What is a coredump ?

A coredump is the state of the programs memory when it crashed. It allows programmers to exactly nail down the line in the code which caused the segfault. See https://en.wikipedia.org/wiki/Core_dump for more information.

Why do I need ulimit and what does it do ?

As coredumps save the programs memory on the hard disk the coredump files can become quite large because some applications consume a lot of memory. On embedded devices (e.g. small routers) a coredump can fill the entire disk easily. Therefore the tool “ulimit” allows you to control what memory size is safe to be saved on disk. A “ulimit -c 20000” saves coredumps of up to 20MB, “ulimit -c unlimited” saves everything no matter how big it is. You can check your systems default by running “ulimit -c” without any value. If the setting is too small or coredumping is deactivated, you have to run “ulimit -c “ each time before you start batman.

How to get a batman coredump ?

  • log into a shell on your device

  • set the ulimit value if necessary (see ulimit section of this document)

  • start batman in this very shell but don’t let it fork into the background using a debug level (-d 3 or -d 4)

  • do not close the shell as it will kill the running batman

  • make batman crash (depending on the bug you experience)

  • retrieve the “core” file from the current directory

Don’t forget to send the used batman binary along with the coredump. Without the correct binary the coredump is useless!

I can’t find the coredump …

  • May be batman did not crash but just exited ? A coredump can be created only on a segmentation fault. Your system logs should contain a log entry similar to “Error - SIGSEGV received, trying to clean up …” otherwise batman did not crash.

  • Did you check the ulimit section ?

  • The coredumping behaviour can be modified by changing some /proc parameters like /proc/sys/kernel/core_uses_pid and /proc/sys/kernel/core_pattern. In most cases the defaults are the right choice. Only modify them if you are sure what you are doing!

  • Some distributions (especially for embedded devices) use busybox which allows to completely disable coredumping (even if ulimit is set). Look for the CONFIG_FEATURE_INIT_COREDUMPS option to learn more about it. OpenWRT allows to enable it via ‘make menuconfig’: Base system -> busybox -> Configuration -> Init Utilities -> Support dumping core for child processes.

  • OpenWRT also may disable ELF core dumping in the kernel which you can activate by running “make kernel_menuconfig” -> General setup -> Configure standard kernel features (for small systems) -> Enable ELF core dumps