编写C ++函数包装器的挑战，以便可以从C代码中使用它们

I am now writing wrappers for C++ functions, such that they can be used from C code.

我现在正在为C ++函数编写包装器,以便可以从C代码中使用它们。

The idea is to compile the cpp files using g++ and the c files using gcc, then link them together (!), but exposing ONLY those functions that are needed, to the C programs, by making them available in a header file 'test.h' (or maybe test.hpp?), like so:

我们的想法是使用g ++编译cpp文件和使用gcc编译c文件,然后将它们链接在一起(!),但只将那些需要的函数暴露给C程序,使它们在头文件'test中可用。 h'(或者可能是test.hpp?),如下:

(Note how I do not expose function 'vector Tokenize(const string& str,const string& delimiters)')

(注意我不公开函数'vector Tokenize(const string&str,const string&delimiters)')

test.h:

/* Header can be read by both C+ and C compilers, just the way we want! */
#ifndef TEST_H
#define TEST_H

#ifdef __cplusplus
extern "C" {
#endif

#if defined(__STDC__) || defined(__cplusplus)
 extern int TokenizeC(const char* text, const char* delim, char ***output);   /* ANSI C prototypes */
 extern void reclaim2D(char ***store, unsigned int itemCount);
#endif

#ifdef __cplusplus
}
#endif

#endif /* TEST_H */

test.cpp:

#include <string>
#include <iostream>
#include <vector>

#include <assert.h>

#include "test.h"

using namespace std;

vector<string> Tokenize(const string& str,const string& delimiters)
{
 vector<string> tokens;

 string::size_type delimPos = 0, tokenPos = 0, pos = 0;

 if(str.length() < 1)  return tokens;

 while(1)
 {
   delimPos = str.find_first_of(delimiters, pos);
   tokenPos = str.find_first_not_of(delimiters, pos);

   if(string::npos != delimPos)
   {
     if(string::npos != tokenPos)
     {
       if(tokenPos < delimPos) tokens.push_back(str.substr(pos,delimPos-pos));
       else tokens.push_back("");
     }
     else tokens.push_back("");

     pos = delimPos + 1;
   }
   else
   {
     if(string::npos != tokenPos) tokens.push_back(str.substr(pos));
     else tokens.push_back("");
     break;
   }
 }

 return tokens;
}

int TokenizeC(const char* text, const char* delim, char ***output)
{
    if((*output) != NULL) return -1; /* I will allocate my own storage, and no one tells me how much. Free using reclaim2D */

    vector<string> s = Tokenize(text, delim);

    // There will always be a trailing element, that will be blank as we keep a trailing delimiter (correcting this issue would not be worth the time, so this is a quick workaround)
    assert(s.back().length() == 0); // This will be nop'ed in release build
    s.pop_back();

    (*output) = (char **)malloc(s.size() * sizeof(char *));

    for(vector <string>::size_type x = 0; x < s.size(); x++)
    {
        (*output)[x] = strdup(s[x].c_str());

        if(NULL == (*output)[x])
        {
            // Woops! Undo all
            // TODO : HOW to test this scenario?

            for(--x; x >= 0; --x)
            {
                free((*output)[x]);
                (*output)[x] = NULL;
            }

            return -2; 
        }
    }

    return x; /* Return the number of tokens if sucessful */
}

void reclaim2D(char ***store, unsigned int itemCount)
{
    for (int x = 0; itemCount < itemCount; ++x)
    {
        free((*store)[x]);
        (*store)[x] = NULL;
    }

    free((*store));
    (*store) = NULL;
}

poc.c:

#include <stdio.h>
#include "test.h"

int main()
{
    const char *text = "-2--4--6-7-8-9-10-11-", *delim = "-";

    char **output = NULL;

    int c = TokenizeC(text, delim, &output);

    printf("[*]%d\n", c);

    for (int x = 0; x < c; ++x)
    {
        printf("[*]%s\n", output[x]);
    }

    reclaim2D(&output, c);

    return 0;
}

Do you notice something wrong?

你注意到了什么问题吗?

For starters, when I ran this program, I got "Unsatisfied code symbol '__gxx_personality_v0'"

对于初学者来说,当我运行这个程序时,我得到了“不满意的代码符号'__gxx_personality_v0'”

Thankfully, there is something here : What is __gxx_personality_v0 for?

值得庆幸的是,这里有一些东西:__gxx_personality_v0是什么?

Once I ran g++ with options " -fno-exceptions -fno-rtti", the output now fails with "Unsatisfied data symbol '_ZNSs4_Rep20_S_empty_rep_storageE'"

一旦我用选项“-fno-exceptions -fno-rtti”运行g ++,输出现在失败并显示“不满意的数据符号'_ZNSs4_Rep20_S_empty_rep_storageE'”

Ofcourse, the two environments (the one to compile - HP-UX B.11.23 ia64 and the one to run the binary on - HP-UX B.11.31 ia64) have different library versions (but same architecture), and this should not be a reason for the errors.

当然,两个环境(编译的那个 - HP-UX B.11.23 ia64和运行二进制文件的环境 - HP-UX B.11.31 ia64)具有不同的库版本(但是相同的架构),这不应该是错误的原因。

I would also like to test out the case marked by "// TODO : HOW to test this scenario?", but that can wait for now.

我还要测试标记为“// TODO:如何测试这种情况?”的案例,但现在可以等待了。

Any pointers?

3 个解决方案

#1

The easiest way to avoid undefined symbols while linking is to link with g++ (not gcc). You can still compile your .c file with gcc, though.

链接时避免未定义符号的最简单方法是链接g ++(而不是gcc)。不过,您仍然可以使用gcc编译.c文件。

Also please use system at a time. The link error may go away if you run all your gcc and g++ commands on the same system (no matter the old or the new one).

另外请一次使用系统。如果在同一系统上运行所有gcc和g ++命令(无论旧的还是新的),链接错误可能会消失。

#2

To call a C++ function from C, you can't have mangled names. Remove the conditional test for __cplusplus where you do the extern "C". Even though your functions will be compiled by a C++ compiler, using extern "C" will cause it to avoid name-mangling.

要从C调用C ++函数,您不能使用损坏的名称。删除__cplusplus的条件测试,执行extern“C”。即使您的函数将由C ++编译器编译,使用extern“C”也会导致它避免名称错误。

Here is an example:

这是一个例子:

The C file.

C文件。

/* a.c */
#include "test.h"

void call_cpp(void)
{
  cpp_func();
}

int main(void)
{
  call_cpp();
  return 0;
}

The header file.

头文件。

/* test.h */
#ifndef TEST_H
#define TEST_H
extern "C" void cpp_func(void);
#endif

The CPP file.

CPP文件。

// test.cpp
#include <iostream>
#include "test.h"

extern "C" void cpp_func(void)
{
  std::cout << "cpp_func" << std::endl;
}

The compiler command line.

编译器命令行。

g++ a.c test.cpp

#3

Is there a reason why you haven't considered modifying Swig to do just this? I seem to remember that there is a developmental branch of Swig to do just this...

有没有理由为什么你没有考虑修改Swig这样做呢?我似乎记得Swig有一个发展分支来做这个...

#1