Question

您好我正在开发一个类似的程序：

./Filters File [filters...]

过滤器可以是很多.so库，我想自己创建并将它们附加到文件中。但是所有库都具有相同的功能process(a1,a2,a3)，只是每个库都做了不同的事情。

我试图像这样使用它：

/*Open the library*/
if (!(descriptor_lib=dlopen(dir_filter, RTLD_LAZY))) {
    fprintf(stderr, MYNAME": %s\n", dlerror());
    return(1);
}

/*We find the function we want to use*/
if (!(fn=dlsym(descriptor_bib, "process"))) {
    fprintf(stderr, MYNAME": %s\n", dlerror());
    return(1);
}

/*And then I try to use the function*/
printf("Number of chars readed: %d", fn(a1, a2, a3));

但是当我尝试编译时遇到此错误：error: too many arguments to function ‘fn’

dir_filter是库的方向，但它是一个变量，因为我创建一个循环来读取所有过滤器，在dir_filter中复制实际的过滤器，然后使用上面的代码。

我认为如果我使用库的名称指定dir_filter，它将起作用，但我不能这样做因为我需要让它适用于不同的过滤器（不同的库），它不会总是一样的。现在我只有3个库，但如果将来我想扩展它，我不想在每次添加新库时扩展代码。

那么我做错了什么，或者使用变量与dlopen合作是不可能的？

编辑：功能过程是这样的：

int process(char*buff_in, char*buff_out, int tam)

EDIT2：使用typedef我可以修复函数指针，这就是问题所在。感谢您的回复，对不起我的英语。

Answer 1

Assuming that your process function is declared (in your plugin code) as

int process(int, int, int);

Then, you'll better define a typedef for its signature (within the main program doing the dlsym):

typedef int process_sig_t (int, int, int);

And you'll use that to declare the function pointer (there are some "direct" ways to declare in C a function pointer without using a typedef for its signature, but I find them less readable).

process_sig_t *fnptr = NULL;

The compiler would then know the signature of the function referenced by that pointer.

^{(the initialization of fnptr to NULL could be omitted; it makes the code more readable and less error prone, since with a more reproducible behavior)}

which you would fill using dlsym e.g.

if (!(fnptr=dlsym(descriptor_bib, "process"))) {
  fprintf(stderr, MYNAME": %s\n", dlerror());
  return(1);
}

The use it

printf("Number of chars read: %d\n", (*fnptr) (a1, a2, a3));

BTW, function pointers could be called directly:

// less readable code, we don't know that fnptr is a function pointer
printf("Number of chars read: %d\n", fnptr (a1, a2, a3));

^{(it is better to end most printf format strings with \n; otherwise call fflush(3) since stdout is often line-buffered)}

BTW, your question is mostly about understanding function pointers (it would be relevant if the function pointer was obtained thru something else than dlsym, e.g. using some JIT compiling library like GCCJIT).

dlopen为不同的图书馆？ C

1 个答案: