Question

I’m trying using pipes with boost libraries, I just want to execute a background program(e.g.: ls) and get it’s output in a string(like you can do with fopen and fread), but I really can’t get why I have no output with this code:

#include <iostream>
#include <cstdio>
#include <sstream>

#include <boost/iostreams/stream.hpp>
#include <boost/iostreams/device/file_descriptor.hpp>

int
main(int argc, char** argv)
{
    using namespace boost::iostreams;

    if(argc < 2)  {
        return -1;
    }

    FILE* p = popen(argv[1], "r");

    if(! p)  {
        std::cerr << "error open pipe" << std::endl;

        return -2;
    }

    int fd = fileno(p);
    std::stringstream ss;
    ss << fd;
    std::string s = ss.str();

    file_descriptor_source pdesc(s);
    stream_buffer<file_descriptor_source> pstream(pdesc);

    std::istream is(&pstream);
    std::string out;

    while(is)  {
        std::getline(is, out);
        std::cout << out << std::endl;
    }

    pstream.close();
    pdesc.close();
    pclose(p);

    return 0;
}

Thanks in advance.

Answer 1

It seems you are trying to open a boost::file_descriptor_source from a “path” which contains the file descriptor number. However, a file of this name probably doesn’t exist. What you probably meant to use is something like this:

if (FILE* p = popen(argv[1], "r")) 
{
    boost::iostreams::file_descriptor_source d(fileno(p), boost::iostreams::close_handle);
    boost::iostreams::stream_buffer<boost::iostreams::file_descriptor_source> pstream(d);
    std::cout << &pstream;
    pclose(p);
}