c - 如何为 cairo_pdf_surface_create_for_stream() 创建正确的回调？

Question

我需要将 PDF 输出到文件描述符，我正在考虑使用 Cairo 和 cairo_pdf_surface_create_for_stream():

我有一个写入函数，具有以下签名:

static cairo_status_t cairowrite(void *fp, unsigned char const *data, unsigned int length);

它返回CAIRO_STATUS_SUCCESS。

现在，我如何将此函数作为回调传递？

我尝试过这个:

cairo_write_func_t (*wfunc)(void *, unsigned char const *, unsigned int);
wfunc = (cairo_write_func_t)&cairowrite;
surface = cairo_pdf_surface_create_for_stream((cairo_write_func_t)wfunc, fp, (double)realwidth, (double)realwidth);

但是我收到编译器警告:警告:来自不兼容的指针类型的赋值这位于我执行wfunc = &cairowrite;的行上。

我做错了什么？

回调函数给出了此 gcc 错误:cairo_status_t，但必须是 cairo_write_func_t 类型。

使用 clang 我收到此错误:

qrencode.c:250:11: warning: incompatible pointer types assigning to 'cairo_write_func_t (*)(void *, unsigned char const *, unsigned int)' from
      'cairo_write_func_t' (aka 'cairo_status_t (*)(void *, unsigned char const *, unsigned int)')
    wfunc = (cairo_write_func_t)&cairowrite;
          ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

我应该做什么？

Answer 1

您可以尝试使用定义函数指针

cairo_status_t (*wfunc)(FILE *, unsigned char *, unsigned int) = &cairowrite;

示例:

#include <iostream>

int funp(int a, int b) {
  return a + b;
}

int call(int a, int b, int (*ptr)(int, int)) {
  int result = ptr(a, b);
  return result;
}

int main() {
  int (*ptr)(int, int) = &funp;
  int r = call(3, 5, ptr);
  std::cout << "Result " << r << std::endl;
  return 0;
}

输出:

Result 8