Re: dhcpd dup my socket?

[Chuck Swiger <cswiger%mac.com@localhost>]

On Jun 23, 2009, at 6:46 AM, Water NB wrote:
[ ... ]
> I think system("/etc/rc.d/dhcpd restart"); is the root issue.
> So how to resolve it?

By default, child processes inherit any open descriptors such as the  
network socket you have opened.  You can either switch to using fork()  
and exec() separately rather than system, and explicitly close the  
descriptor in the child process before running dhcpd, or you can use  
fcntl(fd, FD_SET, FD_CLOEXEC) [1] to have fd closed for you  
automagically...

--
-Chuck

[1]: It's probably safer to take a page from GNU libc recommendation,  
if more flags are added over time:

/* Set the FD_CLOEXEC flag of desc if value is nonzero,
   or clear the flag if value is 0.
   Return 0 on success, or -1 on error with errno set. */

int
set_cloexec_flag (int desc, int value)
{
  int oldflags = fcntl (desc, F_GETFD, 0);
  /* If reading the flags failed, return error indication now.
  if (oldflags < 0)
    return oldflags;
  /* Set just the flag we want to set. */
  if (value != 0)
    oldflags |= FD_CLOEXEC;
  else
    oldflags &= ~FD_CLOEXEC;
  /* Store modified flag word in the descriptor. */
  return fcntl (desc, F_SETFD, oldflags);
}