Question

I am writing a C++ application and I need to read the result of a system command.

I am using popen() more or less as shown here:

    const int MAX_BUFFER = 2048;
    string cmd="ls -l";
    char buffer[MAX_BUFFER];
    FILE *stream = popen(cmd.c_str(), "r");
    if (stream){
       while (!feof(stream))
       {
            if (fgets(buffer, MAX_BUFFER, stream) != NULL)
            {
               //here is all my code
            }
       }
       pclose(stream);
    }

I’ve been trying to re-write this in a different way. I saw some non-standard solutions like:

FILE *myfile;
std::fstream fileStream(myfile);
std::string mystring;
while(std::getline(myfile,mystring))
{
    // .... Here I do what I need
}

My compiler does not accept this though.

How can I read from popen in C++?

Answer 1

Your example:

FILE *myfile;
std::fstream fileStream(myfile);
std::string mystring;
while(std::getline(myfile,mystring))

Does’t work because although you’re very close the standard library doesn’t provide an fstream that can be constructed from a FILE*. Boost iostreams does however provide an iostream that can be constructed from a file descriptor and you can get one from a FILE* by calling fileno.

E.g.:

typedef boost::iostreams::stream<boost::iostreams::file_descriptor_sink>
        boost_stream; 

FILE *myfile; 
// make sure to popen and it succeeds
boost_stream stream(fileno(myfile));
stream.set_auto_close(false); // https://svn.boost.org/trac/boost/ticket/3517
std::string mystring;
while(std::getline(stream,mystring))

Don’t forget to pclose later still.

Note: Newer versions of boost have deprecated the constructor which takes just a fd. Instead you need to pass one of boost::iostreams::never_close_handle or boost::iostreams::close_handle as a mandatory second argument to the constructor.