Question

Basically I need to open and read a list of files I get from another command.
For each line of output of popen
open a file usen ifstream.open

it compiles and if I put the file name directly it works fine, but it doesn’t do anything when using popen output. I’ve seen questions like this but none of this particular way of giving filenames.

here’s the code:

#include <iostream>
#include <sqlite3.h>
#include <stdio.h>
#include <fstream>

using namespace std;


int main () {

    ifstream singlefile;
    FILE *filelist;
    char filename[512];
    string progline;

    if(!(filelist = popen("find `pwd` -name \"*.js\"", "r"))){
        return 1;
    }

    while( fgets(filename, sizeof(filename), filelist)!=NULL)
    {
        cout << filename;
        singlefile.open(filename, ifstream::in);
        while ( singlefile.good() )
        {
            getline (singlefile,progline);
            cout << progline << endl;
        }

        singlefile.close();
    }

    pclose(filelist);

  return 0;

}

next step would be not open each file inside the loop but to store the file list and then open each file.

Thanks

Answer 1

fgets keeps the trailing newline, resulting in a filename of a non-existing file. Also the stream state is only updated after reading. If I replace the while body with the following code, it works for me:

  cout << filename;
  size_t len = strlen(filename);
  // chop off trailing newline
  if (len > 1 && filename[len - 1] == '\n') filename[len - 1] = 0;
  singlefile.open(filename, ifstream::in);
  while ( getline(singlefile, progline) )
    {
      cout << progline << endl;
    }

  singlefile.close();

---

If you actually want to iterate through a list of files, I’d use Boost.Filesystem, which has a nice C++ interface, works for all filenames (even for those with newlines), and is platform-independent.

If this actually is only an example and your actual command is not find, there is still some room for simplification. Here is a suggestion that uses Boost.Iostreams to get rid of most of the C function calls (it would be great to have a device source reading from a process’s standard output, but Boost.Iostreams lacks that):

#include <cstdio>
#include <iostream>
#include <fstream>
#include <stdexcept>
#include <string>

#include <stdio.h>

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

using namespace std;
namespace io = boost::iostreams;

class Popen: private boost::noncopyable {
public:
  explicit Popen(const char* command):
    m_stream(popen(command, "r")) {
    if (!m_stream) throw runtime_error("popen failed");
  }

  ~Popen() {
    pclose(m_stream);
  }

  FILE* stream() const {
    return m_stream;
  }

private:
  FILE* m_stream;
};


int main() {
  Popen pipe_wrapper("find `pwd` -name \"*.cpp\"");
  io::file_descriptor_source pipe_device(fileno(pipe_wrapper.stream()), io::never_close_handle);
  io::stream<io::file_descriptor_source> pipe_stream(pipe_device, 0x1000, 0x1000);
  string filename;
  while (getline(pipe_stream, filename)) {
    cout << filename << endl;
    ifstream file_stream(filename.c_str(), ifstream::in);
    string progline;
    while (getline(file_stream, progline)) {
      cout << progline << endl;
    }
  }
}