Getting the current stack trace on Mac OS X

Question

I'm trying to work out how to store and then print the current stack in my C++ apps on Mac OS X. The main problem seems to be getting dladdr to return the right symbol when given an address inside the main executable. I suspect that the issue is actually a compile option, but I'm not sure.

I have tried the backtrace code from Darwin/Leopard but it calls dladdr and has the same issue as my own code calling dladdr.

Original post: Currently I'm capturing the stack with this code:

int BackTrace(Addr *buffer, int max_frames)
{
    void **frame = (void **)__builtin_frame_address(0);
    void **bp = ( void **)(*frame);
    void *ip = frame[1];
    int i;

    for ( i = 0; bp && ip && i < max_frames; i++ )
    {
        *(buffer++) = ip;
        ip = bp[1];
        bp = (void**)(bp[0]);
    }

    return i;
}

Which seems to work ok. Then to print the stack I'm looking at using dladdr like this:

Dl_info dli;
if (dladdr(Ip, &dli))
{
    ptrdiff_t       offset;
    int c = 0;

    if (dli.dli_fname && dli.dli_fbase)
    {
        offset = (ptrdiff_t)Ip - (ptrdiff_t)dli.dli_fbase;
        c = snprintf(buf, buflen, "%s+0x%x", dli.dli_fname, offset );
    }
    if (dli.dli_sname && dli.dli_saddr)
    {
        offset = (ptrdiff_t)Ip - (ptrdiff_t)dli.dli_saddr;
        c += snprintf(buf+c, buflen-c, "(%s+0x%x)", dli.dli_sname, offset );
    }

    if (c > 0)
        snprintf(buf+c, buflen-c, " [%p]", Ip);

Which almost works, some example output:

/Users/matthew/Library/Frameworks/Lgi.framework/Versions/A/Lgi+0x2473d(LgiStackTrace+0x5d) [0x102c73d]
/Users/matthew/Code/Lgi/LgiRes/build/Debug/LgiRes.app/Contents/MacOS/LgiRes+0x2a006(tart+0x28e72) [0x2b006]
/Users/matthew/Code/Lgi/LgiRes/build/Debug/LgiRes.app/Contents/MacOS/LgiRes+0x2f438(tart+0x2e2a4) [0x30438]
/Users/matthew/Code/Lgi/LgiRes/build/Debug/LgiRes.app/Contents/MacOS/LgiRes+0x35e9c(tart+0x34d08) [0x36e9c]
/Users/matthew/Code/Lgi/LgiRes/build/Debug/LgiRes.app/Contents/MacOS/LgiRes+0x1296(tart+0x102) [0x2296]
/Users/matthew/Code/Lgi/LgiRes/build/Debug/LgiRes.app/Contents/MacOS/LgiRes+0x11bd(tart+0x29) [0x21bd]

It's getting the method name right for the shared object but not for the main app. Those just map to "tart" (or "start" minus the first character).

Ideally I'd like line numbers as well as the method name at that point. But I'll settle for the correct function/method name for starters. Maybe shoot for line numbers after that, on Linux I hear you have to write your own parser for a private ELF block that has it's own instruction set. Sounds scary.

Anyway, can anyone sort this code out so it gets the method names right?

Urn

Answer 1

What releases of OS X are you targetting. If you are running on Mac OS X 10.5 and higher you can just use the backtrace() and backtrace_symbols() libraray calls. They are defined in execinfo.h, and there is a manpage with some sample code.

Edit:

You mentioned in the comments that you need to run on Tiger. You can probably just include the implementation from Libc in your app. The source is available from Apple's opensource site. Here is a link to the relevent file.

Determined

I'm currently targeting the current and previous releases, ie Leopard and Tiger. Also I currently have Tiger on my Macbook. So until snow leopard comes out my code will need to compile/run on Tiger. – Urn

Well, the source code for those function is opensource, so you can probably include the implementation in your app. I am editing the my answer with the relevent info. – Determined

I've had a look at the code you posted links and it eventually uses dladdr just the same as my code. Which gives the same wrong results, in that anything in my executable (i.e. not a shared object) has the symbol "tart". – Urn

In OS 10.6 and later, there is the higher-level wrapper +[NSThread callStackSymbols]. – Lyophilize

Apple manpage link has changed: developer.apple.com/library/archive/documentation/System/… – Overdone

backtrace_symbols returns assembly file and offset, exactly the same information returned by dladdr. The question was about getting source file name and line, which this is zero help to answering. – Regan

Answer 2

On macOS, you can use atos to look up source file and line numbers from debug symbols. atos is a command line utility, so you need to launch a separate process. Something like this will work:

#include <iostream>
#include <sstream>

#include <dlfcn.h>
#include <execinfo.h>
#include <stdio.h>

void print_backtrace() {
  constexpr int kBacktraceDepth = 15;
  void* backtrace_addrs[kBacktraceDepth];

  int trace_size = backtrace(backtrace_addrs, kBacktraceDepth);

  for (int i = 0; i < trace_size; ++i) {
    Dl_info info;
    dladdr(backtrace_addrs[i], &info);

    std::stringstream cmd(std::ios_base::out);
    cmd << "atos -o " << info.dli_fname << " -l " << std::hex
      << reinterpret_cast<uint64_t>(info.dli_fbase) << ' '
      << reinterpret_cast<uint64_t>(backtrace_addrs[i]);

    FILE* atos = popen(cmd.str().c_str(), "r");

    constexpr int kBufferSize = 200;
    char buffer[kBufferSize];

    fgets(buffer, kBufferSize, atos);
    pclose(atos);

    std::cout << buffer;
  }
  std::cout << std::flush;
}

Regan