问题描述
我正在通过 boost::process 开始一个过程。该过程使用std::cout 和std::cerr 来输出一些信息。我需要检索这些信息。在某些时候，我希望能够存储那些保留顺序和严重性的输出（来自 cout 或 cerr 的输出）。
但考虑到 boost::process 重定向输出的方式，我无法做到这一点。我只能将 std::cout 重定向到特定的 ipstream，将 std::cerr 重定向到另一个。然后，在阅读它们时，我无法保留顺序。
这是一个 MCVE 隔离问题：
#include <iostream>
#include <boost/process.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
void doReadOutput( boost::process::ipstream* is,boost::process::ipstream* err,std::ostream* out )
{
std::string line;
if ( std::getline( *is,line ) )
*out << "cout: " << line << std::endl;
if ( std::getline( *err,line ) )
*out << "cerr: " << line << std::endl;
}
void readOutput( boost::process::ipstream* is,std::ostream* out,std::atomic_bool* continueFlag )
{
std::string line;
while ( *continueFlag )
{
doReadOutput( is,err,out );
}
// get last outputs that may remain in buffers
doReadOutput( is,out );
}
int main( int argc,char* argv[] )
{
if ( argc == 1 )
{
// run this same program with "foo" as parameter,to enter "else" statement below from a different process
try
{
boost::process::ipstream is_stream,err_stream;
std::stringstream merged_output;
std::atomic_bool continueFlag = true;
boost::process::child child( argv[0],std::vector<std::string>{ "foo" },boost::process::std_out > is_stream,boost::process::std_err > err_stream );
boost::thread thrd( boost::bind( readOutput,&is_stream,&err_stream,&merged_output,&continueFlag ) );
child.wait();
continueFlag = false;
thrd.join();
std::cout << "Program output was:" << std::endl;
std::cout << merged_output.str();
}
catch ( const boost::process::process_error& err )
{
std::cerr << "Error: " << err.code() << std::endl;
}
catch (...) // @NOCOVERAGE
{
std::cerr << "UnkNown error" << std::endl;
}
}
else
{
// program invoked through boost::process by "if" statement above
std::cerr << "Error1" << std::endl;
std::cout << "Hello World1" << std::endl;
std::cerr << "Error2" << std::endl;
std::cerr << "Error3" << std::endl;
std::cerr << "Error4" << std::endl;
std::cerr << "Error5" << std::endl;
std::cout << "Hello World2" << std::endl;
std::cerr << "Error6" << std::endl;
std::cout << "Hello World3" << std::endl;
}
return 0;
}
当我执行这个程序时（在 Windows 10 下用 Visual Studio 2019 编译），它输出：
Program output was:
cout: Hello World1
cerr: Error1
cout: Hello World2
cerr: Error2
cout: Hello World3
cerr: Error3
cerr: Error4
cerr: Error5
cerr: Error6
虽然我想要：
Program output was:
cerr: Error1
cout: Hello World1
cerr: Error2
cerr: Error3
cerr: Error4
cerr: Error5
cout: Hello World2
cerr: Error6
cout: Hello World3
有什么办法可以实现吗？
#include <iostream>
#include <boost/process.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/thread/mutex.hpp>
void doReadOutput( boost::process::ipstream* str,const std::string& prefix,boost::mutex* mutex )
{
std::string line;
if ( std::getline( *str,line ) )
{
boost::mutex::scoped_lock lock( *mutex );
*out << prefix << ": " << line << std::endl;
}
}
void readOutput( boost::process::ipstream* str,std::string prefix,boost::mutex* mutex,std::atomic_bool* continueFlag )
{
while ( *continueFlag )
{
doReadOutput( str,out,prefix,mutex );
boost::thread::yield();
}
// get last outputs that may remain in buffers
doReadOutput( str,mutex );
}
int main( int argc,err_stream;
std::stringstream merged_output;
std::atomic_bool continueFlag = true;
boost::process::child child( argv[0],boost::process::std_err > err_stream );
boost::mutex mutex;
boost::thread thrdis( boost::bind( readOutput,"cout",&mutex,&continueFlag ) );
boost::thread thrderr( boost::bind( readOutput,"cerr",&continueFlag ) );
child.wait();
continueFlag = false;
thrdis.join();
thrderr.join();
std::cout << "Program output was:" << std::endl;
std::cout << merged_output.str();
}
catch ( const boost::process::process_error& err )
{
std::cerr << "Error: " << err.code() << std::endl;
}
catch (...) // @NOCOVERAGE
{
std::cerr << "UnkNown error" << std::endl;
}
}
else
{
// program invoked through boost::process by "if" statement above
std::cerr << "Error1" << std::endl;
std::cout << "Hello World1" << std::endl;
std::cerr << "Error2" << std::endl;
std::cerr << "Error3" << std::endl;
std::cerr << "Error4" << std::endl;
std::cerr << "Error5" << std::endl;
std::cout << "Hello World2" << std::endl;
std::cerr << "Error6" << std::endl;
std::cout << "Hello World3" << std::endl;
}
return 0;
}
那么输出为：
Program output was:
cerr: Error1
cout: Hello World1
cerr: Error2
cout: Hello World2
cerr: Error3
cout: Hello World3
cerr: Error4
cerr: Error5
cerr: Error6
还是没想到...
解决方法
您将需要非阻塞 IO。库中支持的方式是使用异步管道。
你会为 stderr/stdout 运行一个循环
- async_read 到缓冲区中，直到你得到一行或更多行
- 在行可用时将其从输入缓冲区复制到输出缓冲区
因为你最终会在管道/缓冲区状态上有两次非常相同的循环，所以将它封装成一个类型是有意义的，例如
struct IoPump {
IoPump(io_context& io,std::string& merged) : _pipe(io),_merged(merged) {}
boost::asio::streambuf _buf;
bp::async_pipe _pipe;
std::string& _merged;
void do_loop();
};
io_context io;
std::string merged;
IoPump outp{io,merged},errp{io,merged};
bp::child child(program,std::vector<std::string> { "foo" },bp::std_out > outp._pipe,bp::std_err > errp._pipe);
outp.do_loop(); // prime the pump
errp.do_loop(); // prime the pump
io.run();
仅此而已。嗯，当然，除了 IoPump::do_loop() 的实际作用：
void do_loop() {
boost::asio::async_read_until(_pipe,_buf,"\n",[this,out = boost::asio::dynamic_buffer(_merged)](
error_code ec,size_t xfer) mutable {
if (!ec) {
out.commit(buffer_copy(
out.prepare(xfer),_buf.data(),xfer));
_buf.consume(xfer);
do_loop(); // chain
} else {
std::cerr << "IoPump: " << ec.message() << "\n";
}
});
}
注意
- 您的主应用程序完全是单线程的
- 意味着异步完成处理程序永远不会同时运行
- 意味着直接访问
std::string merged;输出缓冲区是安全的，无需担心同步
现场演示
static void main_program(char const* program);
static void child_program();
int main(int argc,char** argv) {
if (argc == 1)
main_program(argv[0]);
else
child_program();
}
#include <iostream>
static void child_program() {
std::cerr << "Error1" << std::endl;
std::cout << "Hello World1" << std::endl;
std::cerr << "Error2" << std::endl;
std::cerr << "Error3" << std::endl;
std::cerr << "Error4" << std::endl;
std::cerr << "Error5" << std::endl;
std::cout << "Hello World2" << std::endl;
std::cerr << "Error6" << std::endl;
std::cout << "Hello World3" << std::endl;
}
#include <boost/process.hpp>
#include <boost/asio.hpp>
static void main_program(char const* program) {
namespace bp = boost::process;
try {
using boost::system::error_code;
using boost::asio::io_context;
struct IoPump {
IoPump(io_context& io,_merged(merged) {}
boost::asio::streambuf _buf;
bp::async_pipe _pipe;
std::string& _merged;
void do_loop() {
boost::asio::async_read_until(_pipe,out = boost::asio::dynamic_buffer(_merged)](
error_code ec,size_t xfer) mutable {
if (!ec) {
out.commit(buffer_copy(
out.prepare(xfer),xfer));
_buf.consume(xfer);
do_loop(); // chain
} else {
std::cerr << "IoPump: " << ec.message() << "\n";
}
});
}
};
io_context io;
std::string merged;
IoPump outp{io,merged};
bp::child child(program,bp::std_err > errp._pipe);
outp.do_loop(); // prime the pump
errp.do_loop(); // prime the pump
io.run();
std::cout << "Program output was:" << std::endl;
std::cout << merged;
} catch (const bp::process_error& err) {
std::cerr << "Error: " << err.code().message() << std::endl;
} catch (...) { // @NOCOVERAGE
std::cerr << "Unknown error" << std::endl;
}
}
印刷品
IoPump: End of file
IoPump: End of file
和标准输出：
Program output was:
Error1
Error2
Hello World1
Error3
Hello World2
Error4
Hello World3
Error5
Error6
其他示例
我在这个网站上已经有很多例子了。只需寻找async_pipe
开箱即用
您可以简单地在描述符级别将 stderr 重定向到 stdout 并完成！例如
boost::asio::io_context io;
std::future<std::string> merged;
bp::child child(program,bp::std_out > merged,bp::posix::fd.bind(2,1),io);
io.run();
std::cout << "Program output was:" << std::quoted(merged.get()) << "\n";
或者使用逐行阅读循环：
bp::ipstream merged;
bp::child child(program,1));
child.wait();
std::cout << "Program output was:" << std::endl;
for (std::string line; getline(merged,line);)
std::cout << "merged: " << std::quoted(line) << "\n";
打印
Program output was:
merged: "Error1"
merged: "Hello World1"
merged: "Error2"
merged: "Error3"
merged: "Error4"
merged: "Error5"
merged: "Hello World2"
merged: "Error6"
merged: "Hello World3"