[Libvirt-cim] debugging providers with gdb

So we all know that it's annoying when your provider is running in sfcb and it segfaults, because using gdb on a threaded app is not so fun. Well, I re-visited the debugging sfcb section of sfcb's README, and I guess something sunk in this time because I was able to put it into practice. This seemed like the kind of thing to mention here for posterity. Turns out it's pretty simple. (FYI, not sure how much of this requires you to be root, I just did the easy way out and was root for all of it.) The first step is to recompile sfcb with debugging symbols: CFLAGS=-g ./configure make clean && make && make install Then start up sfcbd and tell it which provider you want to debug: SFCB_PAUSE_PROVIDER=Virt_VSMigrationService sfcbd # (of course put your provider's name in here) Now do whatever it is you need to do to get your provider to run (wbemcli call, etc.), and you'll see something like this: -#- Pausing for provider: Virt_VSMigrationService -pid: 13943 Now you can attach gdb. Open up a new terminal and run: gdb sfcbd 13943 # whatever pid you just got You're now attached to the thread; I'll summarize what state everything is in. After gdb attaches, it pauses the thread. The thread itself is in a loop that checks if a local variable (debug_break) is zero. If it is zero, the thread sleeps for five seconds and checks again. We need to get out of that loop so our provider can actually run. So we set a breakpoint at the check. The check is in providerDrv.c; in sfcb-1.2.5, it is line 2577. To double-check, just do a grep: grep -n debug_break providerDrv.c 2569: int debug_break = 0; 2577: if (debug_break) break; That if statement is the one we need. So at the gdb prompt, set the breakpoint, then use "continue" to start the thread. You should hit the breakpoint very quickly: (gdb) break providerDrv.c:2577 Breakpoint 1 at 0x1c1c36: file providerDrv.c, line 2577. (gdb) continue Continuing. [Switching to Thread -1208785216 (LWP 13943)] Breakpoint 1, processProviderInvocationRequests (name=0x9e06300 "Virt_VSMigrationService") at providerDrv.c:2577 2577 if (debug_break) break; Now we can set debug_break to nonzero so it will break out of the sleep loop. This is also a convenient place to set breakpoints in your provider if you wanted to. If you're just waiting for a segfault to hit so you can do a backtrace, you shouldn't have to worry about that. For the example, we'll just get out of the loop: (gdb) set debug_break=1 (gdb) continue And now your provider is running in gdb. For a little help with what you can do with gdb, I found this to be a pretty good resource: http://www.delorie.com/gnu/docs/gdb/gdb_toc.html#SEC_Contents -- -Jay