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Re: Open a file on a specific file descriptor?

by benizi (Hermit)
on Jan 02, 2008 at 08:16 UTC ( #659929=note: print w/replies, xml ) Need Help??

in reply to Re: Open a file on a specific file descriptor?
in thread Open a file on a specific file descriptor?

Perl's open() interface (and for that matter the underlying UNIX/Linux open(2) interface) don't allow you to specify a particular file descriptor.

Ah, my mistake. I was confused by the fact that C's fdopen(3) is on the same man-page as fopen(3). "fdopen" is mentioned in perldoc -f open when discussing open's '>&=6' syntax. I saw fopen(path, mode) and fdopen(fd, mode) and conflated the two's capabilities. (open a FILE* given a path, and open a FILE* given an fd number).

The usual way to handle this would be keep creating handles until the descriptor reaches the desired number.

Hmm. Looking at the source for sslclient, it seems the strategy is:

  1. Close fd's 6 and 7.
  2. Open a pipe.
  3. fd_move the pipe's ends to 6 and 7.

fd_move is a qmail-ism, apparently. It uses fcntl(current-fd,F_DUPFD,desired-fd) behind the scenes.

Note also that the parent should be creating a couple of pipes using the pipe() function to create the connections between parent and child.

Oops, yes. Thanks. Forgot in my for-this-node example that parent's "in" is child's "out" and vice versa.

For the curious, perldoc perlipc also suggests that my case warrants $SIG{PIPE} = 'IGNORE'; and $SIG{CHLD} = sub { }; # do something to handle a dead child

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