Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

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Getting Started

How to print current call stack
Handle terminates, aborts and Segmentation Faults
Better asserts
Exceptions with stacktrace
Enabling and disabling stacktraces
Saving stacktraces by specified format
Getting function information from pointer
Global control over stacktrace output format

boost::stacktrace::stacktrace contains methods for working with call-stack/backtraces/stacktraces. Here's a small example:

#include <boost/stacktrace.hpp>

// ... somewhere inside the `bar(int)` function that is called recursively:
std::cout << boost::stacktrace::stacktrace();

In that example:

  • boost::stacktrace:: is the namespace that has all the classes and functions to work with stacktraces
  • stacktrace() is the default constructor call; constructor stores the current function call sequence inside the stacktrace class.

Code from above will output something like this:

0# bar(int) at /path/to/source/file.cpp:70
1# bar(int) at /path/to/source/file.cpp:70
2# bar(int) at /path/to/source/file.cpp:70
3# bar(int) at /path/to/source/file.cpp:70
4# main at /path/to/main.cpp:93
5# __libc_start_main in /lib/x86_64-linux-gnu/libc.so.6
6# _start
[Note] Note

By default the Stacktrace library is very conservative in methods to decode stacktrace. If your output does not look as fancy as in example from above, see section "Configuration and Build" for allowing advanced features of the library.

Segmentation Faults and std::terminate calls sometimes happen in programs. Programmers usually wish to get as much information as possible on such incidents, so having a stacktrace will significantly improve debugging and fixing.

std::terminate calls std::abort, so we need to capture stack traces on Segmentation Faults and Abort signals.

[Warning] Warning

Writing a signal handler requires high attention! Only a few system calls allowed in signal handlers, so there's no cross platform way to print a stacktrace without a risk of deadlocking. The only way to deal with the problem - dump raw stacktrace into file/socket and parse it on program restart.

Let's write a handler to safely dump stacktrace:

#include <signal.h>     // ::signal, ::raise
#include <boost/stacktrace.hpp>

void my_signal_handler(int signum) {
    ::signal(signum, SIG_DFL);
    boost::stacktrace::safe_dump_to("./backtrace.dump");
    ::raise(SIGABRT);
}

Registering our handler:

::signal(SIGSEGV, &my_signal_handler);
::signal(SIGABRT, &my_signal_handler);

At program start we check for a file with stacktrace and if it exist - we're writing it in human readable format:

if (boost::filesystem::exists("./backtrace.dump")) {
    // there is a backtrace
    std::ifstream ifs("./backtrace.dump");

    boost::stacktrace::stacktrace st = boost::stacktrace::stacktrace::from_dump(ifs);
    std::cout << "Previous run crashed:\n" << st << std::endl;

    // cleaning up
    ifs.close();
    boost::filesystem::remove("./backtrace.dump");
}

Now we'll get the following output on std::terminate call after the program restarts:

Previous run crashed:
 0# 0x00007F2EC0A6A8EF
 1# my_signal_handler(int) at ../example/terminate_handler.cpp:37
 2# 0x00007F2EBFD84CB0
 3# 0x00007F2EBFD84C37
 4# 0x00007F2EBFD88028
 5# 0x00007F2EC0395BBD
 6# 0x00007F2EC0393B96
 7# 0x00007F2EC0393BE1
 8# bar(int) at ../example/terminate_handler.cpp:18
 9# foo(int) at ../example/terminate_handler.cpp:22
10# bar(int) at ../example/terminate_handler.cpp:14
11# foo(int) at ../example/terminate_handler.cpp:22
12# main at ../example/terminate_handler.cpp:84
13# 0x00007F2EBFD6FF45
14# 0x0000000000402209
[Note] Note

Function names from shared libraries may not be decoded due to address space layout randomization. Still better than nothing.

Pretty often assertions provide not enough information to locate the problem. For example you can see the following message on out-of-range access:

../../../boost/array.hpp:123: T& boost::array<T, N>::operator[](boost::array<T, N>::size_type) [with T = int; long unsigned int N = 5ul]: Assertion '(i < N)&&("out of range")' failed.
Aborted (core dumped)

That's not enough to locate the problem without debugger. There may be thousand code lines in real world examples and hundred places where that assertion could happen. Let's try to improve the assertions, and make them more informative:

// BOOST_ENABLE_ASSERT_DEBUG_HANDLER is defined for the whole project
#include <stdexcept>    // std::logic_error
#include <iostream>     // std::cerr
#include <boost/stacktrace.hpp>

namespace boost {
    inline void assertion_failed_msg(char const* expr, char const* msg, char const* function, char const* /*file*/, long /*line*/) {
        std::cerr << "Expression '" << expr << "' is false in function '" << function << "': " << (msg ? msg : "<...>") << ".\n"
            << "Backtrace:\n" << boost::stacktrace::stacktrace() << '\n';
        std::abort();
    }

    inline void assertion_failed(char const* expr, char const* function, char const* file, long line) {
        ::boost::assertion_failed_msg(expr, 0 /*nullptr*/, function, file, line);
    }
} // namespace boost

We've defined the BOOST_ENABLE_ASSERT_DEBUG_HANDLER macro for the whole project. Now all the BOOST_ASSERT and BOOST_ASSERT_MSG will call our functions assertion_failed and assertion_failed_msg in case of failure. In assertion_failed_msg we output information that was provided by the assertion macro and boost::stacktrace::stacktrace:

Expression 'i < N' is false in function 'T& boost::array<T, N>::operator[](boost::array<T, N>::size_type) [with T = int; long unsigned int N = 5ul; boost::array<T, N>::reference = int&; boost::array<T, N>::size_type = long unsigned int]': out of range.
Backtrace:
 0# boost::assertion_failed_msg(char const*, char const*, char const*, char const*, long) at ../example/assert_handler.cpp:39
 1# boost::array<int, 5ul>::operator[](unsigned long) at ../../../boost/array.hpp:124
 2# bar(int) at ../example/assert_handler.cpp:17
 3# foo(int) at ../example/assert_handler.cpp:25
 4# bar(int) at ../example/assert_handler.cpp:17
 5# foo(int) at ../example/assert_handler.cpp:25
 6# main at ../example/assert_handler.cpp:54
 7# 0x00007F991FD69F45 in /lib/x86_64-linux-gnu/libc.so.6
 8# 0x0000000000401139

Now we do know the steps that led to the assertion and can find the error without debugger.

You can provide more information along with exception by embedding stacktraces into the exception. There are many ways to do that, here's how to doe that using Boost.Exception:

  • Declare a boost::error_info typedef that holds the stacktrace:
#include <boost/stacktrace.hpp>
#include <boost/exception/all.hpp>

typedef boost::error_info<struct tag_stacktrace, boost::stacktrace::stacktrace> traced;
  • Write a helper class for throwing any exception with stacktrace:
template <class E>
void throw_with_trace(const E& e) {
    throw boost::enable_error_info(e)
        << traced(boost::stacktrace::stacktrace());
}
  • Use throw_with_trace(E); instead of just throw E;:
if (i >= 4)
    throw_with_trace(std::out_of_range("'i' must be less than 4 in oops()"));
if (i <= 0)
    throw_with_trace(std::logic_error("'i' must not be greater than zero in oops()"));
  • Process exceptions:
try {
    foo(5); // testing assert handler
} catch (const std::exception& e) {
    std::cerr << e.what() << '\n';
    const boost::stacktrace::stacktrace* st = boost::get_error_info<traced>(e);
    if (st) {
        std::cerr << *st << '\n';
    }
}

Code from above will output:

'i' must not be greater than zero in oops()
 0# void throw_with_trace<std::logic_error>(std::logic_error const&) at ../example/throwing_st.cpp:22
 1# oops(int) at ../example/throwing_st.cpp:38
 2# bar(int) at ../example/throwing_st.cpp:54
 3# foo(int) at ../example/throwing_st.cpp:59
 4# bar(int) at ../example/throwing_st.cpp:49
 5# foo(int) at ../example/throwing_st.cpp:59
 6# main at ../example/throwing_st.cpp:76
 7# 0x00007FAC113BEF45 in /lib/x86_64-linux-gnu/libc.so.6
 8# 0x0000000000402ED9

At some point arises a requirement to easily enable/disable stacktraces for a whole project. That could be easily achieved.

Just define BOOST_STACKTRACE_LINK for a whole project. Now you can enable/disable stacktraces by just linking with different libraries:

  • link with boost_stacktrace_noop to disable backtracing
  • link with other boost_stacktrace_* libraries

See section "Configuration and Build" for more info.

boost::stacktrace::stacktrace provides access to individual frames of the stacktrace, so that you could save stacktrace information in your own format. Consider the example, that saves only function addresses of each frame:

#include <boost/stacktrace.hpp>
#include <iostream>     // std::cout

namespace bs = boost::stacktrace;
void dump_compact(const bs::stacktrace& st) {
    for (bs::frame frame: st) {
        std::cout << frame.address() << ',';
    }

    std::cout << std::endl;
}

Code from above will output:

0x7fbcfd17f6b5,0x400d4a,0x400d61,0x400d61,0x400d61,0x400d61,0x400d77,0x400cbf,0x400dc0,0x7fbcfc82d830,0x400a79,

boost::stacktrace::frame provides information about functions. You may construct that class from function pointer and get the function name at runtime:

#include <signal.h>     // ::signal
#include <boost/stacktrace/frame.hpp>
#include <iostream>     // std::cerr
#include <cstdlib>      // std::exit

void print_signal_handler_and_exit() {
    typedef void(*function_t)(int);

    function_t old_signal_function = ::signal(SIGSEGV, SIG_DFL);
    boost::stacktrace::frame f(old_signal_function);
    std::cout << f << std::endl;
    std::exit(0);
}

Code from above will output:

my_signal_handler(int) at boost/libs/stacktrace/example/debug_function.cpp:21

You may override the behavior of default stacktrace output operator by defining the macro from Boost.Config BOOST_USER_CONFIG to point to a file like following:

#ifndef USER_CONFIG_HPP
#define USER_CONFIG_HPP

#include <boost/stacktrace/stacktrace_fwd.hpp>

#include <iosfwd>

namespace boost { namespace stacktrace {

template <class CharT, class TraitsT, class Allocator>
std::basic_ostream<CharT, TraitsT>& do_stream_st(std::basic_ostream<CharT, TraitsT>& os, const basic_stacktrace<Allocator>& bt);

template <class CharT, class TraitsT>
std::basic_ostream<CharT, TraitsT>& operator<<(std::basic_ostream<CharT, TraitsT>& os, const stacktrace& bt) {
    return do_stream_st(os, bt);
}

}}  // namespace boost::stacktrace
#endif // USER_CONFIG_HPP

Implementation of do_stream_st may be the following:

namespace boost { namespace stacktrace {

template <class CharT, class TraitsT, class Allocator>
std::basic_ostream<CharT, TraitsT>& do_stream_st(std::basic_ostream<CharT, TraitsT>& os, const basic_stacktrace<Allocator>& bt) {
    const std::streamsize w = os.width();
    const std::size_t frames = bt.size();
    for (std::size_t i = 0; i < frames; ++i) {
        os.width(2);
        os << i;
        os.width(w);
        os << "# ";
        os << bt[i].name();
        os << '\n';
    }

    return os;
}

}}  // namespace boost::stacktrace

Code from above will output:

Terminate called:
 0# bar(int)
 1# foo(int)
 2# bar(int)
 3# foo(int)

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