How to get the correct line of code with backtrace in C++?

Question

I took this code and modified it to look like this:

std::string Backtrace(int skip = 1)
{
    void *callstack[128];
    const int nMaxFrames = sizeof(callstack) / sizeof(callstack[0]);
    char buf[1024];
    int nFrames = backtrace(callstack, nMaxFrames);
    char **symbols = backtrace_symbols(callstack, nFrames);

    string message = "";
    for (int i = skip; i < nFrames; i++) {
        Dl_info info;
        if (dladdr(callstack[i], &info)) {
            char *demangled = nullptr;
            int status;
            demangled = abi::__cxa_demangle(info.dli_sname, NULL, 0, &status);
            if(demangled != nullptr)
                message += string(demangled) + ": " +
                    to_string((char *)callstack[i] - (char *)info.dli_saddr) + "\n";
            free(demangled);
        }
    }
    free(symbols);
    if (nFrames == nMaxFrames)
        message += "[truncated]\n";
    return message;
}

This is supposed to print a stack trace of my current program to identify where things went awry without having to turn on gdb every time my program crashes.

When I run this code (in a state guaranteed to trigger an issue) I get the following stack trace:

DebugCallback(VkDebugUtilsMessageSeverityFlagBitsEXT, unsigned int, VkDebugUtilsMessengerCallbackDataEXT const*, void*): 146
vk::DispatchLoaderStatic::vkQueueSubmit(VkQueue_T*, unsigned int, VkSubmitInfo const*, VkFence_T*) const: 50
Display::UpdateFrame(): 1088
RenderingPipeline::RenderFrame(vk::Buffer&, vk::Buffer&, Image&, unsigned int): 63
RenderHandler::RenderHandler(Window*, HardwareInterface*, Display*, Memory*): 784

My goal is to try to print as much relevant information as possible. (file, function, line). Now, I thought that the instruction: (char *)callstack[i] - (char *)info.dli_saddr) which I copied from the original script, would get me the line where the code was called, but for example the file where Display::UpdateFrame() is defined doesn;t even have 1000 lines, so trivially that number isn't the number of the calling code in the original file.

Is there a way to obtain this information with stack trace similarly to how GDB does it?

i.e if the function was called in the source code at

File: Display.hpp

Function: Display::UpdateFrame()

Line: 227

Can I retrieve that information at runtime using stacktrace?

Shot in the dark: does the line number shown include the pre-processor expanded header files above it? — Weak to Enuma Elish
– Weak to Enuma Elish, Commented Aug 31, 2019 at 22:56
Maybe, but I don't have a simple way to test it. Although if it is including the headers, i would expect a much larger number — Makogan
– Makogan, Commented Aug 31, 2019 at 23:01
You might be able to get more help with your inquiry by directing it to the gdb mailing list. Can't think of a better place to ask how to do something that gdb does, perhaps via an API interface of some sort, then from directly asking the folks who wrote gdb. I don't see any API of this available on Fedora, as far as I can tell, but maybe you can get lucky. — Sam Varshavchik
– Sam Varshavchik, Commented Sep 1, 2019 at 0:03
Are those line numbers or byte offsets of the machine instruction that crashed? — user1118321
– user1118321, Commented Sep 1, 2019 at 5:26
I am not sure, they are definitely not line numbers. So by elimination, byte offsets? — Makogan
– Makogan, Commented Sep 1, 2019 at 17:29

Carlo Wood · Accepted Answer · 2019-09-01 20:00:27Z

2

The backtrace() returns offsets in bytes relative to the start of some ELF section. In order to get line numbers and function names you need to use a library that can read the debug info of your program and then figure out which source file / line number / function the given offset corresponds to.

Here is an example of how to do this (written by me), using libbfd (assuming you're on linux):

https://github.com/CarloWood/libmemleak/blob/master/src/addr2line.c

answered Sep 1, 2019 at 20:00

Carlo Wood

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