On Mon, Oct 22, 2012 at 02:24:01AM +0200, Thorben Thuermer wrote: > on http://curl.haxx.se/libcurl/features.html we find: > "libcurl is designed and implemented entirely thread safe." > "libcurl uses certain system calls to obtain information. Some of the most > crucial ones are the name resoluition calls (the gethostby* family)." > > i just debugged a problem in a multithreaded application > ( https://github.com/volkszaehler/vzlogger , thread in german: > > http://volkszaehler.org/pipermail/volkszaehler-users/2012-October/000619.html > ) > that appers to be caused by libcurl's (ab)use of alarm() to timeout > gethostbyname() not being thread-safe. > (as described here: https://bugzilla.redhat.com/show_bug.cgi?id=539809 ). > > there appears to be a race-condition when two threads run requests > simultaneously, which reproducibly leads to a crash with > the logging function being called with an invalid buffer/file, > without any actual dns-timeout having occured. > (the application only accesses localhost, which is defined in /etc/hosts, > curiously the problem persists if a numeric IP is used.)
If the problem exists even with a numeric IP, then it's unlikely to be a DNS issue since numeric addresses bypass the external resolver. Are you making SSL connections by chance? There is more you may need to do in that case; http://curl.haxx.se/libcurl/c/libcurl-tutorial.html#Multi-threading > the problem disappeared when i compiled libcurl without HAVE_ALARM. CURLOPT_NOSIGNAL ought to do the same at run-time, but it's odd that this occurs with numeric addresses (assuming SSL is not involved). > i'm not sure if this is a libcurl bug or just a documentation issue, > but it has cost us some time to track down... This is from curl_easy_setopt(3) http://curl.haxx.se/libcurl/c/curl_easy_setopt.html CURLOPT_NOSIGNAL Pass a long. If it is 1, libcurl will not use any functions that install signal handlers or any functions that cause signals to be sent to the process. This option is mainly here to allow multi-threaded unix applications to still set/use all timeout options etc, without risking getting signals. (Added in 7.10) And this is from libcurl-tutorial(3) http://curl.haxx.se/libcurl/c/libcurl-tutorial.html#Multi-threading When using multiple threads you should set the CURLOPT_NOSIGNAL option to 1 for all handles. libcurl now gives a number of choices of DNS resolver back-end at compile time, some of which are immune to this problem. >>> Dan ------------------------------------------------------------------- List admin: http://cool.haxx.se/list/listinfo/curl-library Etiquette: http://curl.haxx.se/mail/etiquette.html
