Re: Re: Re: Closing parent CGI after fork()

You probably want to close STDIN and STDERR too

You probably don't want to do any of this stuff, but you'll find you have to... :(

Even though the valuable information found by following links to merlyn's nodes/articles elsewhere in this thread only talk about closing STDOUT, I found I had to close STDIN and STDERR, as well. This is for Apache 2. I had to jump through some hoops to redirect the child's STDERR to a file and later send anything found there to the apache error_log, rather than lose any STDERR output produced by the child... a bit ugly, but it works.

Also, I found I had to reopen all 3 filehandles (to/from /dev/null for STDOUT and STDIN, to a file for STDERR) as well, since my child is opening other files, and if you just close STD(IN|OUT|ERR), the next time you open a file it will get file descriptor 0, 1, or 2, which is essentially reopening STD(IN|OUT|ERR), and I found this causes apache to hang again! Ugh. forker beware!

HTH

--
edan

Comment on Re: Re: Re: Closing parent CGI after fork()

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Re: Re: Re: Re: Closing parent CGI after fork() by hambo (Novice) on May 11, 2004 at 00:31 UTC
Thanks Edan. Well, I have done the project now, but take this into consideration for the upgrade - which I am already thinking about. My solution was to open a tiny browser window, make it the same colour as the background of the web page, and leave it running "hidden". A real cludge I know, but I really needed to move on here. Thanks for the advice anyway Couple of questions though: How do I go about re-opening STD(IN\|OUT\|ERR)? If I close STDOUT, how do I get hold of my STDOUT from the perl script or more importantly the `system` call? merlyn gave me a useful link to his long-running CGI process, but it did not work as I expected. I rather expected that every couple of seconds it would return, and update the page. Instead, it waited till the traceroute completed altogther before giving the results. I wondered whether this had to do with running it under Apache 2 rather than 1.3? Have you any comments on his long-running process? Thanks for your help though Cheers Hambo -- Open Source - Where everything is QED Perl - where programming is FUN	[reply] [d/l]
Re: Re: Re: Re: Re: Closing parent CGI after fork() by edan (Curate) on May 31, 2004 at 14:04 UTC
How do I go about re-opening STD(IN\|OUT\|ERR)? This is what I did: `# throw away STDIN and STDOUT open STDIN, '<', '/dev/null'; open STDOUT, '>', '/dev/null'; # save STDERR to deal with later my $err_file = "/tmp/err.$$"; open STDERR, '>', $err_file;` [download] If I close STDOUT, how do I get hold of my STDOUT from the perl script or more importantly the system call? Well, as you see, you're not really closing `STDOUT` you are reopening it to some other place. This will cause all output that would ordinarily go to your program's standard `STDOUT` to go to the file specified in the `open` call above. That includes any output generated by `system` calls and the like, since it's going to the same filehandle, only in a child process in that case (`system` simply performs a `fork` and `exec` under the hood). If you needed to get your hands on that output, you'd have to direct it to a real file instead of `/dev/null`, as in the `STDERR` example above. HTH -- edan	[reply] [d/l]