Unable to capture standard output of process using Boost.Process

Question

Currently am using Boost.Process from the Boost sandbox, and am having issues getting it to capture my standard output properly; wondering if someone can give me a second pair of eyeballs into what I might be doing wrong.

I'm trying to take thumbnails out of RAW camera images using DCRAW (latest version), and capture them for conversion to QT QImage's.

The process launch function:

namespace bf = ::boost::filesystem; 
namespace bp = ::boost::process;

QImage DCRawInterface::convertRawImage(string path) {
    // commandline:  dcraw -e -c <srcfile>  -> piped to stdout.
    if ( bf::exists( path ) ) {
        std::string exec = "bin\\dcraw.exe";

        std::vector<std::string> args;
        args.push_back("-v");
        args.push_back("-c");
        args.push_back("-e");
        args.push_back(path);

        bp::context ctx;
        ctx.stdout_behavior = bp::capture_stream();

        bp::child c = bp::launch(exec, args, ctx);

        bp::pistream &is = c.get_stdout();
        ofstream output("C:\\temp\\testcfk.jpg");
        streamcopy(is, output);
    }
    return (NULL);
}


inline void streamcopy(std::istream& input, std::ostream& out) {
    char buffer[4096];
    int i = 0;
    while (!input.eof() ) {
        memset(buffer, 0, sizeof(buffer));
        int bytes = input.readsome(buffer, sizeof buffer);
        out.write(buffer, bytes);
        i++;
    }
}

Invoking the converter:

DCRawInterface DcRaw;
DcRaw.convertRawImage("test/CFK_2439.NEF"); 

The goal is to simply verify that I can copy the input stream to an output file.

Currently, if I comment out the following line:

    args.push_back("-c");

then the thumbnail is written by DCRAW to the source directory with a name of CFK_2439.thumb.jpg, which proves to me that the process is getting invoked with the right arguments. What's not happening is connecting to the output pipe properly.

FWIW: I'm performing this test on Windows XP under Eclipse 3.5/Latest MingW (GCC 4.4).

[UPDATE]

From debugging, it would appear that by the time the code reaches streamcopy, the file/pipe is already closed - bytes = input.readsome(...) is never any value other than 0.

Answer 1

Well I think that you need to redirect correctly the output stream. In my application something like this works :

[...]

bp::command_line cl(_commandLine);
bp::launcher l;

l.set_stdout_behavior(bp::redirect_stream);
l.set_stdin_behavior(bp::redirect_stream);
l.set_merge_out_err(true);

bp::child c = l.start(cl);
bp::pistream& is = c.get_stdout();

string result;
string line;
while (std::getline(is, line) && !_isStopped)
{
    result += line;
}

c.wait();

[...]

Without the redirect the stdout will go nowhere if I remember correctly. It is a good practice to wait for the process end if you want to get the whole output.

EDIT:

I'm on Linux with perhaps an old version of boost.process. i realize that your code is similar to the snippet I gave you. The c.wait() might be the key ...

EDIT: Boost.process 0.1 :-)

Answer 2

If migrating to the "latest" boost.process isn't an issue (as you sure know, there are several variants for this library) ,you could use the following (http://www.highscore.de/boost/process0.5/)

file_descriptor_sink sink("stdout.txt");
execute(
    run_exe("test.exe"),
    bind_stdout(sink)
);