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[英]Why does read() fail with EAGAIN when piping to a program using boost::asio for STDIN/STDOUT?
[英]Piping ffmpeg output into ffplay stdin with boost
我正在嘗試將 ffmpeg 進程的 output 進程轉換為 ffplay 進程(有點像播放)。 我的問題如下:如果我逐個字符地復制 output 字符(按字符我的意思是char
)它可以正常工作,除了它會消耗大量的 CPU 功率。 但是,當我嘗試將 pipe 塊放入其中(通過使用緩沖區)時,ffplay 出於某種原因甚至無法識別輸入。
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);
以下是用於比較的 2 個代碼片段:
這里是逐個char
地復制char
while(ffmpeg.running()) {
char c;
c = iso.get();
in << c;
}
在這里它是在緩沖區的幫助下復制的
char buffer[1024];
while(ffmpeg.running()) {
iso.get(buffer, 1024);
in << buffer;
}
如有必要,我可以提供 ffplay output,但是我沒有看到任何錯誤或類似的東西。
無需手動完成這項工作,您只需連接相同的管道:
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)
);
這會做你所期望的。 這也意味着您不要選擇ipstream
/ opstream
,因為角色是:
//bp::pstream iso, ise, in;
bp::pipe iso, ise, in;
兩者都有效,但您可能不需要 stream 對象的復雜性。 事實上,您可以使用異步版本,它的行為仍然相同(但為您提供更多選項/控制異步運行):
對於生產者/消費者進程,明顯替換/簡化了命令:
#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();
}
按預期打印“hello world”的反面:
dlrow olleh
只是為了展示,這里是您如何手動執行 IO 泵的方法。
#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();
}
印刷
nread: 27 (eof:1)
Closing in
Handlers executed: 6
消費者子進程標准輸出包含
00000000: 6865 6c6c 6f20 776f 726c 640a 594f 484f hello world.YOHO
00000010: 484f 0a4d 4f48 4f48 4f0a 0a HO.MOHOHO..
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