Date: Thu, 13 Jan 2000 05:55:49 +0100 (CET) From: Oliver Fromme <olli@dorifer.heim3.tu-clausthal.de> To: freebsd-questions@FreeBSD.ORG Subject: Re: Giving a sighandler more information Message-ID: <200001130455.FAA70312@dorifer.heim3.tu-clausthal.de> In-Reply-To: <85jgok$1rc9$1@atlantis.rz.tu-clausthal.de>
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Laurence Berland <stuyman@confusion.net> wrote in list.freebsd-questions: > Oliver Fromme wrote: >> Apart from that: You can't use a FILE* within a signal handler >> anyway, because most of stdio's functions are not guaranteed to >> be re-entrant. >> >> If you have to do something with a file within a signal >> handler, use the lower-level file handling functions based on >> file descriptors, i.e. open(), read(), write(), close(). >> Conveniently, a file descriptor fits in a sig_atomic_t, too. >> >> Please refer to the sigaction(2) manpage, it contains a >> complete list of all functions that are "async-signal safe". >> All other functions are _not_ safe to use from within a signal >> handler. > > I take it it's equally unsafe if the signal handler calls a function > that calls things outside of the async-safe list. Correct, because all of that would still happen within the signal handler context. > The issue here is > that I'm writing an ftp daemon for a compsci project, and I'm getting > hung up on what to do when SIGURG is received as part of a TCP sync. > Since I've been using stream based functions (fprintf fgets, etc.) I > would hope to use those in the sighandler too. If that's not possible > (or rather, not safe, as you indicate), how do you suggest I proceed? I > suppose I could do a rewrite using combinations of snprintfs to format > strings and read/write to do input output. This is certainly possible > if necessary, but preferably avoidable. Any suggestions? Your signal handler could simply set a flag bit in some variable. Your "main" code would then have to check for that flag regularly. > <aside>Is there a way to discover these various caveats to issues that > one with as little experience as I might find more easily?</aside> I'd recommend Stevens' book on UNIX Programming. A few important issues can also be found in the UNIX programmers' FAQ: http://www.erlenstar.demon.co.uk/unix/faq_toc.html Regards Oliver -- Oliver Fromme, Leibnizstr. 18/61, 38678 Clausthal, Germany (Info: finger userinfo:olli@dorifer.heim3.tu-clausthal.de) "In jedem Stück Kohle wartet ein Diamant auf seine Geburt" (Terry Pratchett) To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-questions" in the body of the message
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