Use a C struct in C++/CLI, calling conventions issue - c

I'm using lib_usb in my C++/CLI project, and I need to use it's functions usb_get_busses and some others in my managed C++/CLI code.
Using a managed AutoPtr to create the usb_bus struct as follows:
AutoPtr<struct usb_bus> bus;
now I need to call the usb_get_busses function, however the linker seems to be unhappy as to the calling conventions. The CLR uses the clrcall calling convention, and obviously C using __cdecl.
usb_bus* usb_get_busses(void) <--- the signature of the C function.
Now how do I make myself able to use the lib_usb in C++/CLI? Do I have to create a C++/CLI wrapper or something?
The linker spits out this message:
error LNK2031: unable to generate p/invoke for "extern "C" struct
usb_bus * __clrcall usb_get_busses(void)"
(?usb_get_busses##$$J0YMPAUusb_bus##XZ); calling convention missing in
metadata

That's not specified with __cdecl at all. Look at the linker error- it clearly states __clrcall. Try explicitly specifying it with __cdecl.

Related

Dealing with custom types in a library using dlopen

I need to load a library dynamically at runtime, and the code doesn't have the custom types the library uses defined at compile time.
This seems to initialize the struct correctly:
void *data = malloc(128);
InitCustomType(data);
The problem with this approach is that the size of the struct is unknown.
This is an example of how the library is normally used: (CustomType is a struct)
CustomType customType;
InitCustomType(&customType);
// Now customType can be used in the library calls
This is an example of how the library is normally used: (CustomType is a struct)
If that is an example, then nothing should stop you from using CustomType, or using malloc(sizeof(CustomType)).
the code doesn't have the custom types the library uses defined at compile time.
That statement is inconsistent with above. Either you do have a definition of CustomType, or you don't. If you do, you don't have a problem. If you don't, you can't really use this library.

Is there a way in C to have the compiler/linker give an error if a function is not defined?

In my case I am writing a simple plugin system in C using dlfcn.h (linux). The plugins are compiled separately from the main program and result in a bunch of .so files.
There are certain functions that must be defined in the plugin in order for the the plugin to be called properly by the main program. Ideally I would like each plugin to have included in it a .h file or something that somehow states what functions a valid plugin must have, if these functions are not defined in the plugin I would like the plugin to fail compilation.
I don't think you can enforce that a function be defined at compile time. However, if you use gcc toolchain, you can use the --undefined flag when linking to enforce that a symbol be defined.
ld --undefined foo
will treat foo as though it is an undefined symbol that must be defined for the linker to succeed.
You cannot do that.
It's common practice, to only define two exported functions in a library opened by dlopen(), one to import functions in your plugin and one to export functions of your plugin.
A few lines of code are better than any explanation:
struct plugin_import {
void (*draw)(float);
void (*update)(float);
};
struct plugin_export {
int (*get_version)(void);
void (*set_version)(int);
};
extern void import(struct plugin_import *);
extern void export(struct plugin_export *);
int setup(void)
{
struct plugin_export out = {0};
struct plugin_import in;
/* give the plugin our function pointers */
in.draw = &draw, in.update = &update;
import(&in);
/* get our functions out of the plugin */
export(&out);
/* verify that all functions are defined */
if (out.get_version == NULL || out.set_version == NULL)
return 1;
return 0;
}
This is very similar to the system Quake 2 used. You can look at the source here.
With the only difference, Quake 2 only exported a single function, which im- and exports the functions defined by the dynamic library at once.
Well after doing some research and asking a few people that I know of on IRC I have found the following solution:
Since I am using gcc I am able to use a linker script.
linker.script:
ASSERT(DEFINED(funcA), "must define funcA" ) ;
ASSERT(DEFINED(funcB), "must define funcB" ) ;
If either of those functions are not defined, then a custom error message will be output when the program tries to link.
(more info on linker script syntax can be found here: http://www.math.utah.edu/docs/info/ld_3.html)
When compiling simply add the linker script file after the source file:
gcc -o test main.c linker.script
Another possibility:
Something that I didn't think of (seems a bit obvious now) that was brought to my attention is you can create small program that loads your plugin and checks to see that you have valid function pointers to all of the functions that you want your plugin to have. Then incorporate this into your build system, be it a makefile or a script or whatever. This has the benefit that you are no longer limited to using a particular compiler to make this work. As well as you can do some more sophisticated checks for other other things. The only downside being you have a little more work to do to get it set up.

Linking LLVM JIT code to external C++ functions

I'm writing a LLVM scripting engine that JIT compiles scripting code in a custom language. My problem is that I'm unable to call external functions (even the C99 erf() function is failing).
For example if I extern "C" the erf function,
extern "C" double erft(double x){
return erf(x);
}
and create a function with external linkage
std::vector<const Type*> Double1(1,Type::getDoubleTy(getGlobalContext()));
FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),Double1,false);
Function *erft = Function::Create(FT,Function::ExternalLinkage,"erft",TheModule);
get the following error message when running my script with erft(0.0) :
LLVM ERROR: Program used external function 'erft' which could not be resolved!
Doing the mapping manually,
void ExecutionEngine::addGlobalMapping( const GlobalValue * erfF, void * erft);
will get me the following error:
declaration of `void llvm::ExecutionEngine::addGlobalMapping(const llvm::GlobalValue*, void*)' outside of class is not definition
Obviously I'm doing something very wrong. Any help would be much appreciated
Assuming you haven't disabled it (by calling EE->DisableSymbolSearching()) then LLVM will use dlsym() to find the symbols in the JIT program itself. Depending on your platform, that might mean that you need to build your JIT with -fPIC, or that it might not be available at all (such as on Windows).
Aside from automatic symbol searching, you can always register the individual functions yourself using EE->addGlobalMapping(GV, &function) where GV = the llvm::Function* function declaration that matches the native function you're calling. In your case with ertf() that's:
EE->addGlobalMapping(erft, &::erft);
Note that you named the global function erft() and the local variable erft, hence the "::". Please pick different names next time!
This might be happening because you forgot to add the "libm" depedency, try using:
[your module]->addLibrary("m");
See here for more information about the Module::addLibrary().
I don't know llvm, but this make no sense:
void ExecutionEngine::addGlobalMapping( const GlobalValue * erfF, void * erft);
That defines a new function in C++. What you need to do is somehow register your function with LLVM. Defining that function is like trying to add new methods to the LLVM classes, not what you want to do.

Is there a way to get an extern variable (or function) by name

I thought I read about a C standard library function recently that was able to return a pointer to any extern variable whose name was passed to it as a const char *. I think that it works via linker symbols, if that helps.
You could be thinking of dlsym, which is not part of the C standard library but part of the POSIX API.
It depends on the system. dlsym has already been mentioned. Its Windows counterpart is GetProcAddress. In the latter case, the function needs to be not only external but exported.

Confused by Unresolved external symbol error

I'm trying to build a wrapper library with VC++'s compiler.
ErlDriver.c
#define __WIN32__
#define DLL_EXPORT __declspec(dllexport)
#include "erl_driver.h"
DLL_EXPORT int _driver_output(ErlDrvPort port, char *buf, int len) {
return driver_output(port, buf, len);
}
build.bat
cl /I%ERL_DRIVER_H% /LD /MD ErlDriver.c
When I attempt to build this, I get the following linker error:
ErlDriver.obj : error LNK2019: unresolved external symbol _WinDynDriverCallbacks referenced in function __driver_output
erl_win_dyn_driver.h (included in erl_driver.h)
typedef struct {
WDD_FTYPE(driver_output) *driver_output;
// a ton more of those
} TWinDynDriverCallbacks;
extern TWinDynDriverCallbacks WinDynDriverCallbacks;
#define driver_output (WinDynDriverCallbacks.driver_output)
So, as you can see, WinDynDriverCallbacks is defined declared.
What could be causing the linker error, then?
No, it's not defined (at least in what you quoted). It's declared. The "extern" keyword means "the definition for this symbol appears in another compilation unit (source file)." You need to be linking with the object file (or library) produced from compiling the source file that defines that symbol.
There is a subtle difference between "declaring" something and "defining" it in C or C++. When you declare it, it tells the compiler that a certain symbol will be defined somewhere else - this can allow the code to use that symbol without needing to see the actual definition. You still have to define the symbol somewhere in the code that is linked in, or else you will get the error message you are seeing.
For example, this is a declaration of the symbol WinDynDriverCallbacks:
extern TWinDynDriverCallbacks WinDynDriverCallbacks;
Your code has this declaration - it allows the code that uses the symbol to successfully compile (but not link).
You need to add a definition somewhere:
TWinDynDriverCallbacks WinDynDriverCallbacks;
The definition must go into a source code file somewhere (not generally in a header file). This tells the compiler to allocate space in the object code for that object and allows the program to link successfully.
I got a very similar problem building a NIF on Windows. Unresolved external symbol _WinDynNifCallbacks. Turns out this is defined by the ERL_NIF_INIT macro and in my case the entire macro needed to be enclosed in a extern C block.
ie this failed
extern "C" ERL_NIF_INIT(...)
while this succeeded
extern "C"
{
ERL_NIF_INIT(...)
}
I strongly suspect this problem is due to the same issue but with the DRIVER_INIT macro for an erlang port driver.
Driver_Init is the main loop that declares "TWinDynDriverCallbacks WinDynDriverCallbacks;" but it's properly declared in the multiple line define for driver_init. You shouldn't need to wrap it in extern "c".
since this thread came up about a million times while trying to setup my barebones erlang port driver i will say this here. I am working out of Joe Armstrong's programming erlang book, chapter 12 interfacing techniques. Using erl5.9 and vs2010.
The code in the book had a omission and a error in example1_lib.c. Though the error is most likely due to the age of the book versus erlang version changes.
Needed to set (#define WIN32) at the very top of example1_lib.c otherwise erlang defaulted to all the Linux options.
Second needed to change (int bufflen) to (ErlDrvSizeT bufflen) in example_drv_output.
After that it built clean.

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