使用Rust结合pdfium与WinAPI打印PDF无输出问题排查

问题描述 #

用Rust结合pdfium和WinAPI打印PDF，代码无报错、调试输出显示正常，但打印机队列任务仅短暂闪过无实际打印内容；用Microsoft Print to PDF测试时生成0字节且无法打开的文件，怀疑问题出在pdfium的FPDF_RenderPage方法，需排查思路。

核心排查方向 #

1. HDC与打印句柄未关联 #

当前代码分开使用CreateDCA创建HDC、OpenPrinter获取打印机句柄，但两者没有绑定，WinAPI打印流程中需要让打印文档与渲染用的HDC关联：

• 修正方案：移除单独的OpenPrinter调用，直接用CreateDC获取的HDC调用StartDocA/StartPage（而非StartDocPrinterA/StartPagePrinter），让渲染内容直接输出到HDC对应的打印任务。

2. FPDF_RenderPage参数单位不匹配 #

PDF页面尺寸单位是点（1点=1/72英寸），而WinAPI HDC的单位依赖打印机分辨率（默认可能是缇或像素），直接将浮点尺寸转i32会导致渲染尺寸错误：

• 修正步骤：
  1. 用GetDeviceCaps(hdc, LOGPIXELSX)和GetDeviceCaps(hdc, LOGPIXELSY)获取打印机DPI；
  2. 将PDF点尺寸转换为打印机像素：

     let dpi_x = GetDeviceCaps(hdc, LOGPIXELSX);
     let dpi_y = GetDeviceCaps(hdc, LOGPIXELSY);
     let render_width = (resp_page_width * dpi_x as f32 / 72.0) as i32;
     let render_height = (resp_page_height * dpi_y as f32 / 72.0) as i32;
     

  3. 打印场景建议给FPDF_RenderPage的flags参数加上0x1000（FPDF_PRINTING）标志，替换或补充当前的0x800。

3. 打印流程顺序不合理 #

当前在启动打印页面后才初始化pdfium、加载文档，虽不报错但可能引发资源竞争：

• 调整顺序：将FPDF_InitLibraryWithConfig放在打印流程（StartDocPrinterA）之前，确保pdfium资源加载完成后再处理打印任务。

4. 打印数据类型不匹配 #

设置的data_type为XPS_PASS，但GDI渲染的内容对应RAW或系统自动判断类型：

• 测试方案：将DOC_INFO_1A的pDatatype设为PSTR::null()，或改为"RAW\0"，避免数据类型不兼容导致内容丢失。

5. 补充错误检查 #

现有调试仅打印基础返回值，需补充关键节点的有效性检查：

• 检查FPDF_LoadMemDocument和FPDF_LoadPage的返回值是否为空指针；
• WinAPI调用失败时用GetLastError()获取具体错误码，比如StartDocPrinterA返回0时排查错误原因。

原代码及调试输出 #

FFI 代码 #

[repr(C)]
#[derive(Clone, Copy)]
struct FPDF_LIBRARY_CONFIG {
    version: i32,
    m_pUserFontPaths: *const c_char,
    m_pIsolate: *const c_void,
    m_v8EmbedderSlot: u32,
}

#[cfg(target_os = "windows")]
#[link(name = "pdfium")]
extern "C" {
    fn FPDF_InitLibraryWithConfig(config: *const FPDF_LIBRARY_CONFIG) -> c_void;

    fn FPDF_DestroyLibrary() -> c_void;

    fn FPDF_LoadMemDocument(
        file_content: *const c_void,
        content_size: i32,
        password: *const c_char,
    ) -> *const c_void;

    fn FPDF_LoadPage(document: *const c_void, page_index: i32) -> *const c_void;

    fn FPDF_GetPageWidthF(page: *const c_void) -> c_float;

    fn FPDF_GetPageHeightF(page: *const c_void) -> c_float;

    fn FPDF_RenderPage(
        hdc: HDC,
        page: *const c_void,
        start_x: i32,
        start_y: i32,
        size_x: i32,
        size_y: i32,
        rotate: i32,
        flags: i32,
    ) -> c_void;
}

打印实现代码 #

pub fn print(file: &[u8]) -> anyhow::Result<()> {
    use std::ptr::null;

    use windows::{
        core::{s, PCSTR, PSTR},
        Win32::{
            Foundation::HANDLE,
            Graphics::{
                Gdi::{CreateDCA, DeleteDC},
                Printing::{
                    ClosePrinter, EndDocPrinter, EndPagePrinter, OpenPrinterA, StartDocPrinterA, StartPagePrinter, DOC_INFO_1A
                },
            },
        },
    };

    unsafe {
        let hdc = CreateDCA(PCSTR::null(), s!("Microsoft Print to PDF"), None, None);
        println!("create dca: {} {:?}", hdc.is_invalid(), hdc);

        let mut printer_handle = HANDLE::default();
        
        let data_type = "XPS_PASS\0"; // also tried "RAW" and "Text"

        let resp_open_printer = OpenPrinterA(s!("Microsoft Print to PDF"), &mut printer_handle, None);
        println!("open printer: {} {:?}", resp_open_printer.is_ok(), printer_handle);

        let doc_name = "Test Document\0";
        let doc_Info = DOC_INFO_1A {
            pDocName: PSTR::from_raw(doc_name.as_ptr().cast_mut()),
            pOutputFile: PSTR::null(),
            pDatatype: PSTR::from_raw(data_type.as_ptr().cast_mut()),
        };

        let resp_start_doc_printer = StartDocPrinterA(printer_handle, 1, &doc_Info);
        println!("start doc printer: {}", resp_start_doc_printer);

        let resp_start_page_printer = StartPagePrinter(printer_handle);
        println!("start page printer: {}", resp_start_page_printer.as_bool());

        let config = FPDF_LIBRARY_CONFIG {
            version: 2,
            m_pUserFontPaths: null(),
            m_pIsolate: null(),
            m_v8EmbedderSlot: 0,
        };

        FPDF_InitLibraryWithConfig(&config);

        let doc = FPDF_LoadMemDocument(file.as_ptr() as *const c_void, file.bytes().count() as i32, null());
        println!("load mem doc: {:?} {}", doc, file.bytes().count() as i32);

        let page = FPDF_LoadPage(doc, 0);
        println!("load page: {:?}", page);

        let resp_page_width = FPDF_GetPageWidthF(page);
        println!("page width: {}", resp_page_width);

        let resp_page_height = FPDF_GetPageHeightF(page);
        println!("page height: {}", resp_page_height);

        FPDF_RenderPage(hdc, page, 0, 0, resp_page_width as i32, resp_page_height as i32, 0, 0x800);

        let resp_end_page_printer = EndPagePrinter(printer_handle);
        println!("end page printer: {}", resp_end_page_printer.as_bool());

        let resp_end_doc_printer = EndDocPrinter(printer_handle);
        println!("end doc printer: {}", resp_end_doc_printer.as_bool());

        let resp_close_printer = ClosePrinter(printer_handle);
        println!("close printer: {}", resp_close_printer.is_ok());

        let resp_delete_dc = DeleteDC(hdc);
        println!("delete dc: {}", resp_delete_dc.as_bool());

        FPDF_DestroyLibrary();
    }
    
    ...
}

调试输出 #

create dca: false HDC(-551480207) // false is good
open printer: true HANDLE(1372067071760)
start doc printer: 73
start page printer: true
load mem doc: 0x13f780cc190 4953
load page: 0x13f758f9730
page width: 297.64 // correct width
page height: 575.43005 // correct height
end page printer: true
end doc printer: true
close printer: true
delete dc: true

内容的提问来源于stack exchange，提问作者e-Ra