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442 lines
15 KiB
Markdown
442 lines
15 KiB
Markdown
Backward-cpp [](http://www.conan.io/source/backward/1.3.0/Manu343726/testing)
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============
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Backward is a beautiful stack trace pretty printer for C++.
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If you are bored to see this:
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Backward will spice it up for you:
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There is not much to say. Of course it will be able to display the code
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snippets only if the source files are accessible (else see trace #4 in the
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example).
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All "Source" lines and code snippet prefixed by a pipe "|" are frames inline
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the next frame.
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You can see that for the trace #1 in the example, the function
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`you_shall_not_pass()` was inlined in the function `...read2::do_test()` by the
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compiler.
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## Installation
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#### Install backward.hpp
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Backward is a header only library. So installing Backward is easy, simply drop
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a copy of `backward.hpp` along with your other source files in your C++ project.
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You can also use a git submodule or really any other way that best fits your
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environment, as long as you can include `backward.hpp`.
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#### Install backward.cpp
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If you want Backward to automatically print a stack trace on most common fatal
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errors (segfault, abort, un-handled exception...), simply add a copy of
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`backward.cpp` to your project, and don't forget to tell your build system.
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The code in `backward.cpp` is trivial anyway, you can simply copy what it's
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doing at your convenience.
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Note for [folly](https://github.com/facebook/folly) library users: must define `backward::SignalHandling sh;` after `folly::init(&argc, &argv);`.
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## Configuration & Dependencies
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### Integration with CMake
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If you are using CMake and want to use its configuration abilities to save
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you the trouble, you can easily integrate Backward, depending on how you obtained
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the library.
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#### As a subdirectory:
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In this case you have a subdirectory containing the whole repository of Backward
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(eg.: using git-submodules), in this case you can do:
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```
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add_subdirectory(/path/to/backward-cpp)
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# This will add backward.cpp to your target
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add_executable(mytarget mysource.cpp ${BACKWARD_ENABLE})
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# This will add libraries, definitions and include directories needed by backward
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# by setting each property on the target.
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add_backward(mytarget)
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```
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#### Modifying CMAKE_MODULE_PATH
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In this case you can have Backward installed as a subdirectory:
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```
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list(APPEND CMAKE_MODULE_PATH /path/to/backward-cpp)
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find_package(Backward)
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# This will add libraries, definitions and include directories needed by backward
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# through an IMPORTED target.
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target_link_libraries(mytarget PUBLIC Backward::Backward)
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```
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Notice that this is equivalent to using the the approach that uses `add_subdirectory()`,
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however it uses cmake's [imported target](https://cmake.org/Wiki/CMake/Tutorials/Exporting_and_Importing_Targets) mechanism.
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#### Installation through a regular package manager
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In this case you have obtained Backward through a package manager.
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Packages currently available:
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- [conda-forge](https://anaconda.org/conda-forge/backward-cpp)
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```
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find_package(Backward)
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# This will add libraries, definitions and include directories needed by backward
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# through an IMPORTED target.
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target_link_libraries(mytarget PUBLIC Backward::Backward)
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```
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### Libraries to unwind the stack
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On Linux and macOS, backtrace can back-trace or "walk" the stack using the
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following libraries:
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#### unwind
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Unwind comes from libgcc, but there is an equivalent inside clang itself. With
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unwind, the stacktrace is as accurate as it can possibly be, since this is
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used by the C++ runtine in gcc/clang for stack unwinding on exception.
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Normally libgcc is already linked to your program by default.
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#### libunwind from the [libunwind project](https://github.com/libunwind/libunwind)
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apt-get install binutils-dev (or equivalent)
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Libunwind provides, in some cases, a more accurate stacktrace as it knows
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to decode signal handler frames and lets us edit the context registers when
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unwinding, allowing stack traces over bad function references.
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For best results make sure you are using libunwind 1.3 or later, which added
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`unw_init_local2` and support for handling signal frames.
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CMake will warn you when configuring if your libunwind version doesn't support
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signal frames.
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On macOS clang provides a libunwind API compatible library as part of its
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environment, so no third party libraries are necessary.
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### Compile with debug info
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You need to compile your project with generation of debug symbols enabled,
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usually `-g` with clang++ and g++.
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Note that you can use `-g` with any level of optimization, with modern debug
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information encoding like DWARF, it only takes space in the binary (it's not
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loaded in memory until your debugger or Backward makes use of it, don't worry),
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and it doesn't impact the code generation (at least on GNU/Linux x86\_64 for
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what I know).
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If you are missing debug information, the stack trace will lack details about
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your sources.
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### Libraries to read the debug info
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Backward supports pretty printed stack traces on GNU/Linux, macOS and Windows,
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it will compile fine under other platforms but will not do anything. **Pull
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requests are welcome :)**
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Also, by default you will get a really basic stack trace, based on the
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`backtrace_symbols` API:
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You will need to install some dependencies to get the ultimate stack trace.
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Three libraries are currently supported, the only difference is which one is the
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easiest for you to install, so pick your poison:
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#### libbfd from the [GNU/binutils](http://www.gnu.org/software/binutils/)
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apt-get install binutils-dev (or equivalent)
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And do not forget to link with the lib: `g++/clang++ -lbfd -ldl ...`
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This library requires dynamic loading. Which is provided by the library `dl`.
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Hence why we also link with `-ldl`.
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Then define the following before every inclusion of `backward.hpp` (don't
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forget to update `backward.cpp` as well):
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#define BACKWARD_HAS_BFD 1
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#### libdw from the [elfutils](https://fedorahosted.org/elfutils/)
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apt-get install libdw-dev (or equivalent)
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And do not forget to link with the lib and inform Backward to use it:
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#define BACKWARD_HAS_DW 1
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Of course you can simply add the define (`-DBACKWARD_HAS_...=1`) and the
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linkage details in your build system and even auto-detect which library is
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installed, it's up to you.
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#### [libdwarf](https://sourceforge.net/projects/libdwarf/) and [libelf](http://www.mr511.de/software/english.html)
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apt-get install libdwarf-dev (or equivalent)
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And do not forget to link with the lib and inform Backward to use it:
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#define BACKWARD_HAS_DWARF 1
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There are several alternative implementations of libdwarf and libelf that
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are API compatible so it's possible, although it hasn't been tested, to
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replace the ones used when developing backward (in bold, below):
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* **_libelf_** by [Michael "Tired" Riepe](http://www.mr511.de/software/english.html)
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* **_libdwarf_** by [David Anderson](https://www.prevanders.net/dwarf.html)
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* libelf from [elfutils](https://fedorahosted.org/elfutils/)
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* libelf and libdwarf from FreeBSD's [ELF Tool Chain](https://sourceforge.net/p/elftoolchain/wiki/Home/) project
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Of course you can simply add the define (`-DBACKWARD_HAS_...=1`) and the
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linkage details in your build system and even auto-detect which library is
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installed, it's up to you.
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That's it, you are all set, you should be getting nice stack traces like the
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one at the beginning of this document.
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## API
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If you don't want to limit yourself to the defaults offered by `backward.cpp`,
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and you want to take some random stack traces for whatever reason and pretty
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print them the way you love or you decide to send them all to your buddies over
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the Internet, you will appreciate the simplicity of Backward's API.
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### Stacktrace
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The StackTrace class lets you take a "snapshot" of the current stack.
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You can use it like this:
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```c++
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using namespace backward;
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StackTrace st; st.load_here(32);
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Printer p; p.print(st);
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```
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The public methods are:
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```c++
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class StackTrace { public:
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// Take a snapshot of the current stack, with at most "trace_cnt_max"
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// traces in it. The first trace is the most recent (ie the current
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// frame). You can also provide a trace address to load_from() assuming
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// the address is a valid stack frame (useful for signal handling traces).
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// Both function return size().
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size_t load_here(size_t trace_cnt_max)
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size_t load_from(void* address, size_t trace_cnt_max)
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// The number of traces loaded. This can be less than "trace_cnt_max".
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size_t size() const
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// A unique id for the thread in which the trace was taken. The value
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// 0 means the stack trace comes from the main thread.
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size_t thread_id() const
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// Retrieve a trace by index. 0 is the most recent trace, size()-1 is
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// the oldest one.
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Trace operator[](size_t trace_idx)
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};
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```
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### TraceResolver
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The `TraceResolver` does the heavy lifting, and intends to transform a simple
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`Trace` from its address into a fully detailed `ResolvedTrace` with the
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filename of the source, line numbers, inlined functions and so on.
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You can use it like this:
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```c++
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using namespace backward;
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StackTrace st; st.load_here(32);
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TraceResolver tr; tr.load_stacktrace(st);
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for (size_t i = 0; i < st.size(); ++i) {
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ResolvedTrace trace = tr.resolve(st[i]);
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std::cout << "#" << i
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<< " " << trace.object_filename
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<< " " << trace.object_function
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<< " [" << trace.addr << "]"
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<< std::endl;
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}
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```
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The public methods are:
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```c++
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class TraceResolver { public:
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// Pre-load whatever is necessary from the stack trace.
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template <class ST>
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void load_stacktrace(ST&)
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// Resolve a trace. It takes a ResolvedTrace, because a `Trace` is
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// implicitly convertible to it.
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ResolvedTrace resolve(ResolvedTrace t)
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};
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```
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### SnippetFactory
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The SnippetFactory is a simple helper class to automatically load and cache
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source files in order to extract code snippets.
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```c++
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class SnippetFactory { public:
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// A snippet is a list of line numbers and line contents.
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typedef std::vector<std::pair<size_t, std::string> > lines_t;
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// Return a snippet starting at line_start with up to context_size lines.
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lines_t get_snippet(const std::string& filename,
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size_t line_start, size_t context_size)
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// Return a combined snippet from two different locations and combine them.
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// context_size / 2 lines will be extracted from each location.
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lines_t get_combined_snippet(
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const std::string& filename_a, size_t line_a,
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const std::string& filename_b, size_t line_b,
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size_t context_size)
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// Tries to return a unified snippet if the two locations from the same
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// file are close enough to fit inside one context_size, else returns
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// the equivalent of get_combined_snippet().
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lines_t get_coalesced_snippet(const std::string& filename,
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size_t line_a, size_t line_b, size_t context_size)
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```
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### Printer
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A simpler way to pretty print a stack trace to the terminal. It will
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automatically resolve the traces for you:
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```c++
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using namespace backward;
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StackTrace st; st.load_here(32);
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Printer p;
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p.object = true;
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p.color_mode = ColorMode::always;
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p.address = true;
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p.print(st, stderr);
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```
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You can set a few options:
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```c++
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class Printer { public:
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// Print a little snippet of code if possible.
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bool snippet = true;
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// Colorize the trace
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// - ColorMode::automatic: Activate colors if possible. For example, when using a TTY on linux.
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// - ColorMode::always: Always use colors.
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// - ColorMode::never: Never use colors.
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bool color_mode = ColorMode::automatic;
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// Add the addresses of every source location to the trace.
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bool address = false;
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// Even if there is a source location, also prints the object
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// from where the trace came from.
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bool object = false;
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// Resolve and print a stack trace to the given C FILE* object.
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// On linux, if the FILE* object is attached to a TTY,
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// color will be used if color_mode is set to automatic.
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template <typename StackTrace>
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FILE* print(StackTrace& st, FILE* fp = stderr);
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// Resolve and print a stack trace to the given std::ostream object.
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// Color will only be used if color_mode is set to always.
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template <typename ST>
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std::ostream& print(ST& st, std::ostream& os);
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```
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### SignalHandling
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A simple helper class that registers for you the most common signals and other
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callbacks to segfault, hardware exception, un-handled exception etc.
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`backward.cpp` simply uses it like that:
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```c++
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backward::SignalHandling sh;
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```
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Creating the object registers all the different signals and hooks. Destroying
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this object doesn't do anything. It exposes only one method:
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```c++
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bool loaded() const // true if loaded with success
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```
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### Trace object
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To keep the memory footprint of a loaded `StackTrace` on the low-side, there a
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hierarchy of trace object, from a minimal `Trace `to a `ResolvedTrace`.
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#### Simple trace
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```c++
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struct Trace {
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void* addr; // address of the trace
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size_t idx; // its index (0 == most recent)
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};
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```
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#### Resolved trace
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A `ResolvedTrace` should contains a maximum of details about the location of
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the trace in the source code. Note that not all fields might be set.
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```c++
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struct ResolvedTrace: public Trace {
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struct SourceLoc {
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std::string function;
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std::string filename;
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size_t line;
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size_t col;
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};
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// In which binary object this trace is located.
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std::string object_filename;
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// The function in the object that contains the trace. This is not the same
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// as source.function which can be an function inlined in object_function.
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std::string object_function;
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// The source location of this trace. It is possible for filename to be
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// empty and for line/col to be invalid (value 0) if this information
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// couldn't be deduced, for example if there is no debug information in the
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// binary object.
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SourceLoc source;
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// An optional list of "inliners". All of these sources locations where
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// inlined in the source location of the trace (the attribute right above).
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// This is especially useful when you compile with optimizations turned on.
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typedef std::vector<SourceLoc> source_locs_t;
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source_locs_t inliners;
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};
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```
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## Contact and copyright
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François-Xavier Bourlet <bombela@gmail.com>
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Copyright 2013-2017 Google Inc. All Rights Reserved.
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MIT License.
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### Disclaimer
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Although this project is owned by Google Inc. this is not a Google supported or
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affiliated project.
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