Debugger

Debugging Net-SNMP Applications

This page contains some tips on debugging Net-SNMP Applications. Often, when an application crashes, you may be asked to run the application under a debugger, or provide details from a core dump. This page presents a simple introduction to one debugger, GDB.

GDB / DDD

GDB is the GNU debugger, which is a terminal-based debugger. DDD is a graphical (GUI) front end to GDB.

Normal invocation

The simplest way to run an application under GDB is like so:

$ gdb /usr/sbin/snmpd
(gdb) run -f -Lo

The '-f' is necessary to prevent snmpd from forking into the background, and '-Lo' tells snmpd to send log messages to STDOUT (i.e. print them in the GDB console window).

Libtool invocation

Net-SNMP uses libtool while building it's applications. This means that sometimes the applications in the source/build directory are not actually binaries, but shell scripts. The libtool script does some magic so that the applications will run using the shared libraries in the build directory, instead of any libraries installed on the system. To run an application with GDB, you have to run GDB through libtool, like so:

$ ./libtool gdb agent/snmpd
(gdb) run -f -Lo

Getting a backtrace (aka stack trace)

Once you have got an application running under GDB, you can easily get a backtrace, which will display the sequence of functions that were called to arrive at the point where the debugger is currently stopped.

(gdb) bt
#0  0x10153d78 in init_snmp(type=0x101bb140 "snmp") at snmp_api.c:854
#1  0x1000524c in main(argc=3, argv=0x7fe04636) at snmpd.c:910

Loading a core file

If an application has crashed, often it will leave a core file. This core file can usually be loaded into GDB to get debugging information after the fact. Simply add the path to the core file after the path to the application when starting GDB.

$ libtool gdb agent/snmpd /tmp/core.70816

Often this will tell you why the application crashed (e.g. SIGSEGV, aka signal 11). You can then get a backtrace to send to one of the mailing lists for interpretation.