Closing stdin stdout and stderr the ptunnel way

Question

I was intrigued by the way ptunnel closes stdin, stdout and stderr:

if (daemonize)
{
    ...
    freopen("/dev/null", "r", stdin);
    freopen("/dev/null", "w", stdout);
    freopen("/dev/null", "w", stderr);
}

Is this a good way to close them? I am confused because freopen will open a file descriptor, which is not closed in that case.

Answer 1

No. It's not entirely safe.

It assumes the freopen() reuses the same file descriptors which is not guaranteed. So if freopen() uses a different file descriptor, say for example, for stdout other than 1 then your subsequent write() 's using that file descriptor will not work as expected. Because POSIX read/write functions use *_FILENO defined as:

/* Standard file descriptors.  */
#define STDIN_FILENO    0       /* Standard input.  */
#define STDOUT_FILENO   1       /* Standard output.  */
#define STDERR_FILENO   2       /* Standard error output.  */

for respective IO operations.

Instead you could do:

#include<unistd.h>

  fd = open("/dev/null",O_RDWR);
  dup2(fd,0);
  dup2(fd,1);
  dup2(fd,2); 

to achieve the same. Obvious downside is that open() and dup2() are POSIX functions and are not part of the C standard.

But you are safe as long as freopen() reuses the file descriptors 0, 1 & 2 respectively or you don't do any IO with that potentially incorrect file descriptor(s).