At this point there are several different ways to dig further. So how does our segfault look now? I don't know (I'd need to read more source). It also inspired me to share a full gdb debugging example, with output and every step involved, including dead ends. Ok, more clues...I think. With the debug info package installed, gdb can list the source along with the assembly: Great! entries can be fixed by adding the python-dbg package. Ok, so what's CUR? You'll probably want you program to stop at some point so that you can review the condition of your program. If it doesn't, the bad news is you'll need to write your own. I wanted to show this error to explain why we often start out with a breakpoint on main, at which point the symbols are likely loaded, and then setting the real breakpoint of interest. (I'll get back to this in section 15.). So this experiment ended in another segfault. The stack trace looks a bit different: we aren't really in doupdate(), but ClrBlank(), which has been inlined in ClrUpdate(), and inlined in doupdate(). I'll move back in time two instructions, then print registers: So, back to finding the "cur_term" clue. Here's an example. Another advantage is that I can trace a few events or a few thousand just as easily. I'd like a core dump to debug this. d N - Deletes breakpoint number N WARNING: Writing memory is not safe! It worked! But the commands and procedures I used to debug it were mostly routine: viewing stack traces, checking registers, setting breakpoints, stepping, and browsing source. Browsing the code paths that set it might provide more clues, to help answer why it isn't being set, or why it is set to zero. The problem with the initial stack trace is that we're seeing Python internals that are executing the methods, but not the methods themselves. That will be ok for now, but I'll show how to set this up for a global location: You can customize that core_pattern further; eg, %h for hostname and %t for time of dump. gdb has improved a lot since then, as have my gdb skills, and I now see it as a powerful modern debugger. Recording adds considerable overhead, so I don't want to add it on main. The /proc/.../core_pattern is set to just "core", which will drop a core dump file called "core" in the current directory. (I could also have just typed "disas" and it would have defaulted to doupdate.). One advantage of using tracers is that they don't pause the target process, like gdb does (although that doesn't matter for this cachetop.py example). I also used i b here (info breakpoints) to list them with information. Here's an example. Same place. More source code browsing using cscope, this time in llvm. It works by playing back register state from our recording. b fn - Puts a breakpoint at the beginning of function "fn". You can double check if zero is valid using i proc m (short for info proc mappings): The first valid virtual address is 0x400000. Then frame 4 is the _curses library, then we're in libncursesw. Plus if I took a closer look at the source, I would have noticed it was building it for libtinfo. I could also use an external tracer to grab data and stack traces on segfault events. Using the first option in cscope: I added the highlighting. So why didn't I run it right away, when I first created the "pending" breakpoint? gdb For the above example with a program named main, the command becomes gdb main Setting Breakpoints. Looking up definitions in cscope is a breeze. This is libncursesw, and I don't have debug info installed (Ubuntu): Good, those versions match. (And we should get that llvm bug fixed.). Tip You may wonder how gdb determines which variable named my var to watch if there is more than one declared in your program. This isn't a particularly interesting or exotic issue, it's just a routine gdb debugging session. We're in ClrBlank(), so I'll list that source code: Ah, that's not defined in the function, so it's a global? I'll run the program and break on set_curterm() as usual: Now I'll turn breakpoint 1 into a conditional breakpoint, so that it only fires when the %rdi register is zero: Neat! Core dump analysis is one approach for debugging, but not the only one. The bcc collection of BPF tools had a pull request for cachetop, which uses a top-like display to show page cache statistics by process. The FileDescriptorHasColors() function has: Here's what that code used to be in an earlier version: It became a "silly dance" involving calling set_curterm() with a null pointer. entries are where symbol translation failed. But it covers the basics and could serve as a tutorial of sorts, bearing in mind there's a lot more to gdb than I used here. I've found conditionals don't work on pending breakpoints, at least on this gdb version. gdb is the acronym for GNU Debugger. I already can guess what libncursesw is for, but if that were foreign to you, then being under "/lib" and ending in ".so. I hope anyone searching for gdb examples finds the full output I've shared to be useful, as well as the various caveats I discussed along the way. I'll go straight to doupdate function entry, run the problem, then set the offset breakpoint once it hits the function: If you haven't done this before, the r (run) command takes arguments that will be passed to the gdb target we specified earlier on the command line (python). Best case, your application crashes immediately, and you realize your mistake. Now TERMINAL is capitalized. This particular stack doesn't look very helpful: frames 5 to 17 (indexed on the left) are Python internals, although we can't see the Python methods (yet). Note that I've inspected just the first invocation of doupdate(), but it could be called multiple times, and the issue may be a later invocation. I'll take us to the set_curterm() 0x0 breakpoint as before, and then issue a ret (short for return), which will return from the function immediately and not execute it. Substitute non-root and sudo as desired. Is there something special about back_color_erase? The help command will list the major sections: You can then run help on each command class. Looks like wgetch()->wrefresh()->doupdate(). I was reminded of the lack of example output when watching the Give me 15 minutes and I'll change your view of GDB talk by Greg Law at CppCon 2015, which, thankfully, includes output! I could run the program live in gdb to inspect the issue. Guide to use GDB and learn debugging techniques. If symbols or stacks are too badly broken to make sense of the stack trace, then there are usually ways to fix it: installing debug info packages (giving gdb more symbols, and letting it do DWARF-based stack walks), or recompiling the software from source with frame pointers and debugging information (-fno-omit-frame-pointer -g). That doesn't seem possible. There's our problem! I should be able to trace calls to set_curterm() in libncursesw, and even print the first argument: Well, that didn't work. This sequence is for walking data structures. I tried setting that to vt100 and running the program, but it hit the same segfault. Just based on the names, I'd guess a window refresh. While I was messing with writes and returns, he suggested adding the llvm option -fno-color-diagnostics to bcc, to avoid this problem code path.
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