HowToReportABug » History » Revision 13

Revision 12 (darix, 2006-09-28 12:37) → Revision 13/20 (simmel, 2009-02-24 09:53)

h1. How to report a bug 


 If you find a bug in lighttpd you can help alot to find a fix for it by following some simple steps: 

 * "Don't Panic":'t_panic 
 * if it is a crash generate a Backtrace 
 * if it behaves in a unexpected way prepare a strace 


 h2. Backtrace 


 A backtrace is a trace of the function which have been called before lighttpd died.  

 The best way to generate such a backtrace is using "valgrind": 




 $ valgrind --tool=memcheck -v --log-file=lighttpd --num-callers=8 lighttpd -D -f ./lighttpd.conf 


 valgrind does many useful things while it waits for the server to crash: 

 * checks for uninitialized variables 
 * checks for out-of-bounds access  
 * and many more 

 valgrind does not normally produce large log files and does not seem to degrade lighttpd's throughput much. Meaning, it's a very good alternative when debugging on busy servers as compared to strace. 

 If you can't use valgrind there is another way to generate a backtrace: 




 $ gdb lighttpd 
 (gdb) handle SIGPIPE pass nostop noprint 
 (gdb) r -D -f lighttpd.conf 
 (gdb) bt 


 h2. strace 


 strace is the name of the syscall-tracer on Linux. Every Unix-like OS provides a utility to collect information about system call like open, write, close and so on. 

 The have different names like  

 * strace 
 * truss 
 * ktrace (then kdump to get output) 

 If strace is available it is prefered as it provides the most useful information. Execute it like: 




 $ strace -olighttpd.trace -tt -s 4000 lighttpd -D -f ./lighttpd.conf  


 It will generate a file called lighttpd.trace. 

 If you have a running instance of lighttpd and want to see what lighttpd is doing you can attach strace to the process: 




 $ strace -olighttpd.trace -tt -s 4000 -p $pidof-lighttpd 


 Alternatively, if you want to debug a possible problem in lighttpd that is not related to networking and doing so on a busy server, try this: 




 $ strace -tt -s 4000 -etrace='!open,epoll_wait,epoll_ctl,sendfile64,read,fcntl,write,fcntl64, 
 close,time,stat64,stat,writev,setsockopt,accept,getsockname,ioctl,socket,connect' \ 
  -p $pidof-lighttpd 


 The above would tell strace to exclude the system calls listed from the trace, thus reducing the performance impact on lighttpd as well as producing much less output that may actually not be necessary in order to debug the problem at hand. 

 h2. == ktrace ==  

 On all BSDs incl. MacOS X you can use ktrace as replacement for strace. The output not as informative as strace, but still very important for tracking down problems. 

 ktrace will, once enabled, trace data until the process exists or the 
 trace point is cleared. A traced process can generate enormous amounts 
 of log data quickly; it is strongly suggested that users memorize how 
 to disable tracing before attempting to trace a process. The following 
 command is sufficient to disable tracing on all user owned processes, 
 and, if executed by root, all processes. 

 <pre> {{{ 
 $ ktrace -C 


 To enable tracing run: 




 $ ktrace -p $ -f lighttpd.trace 


 to attach to the process. 

 Use kdump to get useful data: 




 $ kdump -f lighttpd.trace -T -m 4000 -d 


 For more ways on limiting the output and such, see and 


 h2. Memleaks 


 If you experience that lighttpd is using more than 1Gb of RAM you might have spotted a memleak. Valgrind, our valuable helper, can help you to provide the necessary information to fix it. 




 $ valgrind --tool=memcheck -v --log-file=lighttpd --num-callers=8 \ 
   --leak-check=yes --show-reachable=yes \ 
   lighttpd -D -f ./lighttpd.conf 


 Beware that this will use alot of memory and will slow down the process. You might not be able to run this on a production system with real load. 


 h2. Writing a bug report 


 If you have generated a backtrace or a syscall-trace please make sure that you can reproduce the problem. Otherwise it will be really hard for the developers to fix the problem for you. 

 If possible open a "new ticket": 

 If the report contains sensitive data you might also send it directly to: