Question

I am spawning a process in python using subprocess and want to read output from the program using pipes. The C++ program does not seem to close the pipe though, even when explicitly telling it to close.

#include <cstdlib>
#include <ext/stdio_filebuf.h>
#include <iostream>

int main(int argc, char **argv) {
  int fd = atoi(argv[1]);
  __gnu_cxx::stdio_filebuf<char> buffer(fd, std::ios::out);
  std::ostream stream(&buffer);
  stream << "Hello World" << std::endl;
  buffer.close();
  return 0;
}

I invoke this small program with this python snippet:

import os                                                                                         
import subprocess                                                                                 

read, write = os.pipe()                                                                           
proc = subprocess.Popen(["./dummy", str(write)])                                                  
data = os.fdopen(read, "r").read()                                                                
print data                                                                                        

The read() method does not return, as the fd is not closed. Opening and closing the write fd in python solves the problem. But it seems like a hack to me. Is there a way to close the fd in my C++ process?

Thanks a lot!

Answer 1

Spawning a child process on Linux (all POSIX OSes, really) is usually accomplished via fork and exec. After fork, both processes have the file open. The C++ process closes it, but the file remains open until the parent process closes the fd also. This is normal for code using fork, and usually is handled by a wrapper around fork. Read the man page for pipe. I guess python has no way of knowing which files are being transferred to the child, though, and therefore doesn’t know what to close in the parent vs the child process.