从C初始化C ++结构

Question

I'm trying to use opencv C++ library with another lib in C . So I've defined a structure in an .hpp file which I've added to the .h of the C library

typedef struct {
  cv::FileStorage fs;
  cv::Mat mat;
} myCPPStruct;

#ifdef __cplusplus
extern "C" {
#endif

typedef struct {    
    myCPPStruct * mycppstruct;    
} myCStruct;

void initialize_myCPPStruct(myCPPStruct * mycppstruct);

#ifdef __cplusplus
}
#endif

In the .c files the initializer for myCStruct calls initialize_myCPPStruct which is defined in a .cpp file somewhat as:

void initialize_myCPPStruct(myCPPStruct * mycppstruct){
  mycppstruct = {};
  mycppstruct->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
  mycppstruct->mat = cv::Mat(3,3,CV_8UC3);
}

But once it tries to allocate a value to mycppstruct->fs , it gets a segmentation fault. I assume this is because there is no memory allocation in C, but I've tried

std::memset(stitcher->fs,0,sizeof(cv::FileStorage));

which also doesn't work as it can't parse cv:FileStorage as void * .

Will I have to use the deprecated C OpenCV library to make this work?

EDIT - More details about the compilation.

For the C lib, I recompile (without linking ie with the -c option)all the .c files where I've added C++ functions or structures using g++ while making sure I add the __cplusplus guards in the .h files. All the .c files without C++ code already have .o files compiled with gcc . I then compile the whole program with g++ while making sure to include the relevant library files. There are no compiler errors.

Answer 1

It seems the structure itself has not been created.
You might try this (or something similar):

myCPPStruct* initialize_myCPPStruct()
{
  myCPPStruct* result;
  result = new myCPPStruct();
  result->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
  result->mat = cv::Mat(3,3,CV_8UC3);
  return (result);
}

Answer 2

First, let me point out the sentence which confuse me.

there is no memory allocation in C

Yes, there is. Have a look to malloc function.

char *my_allocated_string = malloc(sizeof(char) * 42);

Here you are, you've allocated an array of character of size 42.

Now, have a look to : mycppstruct = {};

That's not how you allocate a structure in C. You have to call... malloc() !

my_struct *s = malloc(sizeof(my_struct));
s->fs = 42;

Ok. Done. Feels better.

---

Well, first of all, you have to create a wrapper around your C++ code. Why ? Because C++ allows multiple definition of a given function (also call symbol ) :

int my_func(int);
int my_func(char);

This is valid in C++. But, think about it, how is the compiler able to let two functions with the same name exist ? Well it's not. It uses a technique named mangling when evaluating the functions to create 2 different names. Mangling is use on everything function and method. Event on single functions.

C is not able (and willing) to create several functions with the same name. Otherwise, you will experience a kind of function-already-implemented error. When you declare :

int my_func(int);

the C compiler will create the symbol : my_func . No mangling.

To make both language interact, you need to reference to a symbol understandable by the C compiler. If you call my_func from a C source file, the C compiler will look for my_func symbol. Bu since C++ will modify its my_func symbol into something like _ZN9myfuncE , the linkage will fail. That's why you have to says to the C++ compiler to not use mangling on the function you expose to C. That's why you need extern "C" { } .

Feeww, so far so good...

Now you have to embed you C API into an extern "C" block :

In my_c_api.h :

void my_func(int);
void my_func(char);

In my_c_api.cpp :

#include "my_c_api.h"

extern "C" void my_func_i(int i) { my_func(i); }
extern "C" void my_func_c(char c) { my_func(c); }

You compile it to create your C++ library.

In your C compilation pipeline, you link against your new C++ library and header files. Then :

#include "<path_to_my_c_api.h>"

void c(int i,char s)
{
    my_func_i(i);
    my_func_c(c);
}

You cannot compile C++ code with C compiler. You have to compile te C++ code independently.

More on ISO CPP.

Answer 3

You need to allocate the memory for your structure:

    myCPPStruct = malloc(sizeof(myCPPStruct));

Use the above line instead of:

    mycppstruct = {};

Also,since your function is passing in the pointer and allocating the memory internally you need to pass a pointer to the pointer or the allocation will not be passed back:

    void initialize_myCPPStruct(myCPPStruct** mycppstruct){
        if ( mycppstruct == NULL ) {
     //Abort as no address of the pointer to allocate passed
            return;
        }
        *myCPPStruct = malloc(sizeof(myCPPStruct));
        (*mycppstruct)->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
        (*mycppstruct)->mat = cv::Mat(3,3,CV_8UC3);
    }

Or you could just change the above to:

    myCPPStruct* initialize_myCPPStruct(void) {
        myCPPStruct* ptr = malloc(sizeof(myCPPStruct));
        ptr->fs = cv::FileStorage( "file.txt", cv::FileStorage::READ );
        ptr->mat = cv::Mat(3,3,CV_8UC3);
        return ptr;
    }