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

This is the documentation for a snapshot of the master branch, built from commit 00546ea44c.
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Getting Started

How to print current call stack
Better asserts
Handle terminates
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.

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.

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

Here's how to write a terminate handler that dumps stacktrace:

#include <cstdlib>       // std::abort
#include <exception>     // std::set_terminate
#include <iostream>      // std::cerr

#include <boost/stacktrace.hpp>

void my_terminate_handler() {
    try {
        std::cerr << boost::stacktrace::stacktrace();
    } catch (...) {}
    std::abort();
}

Here's how to register it:

std::set_terminate(&my_terminate_handler);

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

Previous run crashed:
 0# my_terminate_handler(int) at ../example/terminate_handler.cpp:37
 1# __cxxabiv1::__terminate(void (*)()) at ../../../../src/libstdc++-v3/libsupc++/eh_terminate.cc:48
 2# 0x00007F3CE65E5901 in /usr/lib/x86_64-linux-gnu/libstdc++.so.6
 3# bar(int) at ../example/terminate_handler.cpp:18
 4# foo(int) at ../example/terminate_handler.cpp:22
 5# bar(int) at ../example/terminate_handler.cpp:14
 6# foo(int) at ../example/terminate_handler.cpp:22
 7# main at ../example/terminate_handler.cpp:84
 8# __libc_start_main in /lib/x86_64-linux-gnu/libc.so.6
 9# 0x0000000000402209
[Warning] Warning

There's a temptation to write a signal handler that prints the stacktrace on SIGSEGV or abort. Unfortunately, there's no cross platform way to do that without a risk of deadlocking. Not all the platforms provide means for even getting stacktrace in async signal safe way.

Signal handler is often invoked on a separate stack and trash is returned on attempt to get a trace!

Generic recommendation is to avoid signal handlers! Use platform specific ways to store and decode core files.

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 do 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 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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