to produce a function with a particular name which accepts arguments of some particular types and returns a value of some particular type. In some cases, an ABI may define more than one "calling convention"; compilers for such systems often provide a means of indicating which calling convention should be used for a particular function. For example, on the Macintosh, most Toolbox routines use the Pascal calling convention, so the prototype for something like "LineTo" would be something like:
/* Note that there are no underscores before the "pascal" keyword because
the Toolbox was written in the early 1980s, before the Standard and its
underscore convention were published */
pascal void LineTo(short x, short y);
If all of the code in a project was compiled using the same compiler, it
wouldn't matter what name the compiler exported for each function, but in
many situations it will be necessary for C code to call functions that were
compiled using other tools and cannot be recompiled with the present compiler
[and may very well not even be in C]. Being able to define the linker name
is thus critical to the use of such functions.
I'll add one other answer, to address some of the tangential discussions that took place.
The C ABI (application binary interface) originally called for passing arguments on the stack in reverse order (i.e. - pushed from right to left), where the caller also frees the stack storage. Modern ABI actually uses registers for passing arguments, but many of the mangling considerations go back to that original stack argument passing.
The original Pascal ABI, in contrast, pushed the arguments from left to right, and the callee had to pop the arguments. The original C ABI is superior to the original Pascal ABI in two important points. The argument push order means that the stack offset of the first argument is always known, allowing functions that have an unknown number of arguments, where the early arguments control how many other arguments there are (ala printf).
The second way in which the C ABI is superior is the behavior in case the caller and callee do not agree on how many arguments there are. In the C case, so long as you don't actually access arguments past the last one, nothing bad happens. In Pascal, the wrong number of arguments is popped from the stack, and the entire stack is corrupted.
The original Windows 3.1 ABI was based on Pascal. As such, it used the Pascal ABI (arguments in left to right order, callee pops). Since any mismatch in argument number might lead to stack corruption, a mangling scheme was formed. Each function name was mangled with a number indicating the size, in bytes, of its arguments. So, on 16 bit machine, the following function (C syntax):
int function(int a)
Was mangled to function#2, because int is two bytes wide. This was done so that if the declaration and definition mismatch, the linker will fail to find the function rather than corrupt the stack at run time. Conversely, if the program links, then you can be sure the correct number of bytes is popped from the stack at the end of the call.
32 bit Windows and onward use the stdcall ABI instead. It is similar to the Pascal ABI, except push order is like in C, from right to left. Like the Pascal ABI, the name mangling mangles the arguments byte size into the function name to avoid stack corruption.
Unlike claims made elsewhere here, the C ABI does not mangle the function names, even on Visual Studio. Conversely, mangling functions decorated with the stdcall ABI specification isn't unique to VS. GCC also supports this ABI, even when compiling for Linux. This is used extensively by Wine, that uses it's own loader to allow run time linking of Linux compiled binaries to Windows compiled DLLs.
C++ compilers use name mangling in order to allow for unique symbol names for overloaded functions whose signature would otherwise be the same. It basically encodes the types of arguments as well, which allows for polymorphism on a function-based level.
C does not require this since it does not allow for the overloading of functions.
Note that name mangling is one (but certainly not the only!) reason that one cannot rely on a 'C++ ABI'.
C++ wants to be able to interop with C code that links against it, or that it links against.
C expects non-name-mangled function names.
If C++ mangled it, it would not find the exported non-mangled functions from C, or C would not find the functions C++ exported. The C linker must get the name it itself expects, because it does not know it is coming from or going to C++.
Mangling the names of C functions and variables would allow their types to be checked at link time. Currently, all (?) C implementations allow you to define a variable in one file and call it as a function in another. Or you can declare a function with a wrong signature (e.g. void fopen(double) and then call it.
I proposed a scheme for the type-safe linkage of C variables and functions through the use of mangling back in 1991. The scheme was never adopted, because, as other have noted here, this would destroy backward compatibility.
Just some opitmization considerations:
Does anyone know it for sure whether intel C++ compiler (such as ICC 13.0, and of cause, compiled with some optimzation options like /O3 etc) will automatically optimize out any unused/uncalled struct/class/functions/variables in the codes like examplefun() below:
//...defining examplefunc()....//
const int a=0;
if (a>0)
int b=examplefunc();
The compiler does not usually optimize out unused functions unless they are static and, therefore, only accessible within a specific module. However, the linker might dead strip the function if linking is done at the function level and not the module level.
You can check the assembly output, linker map, or use something like objdump to check if the function was included in the linked binary.
I don't think the question is correctly stated. While the question literally asks whether the compiler will optimize away a function that is not used, but that is something that only the linker can do.
So what can the compiler do? The compiler can optimize away dead code, so for example in your code above, and because a is known to be 0, the compiler can remove the if statement altogether. For most uses, that is good enough (whether a function makes it to the executable or not won't affect performance much, whether a branch is avoided or not will affect the performance of the function --in particular with branch mispredictions).
Additionally, if the compiler optimizes the branch above, there will be one less reference to the exampleFunc function in the program, and when the linker processes the generated binaries, if there is no reference to a function in the whole program, it can drop the symbol altogether. Note that this can only be done as part of the program linkage, for libraries, even if the function is not called now, a program linked with the library at a later time might use it.
So getting back to the original question, the compiler will optimize away the branch, and the linker might or not remove the function from the binary, but that should not matter as much.
Regarding the other constructs, for struct and class, the only thing that makes it to the binary are the member functions, and the same thing applies there: if you are linking the program and none of the functions is used, the linker can drop the symbols.
I have found a book that states that if you want to use a function from the C standard library which takes a function pointer as an argument (for example qsort), the function of which you want to pass the function pointer needs to be a C function and therefore declared as extern "C".
e.g.
extern "C" {
int foo(void const* a, void const* b) {...}
}
...
qsort(some_array, some_num, some_size, &foo);
I would not be surprised if this is just wrong information, however - I'm not sure, so: is this correct?
A lot depends on whether you're interested in a practical answer for the compiler you're using right now, or whether you care about a theoretical answer that covers all possible conforming implementations of C++. In theory it's necessary. In reality, you can usually get by without it.
The real question is whether your compiler uses a different calling convention for calling a global C++ function than when calling a C function. Most compilers use the same calling convention either way, so the call will work without the extern "C" declaration.
The standard doesn't guarantee that though, so in theory there could be a compiler that used different calling conventions for the two. At least offhand, I don't know of a compiler like that, but given the number of compilers around, it wouldn't surprise me terribly if there was one that I don't know about.
OTOH, it does raise another question: if you're using C++, why are you using qsort at all? In C++, std::sort is almost always preferable -- easier to use and usually faster as well.
This is incorrect information.
extern C is needed when you need to link a C++ library into a C binary; it allows the C linker to find the function names. This is not an issue with function pointers (as the function is not referenced by name in the C code).
If there is no function overloading, the function name serves as the address of the function code, and when a function is being called, its address is easy to find using its name. However with function overloading, how exactly can the program find the correct function address? Is there a hidden table similar to virtual tables that stores the overloaded functions with their address? Thanks a lot!
Name mangling.
It's all done at compile time. The C++ compiler actually modifies the function names you give it internally, so that a function like
int foo(int a, float b, char c)
internally gets a name equivalent to
func_foo_int_float_char()
(the real symbol is usually some gobbledygook like ?CFoo#Foo##QAAX_N#Z ).
As you can see, the name is decorated depending on the exact number and types of parameters passed. So, when you call a function, it's easy for the compiler to look at the parameters you are passing, decorate the function name with them, and come up with the correct symbol. For example,
int a, b; float f; char c;
foo(a,f,c) ; // compiler looks for an internal symbol called func_foo_int_float_char
foo(a,b,c) ; // compiler looks for a symbol called func_foo_int_int_char
Again, it's all done completely at compile time.
The compiler can look at the call, and match that against the known existing overloaded implementations, and pick the right one. No need for a dynamic table, it's all perfectly doable statically at compile-time.
Update: removed my attempt at illustrating the concept by showing differently-named functions that the compiler can choose between.
If you are talking about overloaded methods of the same class, like so:
void function(int n);
void function(char *s);
...
objectInstance->function("Hello World")
It is a compile time thingy. The compiler knows (or in some situations, makes a best guess) at this point which method to call.
A comment I made in the question, I repeat here.
People who suggest name mangling are misguided I think. It is not as if the compiler mangles the name and just does a lookup among the mangled names. It needs to infer the proper types from the available methods. Once it does that, it already knows which method to call. It then uses the mangled name as the last step. Name mangling is not a prerequisite for determining which overloaded function to call.
Overloaded functions are resolved at compile-time. The compiler finds a suitable match for the given set of parameters and simply calls the corresponding function by its address (void foo(int) and void foo() are practically two totally independent functions - if you have foo(4) in your code, the compiler knows which function to call).
It is, I believe, achieved through name mangling:
the functions you know as foo(int) and foo(double) are actually named something like int_foo() and double_foo() (or similar, I'm not entirely sure of the particular semantics employed for C++). This means that C++ symbols are usually an order of magnitude larger than the names they are given in code.
C++ compilers use name mangling (different name for each overload) to distinguish between the functions in the object file. For example
int test(int a){}
int test(float a,float b){}
int test(double a){}
int testbam(double a){}
would produce the symbol names __Z4testi, __Z4testff, __Z4testd, __Z7testbamd. This name mangling is highly compiler-dependent (sadly) and one of many reasons why often C is preferred over C++.
When calling the function test, the compiler matches the given argument types and number of arguments against each function overload. The function prototypes are then used to find out which one should be called.
The function signature is composed of the function name + parameter(s) type(s)
Even if no function overload, compilers usually mangle function and variable names. It is called name mangling. It happens in both C and C++. Function name can be decorated by notably (1) calling convention, (2) C++ function overloading, (3) class member function.
GNU binutil c++filt can undecorate this mangled name, and in Windows, there is UnDecorateSymbolName