That looks a lot like a double free [...]
there's definitely something about initializing libcurl: http://curl.haxx.se/libcurl/c/curl_easy_init.html uses nice phrases like "may be letal in multi-threading" the documentation of Haskell curl http://hackage.haskell.org/packages/archive/curl/1.3.5/doc/html/Network- Curl.html just says "withCurlDo should be called once" while in fact it should be much stronger: "must be called exactly once"? anyway I temporarily dropped curl (replaced by system "wget" ) and the erratic behaviour persists. Now it looks like this: Electric Fence 2.1 Copyright (C) 1987-1998 Bruce Perens. [New Thread 0x7fc37b70f6e0 (LWP 7281)] [New Thread 0x4122a950 (LWP 7284)] [New Thread 0x4214a950 (LWP 7285)] Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x7fc37b70f6e0 (LWP 7281)] 0x00007fc37b715aae in memalign () from /usr/lib/libefence.so.0 (gdb) where #0 0x00007fc37b715aae in memalign () from /usr/lib/libefence.so.0 #1 0x00007fc37b715c97 in malloc () from /usr/lib/libefence.so.0 #2 0x00007fc37a492a55 in gethostbyname () from /lib/libc.so.6 #3 0x000000000089cc88 in networkzm2zi2zi1zi7_NetworkziBSD_zdwlvl_info () #4 0x0000000000000000 in ?? () and indeed, gethostbyname is famous for being non re-entrant. (packages I use are HTTP, hxt, happstack-server; and I can't drop all of them ...)