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## Getting Started

### How to print current call stack

`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
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.

### Handle terminates, aborts and Segmentation Faults

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
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.

| Warning
Not all the platforms provide means for even getting stacktrace in async signal safe way. No stack trace will be saved on such platforms.

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

### Better asserts

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.

### Exceptions with stacktrace

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 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

### Enabling and disabling stacktraces

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.

### Saving stacktraces by specified format

`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,

### Getting function information from pointer

`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

### Global control over stacktrace output format

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)