c++ - 使用 boost 将 ffmpeg 输出通过管道传输到 ffplay 标准输入

Question

我正在尝试将 ffmpeg 进程的输出通过管道传输到 ffplay 进程中(有点像回放)。我的问题如下:如果我逐个字符地复制输出字符(我的意思是 char )它可以正常工作，除了它会消耗大量的 CPU 功率。但是，当我尝试将 block 传输到其中(通过使用缓冲区)时，ffplay 出于某种原因甚至无法识别输入。

bp::ipstream iso;
bp::ipstream ise;
bp::opstream in;
    
bp::child ffmpeg(bp::search_path("ffmpeg"), bp::args({"-loglevel", "quiet", "-f", "pulse", "-i", "default", "-f", "wav", "-bitexact", "-nostdin", "-"}), bp::std_out > iso, bp::std_err > ise);
bp::child ffplay(bp::search_path("ffplay"), bp::args({"-loglevel", "verbose", "-nodisp", "-f", "wav", "-i", "-"}), bp::std_in < in, bp::std_out > bp::null);

以下是用于比较的 2 个代码片段:
这里是复制char通过 char

while(ffmpeg.running()) {
    char c;
    c = iso.get();
    in << c;
}

在这里它是在缓冲区的帮助下复制的

char buffer[1024];
while(ffmpeg.running()) {
    iso.get(buffer, 1024);
    in << buffer;
}

如有必要，我可以提供 ffplay 输出，但是我没有看到任何错误或类似的东西。

Answer 1

无需手动完成这项工作，您只需连接相同的管道:

bp::child ffmpeg(
    pg, bp::search_path(prod.cmd), bp::args(prod.args),
    bp::std_out > iso,
    bp::std_err > ise
);

bp::child ffplay(
    pg, bp::search_path(cons.cmd), bp::args(cons.args),
    (bp::std_in < iso)
    //, (bp::std_out> bp::null)
);

那会做你所期望的。这也意味着您不选择 ipstream/opstream ，因为角色既是:

//bp::pstream iso, ise, in;
bp::pipe iso, ise, in;

两者都有效，但您可能不需要流对象的复杂性。事实上，您可以使用异步版本，它的行为仍然相同(但为您提供更多选项/控制异步运行):

现场在线演示
对于生产者/消费者进程，明显替换/简化了命令:
Live On Coliru

#include <boost/process.hpp>

namespace bp = boost::process;
using namespace std::chrono_literals;
using boost::system::error_code;

int main() {
    //bp::pstream iso, ise, in;
    bp::pipe iso, ise, in;
    //boost::asio::io_context io;
    //bp::async_pipe iso(io), ise(io), in(io);
    bp::group pg;

    struct { std::string cmd; std::vector<std::string> args; } tasks[] = {
#if 1
         { "bash", { "-c", "echo 'hello world'"} },
         { "rev", {} },
#else
         { "ffmpeg", {"-loglevel", "quiet", "-f", "pulse", "-i", "default", "-f", "wav", /*"-bitexact",*/ "-nostdin", "-"} },
         { "ffplay", {"-loglevel", "verbose", "-nodisp", "-f", "wav", "-i", "-"} },
#endif
    };

    auto& [prod, cons] = tasks;

    bp::child ffmpeg(
        pg, bp::search_path(prod.cmd), bp::args(prod.args),
        bp::std_out > iso,
        bp::std_err > ise
    );

    bp::child ffplay(
        pg, bp::search_path(cons.cmd), bp::args(cons.args),
        (bp::std_in < iso)
        //, (bp::std_out> bp::null)
    );

    if (!pg.wait_for(4s))
        pg.terminate();
}

按预期打印“hello world”的反面:

dlrow olleh

做坏事
只是为了展示，这里是你如何手动执行 IO 泵的方法。
Live On Coliru

#include <boost/process.hpp>
#include <boost/asio.hpp>
#include <iostream>

namespace bp = boost::process;
using namespace std::chrono_literals;
using boost::system::error_code;

int main() {
    //bp::pstream iso, ise, in;
    //bp::pipe iso, ise, in;
    boost::asio::io_context io;
    bp::async_pipe iso(io), ise(io), in(io);
    bp::group pg;

    struct { std::string cmd; std::vector<std::string> args; } tasks[] = {
#if 1
         { "bash", { "-c", "echo -e 'hello world\\nYOHOHO\\nMOHOHO\\n'"} },
         { "xxd", {} },
#else
         { "ffmpeg", {"-loglevel", "quiet", "-f", "pulse", "-i", "default", "-f", "wav", /*"-bitexact",*/ "-nostdin", "-"} },
         { "ffplay", {"-loglevel", "verbose", "-nodisp", "-f", "wav", "-i", "-"} },
#endif
    };

    auto& [prod, cons] = tasks;

    bp::child ffmpeg(
        io, pg, bp::search_path(prod.cmd), bp::args(prod.args),
        bp::std_out > iso,
        bp::std_err > ise
    );

    bp::child ffplay(
        io, pg, bp::search_path(cons.cmd), bp::args(cons.args),
        (bp::std_in < in)
        //, (bp::std_out> bp::null)
    );

    std::function<void()> stream_pump;
    stream_pump = [&, buf=std::vector<char>(1024)]() mutable {
      boost::asio::async_read(iso, boost::asio::buffer(buf), [&](error_code ec, size_t const nread) {
        bool const eof = (ec == boost::asio::error::eof);
        std::cerr << "nread: " << nread << " (eof:" << eof << ")\n";
        if (ec && !eof)
          std::cerr << "async_read: " << ec.message() << " (" << nread << ")\n";
        else
          boost::asio::async_write(in, boost::asio::buffer(buf, nread),
            [=,&in,&stream_pump](error_code ec, size_t /*nwritten*/) {
              if (ec || eof) {
                std::cerr << "Closing in\n";
                in.close();
              }
              else
                stream_pump(); // continue the pump
          });
      });
    };

    stream_pump(); // prime the pump

    auto nhandlers = io.run_for(4s);
    std::cerr << "Handlers executed: " << nhandlers << "\n";
    ffmpeg.terminate();

    if (ffplay.running())
        ffplay.wait_for(1s);

    if (ffplay.running())
        pg.terminate();
}

打印

nread: 27 (eof:1)
Closing in
Handlers executed: 6

消费者子进程标准输出包含

00000000: 6865 6c6c 6f20 776f 726c 640a 594f 484f  hello world.YOHO
00000010: 484f 0a4d 4f48 4f48 4f0a 0a              HO.MOHOHO..