| <?xml version="1.0" encoding="UTF-8" standalone="no"?> |
| <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Exceptions</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="C++, exception, error, exception neutrality, exception safety, exception propagation, -fno-exceptions" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_concurrency.html" title="Concurrency" /><link rel="next" href="debug.html" title="Debugging Support" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Exceptions</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.exceptions"></a>Exceptions</h2></div></div></div><p> |
| The C++ language provides language support for stack unwinding |
| with <code class="literal">try</code> and <code class="literal">catch</code> blocks and |
| the <code class="literal">throw</code> keyword. |
| </p><p> |
| These are very powerful constructs, and require some thought when |
| applied to the standard library in order to yield components that work |
| efficiently while cleaning up resources when unexpectedly killed via |
| exceptional circumstances. |
| </p><p> |
| Two general topics of discussion follow: |
| exception neutrality and exception safety. |
| </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.safety"></a>Exception Safety</h3></div></div></div><p> |
| What is exception-safe code? |
| </p><p> |
| Will define this as reasonable and well-defined behavior by classes |
| and functions from the standard library when used by user-defined |
| classes and functions that are themselves exception safe. |
| </p><p> |
| Please note that using exceptions in combination with templates |
| imposes an additional requirement for exception |
| safety. Instantiating types are required to have destructors that |
| do no throw. |
| </p><p> |
| Using the layered approach from Abrahams, can classify library |
| components as providing set levels of safety. These will be called |
| exception guarantees, and can be divided into three categories. |
| </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> |
| One. Don't throw. |
| </p><p> |
| As specified in 23.2.1 general container requirements. Applicable |
| to container and string classes. |
| </p><p> |
| Member |
| functions <code class="function">erase</code>, <code class="function">pop_back</code>, <code class="function">pop_front</code>, <code class="function">swap</code>, <code class="function">clear</code>. And <span class="type">iterator</span> |
| copy constructor and assignment operator. |
| </p></li><li class="listitem"><p> |
| Two. Don't leak resources when exceptions are thrown. This is |
| also referred to as the <span class="quote">“<span class="quote">basic</span>”</span> exception safety guarantee. |
| </p><p> |
| This applicable throughout the standard library. |
| </p></li><li class="listitem"><p> |
| Three. Commit-or-rollback semantics. This is |
| referred to as <span class="quote">“<span class="quote">strong</span>”</span> exception safety guarantee. |
| </p><p> |
| As specified in 23.2.1 general container requirements. Applicable |
| to container and string classes. |
| </p><p> |
| Member functions <code class="function">insert</code> of a single |
| element, <code class="function">push_back</code>, <code class="function">push_front</code>, |
| and <code class="function">rehash</code>. |
| </p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.propagating"></a>Exception Neutrality</h3></div></div></div><p> |
| Simply put, once thrown an exception object should continue in |
| flight unless handled explicitly. In practice, this means |
| propagating exceptions should not be swallowed in |
| gratuitous <code class="literal">catch(...)</code> blocks. Instead, |
| matching <code class="literal">try</code> and <code class="literal">catch</code> |
| blocks should have specific catch handlers and allow un-handed |
| exception objects to propagate. If a |
| terminating <code class="literal">catch(...)</code> blocks exist then it |
| should end with a <code class="literal">throw</code> to re-throw the current |
| exception. |
| </p><p> |
| Why do this? |
| </p><p> |
| By allowing exception objects to propagate, a more flexible |
| approach to error handling is made possible (although not |
| required.) Instead of dealing with an error immediately, one can |
| allow the exception to propagate up until sufficient context is |
| available and the choice of exiting or retrying can be made in an |
| informed manner. |
| </p><p> |
| Unfortunately, this tends to be more of a guideline than a strict |
| rule as applied to the standard library. As such, the following is |
| a list of known problem areas where exceptions are not propagated. |
| </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> |
| Input/Output |
| </p><p> |
| The destructor <code class="function">ios_base::Init::~Init()</code> |
| swallows all exceptions from <code class="function">flush</code> called on |
| all open streams at termination. |
| </p><p> |
| All formatted input in <code class="classname">basic_istream</code> or |
| formatted output in <code class="classname">basic_ostream</code> can be |
| configured to swallow exceptions |
| when <code class="function">exceptions</code> is set to |
| ignore <span class="type">ios_base::badbit</span>. |
| </p><p> |
| Functions that have been registered |
| with <code class="function">ios_base::register_callback</code> swallow all |
| exceptions when called as part of a callback event. |
| </p><p> |
| When closing the underlying |
| file, <code class="function">basic_filebuf::close</code> will swallow |
| (non-cancellation) exceptions thrown and return <code class="literal">NULL</code>. |
| </p></li><li class="listitem"><p> |
| Thread |
| </p><p> |
| The constructors of <code class="classname">thread</code> that take a |
| callable function argument swallow all exceptions resulting from |
| executing the function argument. |
| </p></li></ul></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.alloc"></a>Memory allocation</h3></div></div></div><p> |
| When the program throws an exception the runtime will obtain storage for |
| a <code class="code">__cxa_exception</code> header and the thrown object itself. |
| Libstdc++ will try to use <code class="code">malloc</code> to obtain storage, |
| but provides an emergency buffer to be used if malloc fails, |
| as described by the <a class="link" href="https://itanium-cxx-abi.github.io/cxx-abi/abi-eh.html#imp-emergency" target="_top">Itanium |
| exception handling ABI</a>. |
| </p><p> |
| Contrary to the ABI, the libstdc++ emergency buffer is not always 64kB, |
| and does not always allocate 1kB chunks. The buffer is used as a pool for |
| variable-sized allocations, so that it doesn't waste space for smaller |
| exception objects such as <code class="code">std::bad_alloc</code>. |
| The total size of the buffer is scaled appropriately for the target. |
| Specifically it depends on <code class="code">sizeof(void*)</code>, so that a 64-bit |
| system uses a larger pool than a 32-bit system. This is done because for |
| 32-bit systems the exception objects (and the exception header) require |
| less space, and core counts and thread counts are typically lower as well. |
| </p><p> |
| By default, libstdc++ will use <code class="code">malloc</code> to allocate the buffer |
| on program startup. |
| <a class="xref" href="configure.html" title="Configure">Configuring</a> libstdc++ with the |
| <code class="code">--enable-libstdcxx-static-eh-pool</code> option will make it |
| use a static buffer instead of using <code class="code">malloc</code>. |
| </p><p> |
| The buffer size is chosen automatically, but can be overridden |
| by configuring with <code class="code">--with-libstdcxx-eh-pool-obj-count=NUM</code>, |
| where <code class="code">NUM</code> is the number of simultaneous allocations that |
| should be supported. The size of the pool will be sufficient for |
| <code class="code">NUM</code> exceptions of <code class="code">6 * sizeof(void*)</code> bytes, |
| plus another <code class="code">NUM</code> exceptions captured in |
| <code class="classname">std::exception_ptr</code> and rethrown using |
| <code class="code">std::rethrow_exception</code>. The buffer size determined by the |
| obj-count value applies whether the buffer is reserved as static storage |
| or is allocated dynamically. |
| Setting obj-count to zero will disable the pool, so that no emergency |
| buffer is present. |
| </p><p> |
| For a dynamic buffer, the default size can also be changed at runtime, |
| per-process, via the <code class="literal">GLIBCXX_TUNABLES</code> environment |
| variable. |
| The <code class="literal">GLIBCXX_TUNABLES</code> environment variable should be |
| a string of colon-separated <span class="emphasis"><em>name=value</em></span> pairs. The |
| following names will be recognized, with the specified semantics: |
| </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="code">glibcxx.eh_pool.obj_count</code></span></dt><dd> |
| The number of exception objects to provide space for in the pool. |
| The value must be a non-negative integer and has the same meaning as the |
| <code class="code">--with-libstdcxx-eh-pool-obj-count</code> option for |
| <code class="filename">configure</code>. |
| </dd><dt><span class="term"><code class="code">glibcxx.eh_pool.obj_size</code></span></dt><dd> |
| The expected size of exception objects that the pool might get used for. |
| The value must be a positive integer, and is measured in units of |
| <code class="code">sizeof(void*)</code>. The default value is <code class="literal">6</code> |
| which is large enough to store any exception type thrown by libstdc++. |
| Exceptions larger than this can still be allocated from the pool, |
| but larger exceptions will exhaust the pool more rapidly. |
| </dd></dl></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.no"></a>Doing without</h3></div></div></div><p> |
| C++ is a language that strives to be as efficient as is possible |
| in delivering features. As such, considerable care is used by both |
| language implementer and designers to make sure unused features do |
| not impose hidden or unexpected costs. The GNU system tries to be |
| as flexible and as configurable as possible. So, it should come as |
| no surprise that GNU C++ provides an optional language extension, |
| spelled <code class="literal">-fno-exceptions</code>, as a way to excise the |
| implicitly generated magic necessary to |
| support <code class="literal">try</code> and <code class="literal">catch</code> blocks |
| and thrown objects. (Language support |
| for <code class="literal">-fno-exceptions</code> is documented in the GCC |
| <a class="link" href="https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options" target="_top">manual</a>.) |
| </p><p>Before detailing the library support |
| for <code class="literal">-fno-exceptions</code>, first a passing note on |
| the things lost when this flag is used: it will break exceptions |
| trying to pass through code compiled |
| with <code class="literal">-fno-exceptions</code> whether or not that code |
| has any <code class="literal">try</code> or <code class="literal">catch</code> |
| constructs. If you might have some code that throws, you shouldn't |
| use <code class="literal">-fno-exceptions</code>. If you have some code that |
| uses <code class="literal">try</code> or <code class="literal">catch</code>, you |
| shouldn't use <code class="literal">-fno-exceptions</code>. |
| </p><p> |
| And what is to be gained, tinkering in the back alleys with a |
| language like this? Exception handling overhead can be measured |
| in the size of the executable binary, and varies with the |
| capabilities of the underlying operating system and specific |
| configuration of the C++ compiler. On recent hardware with GNU |
| system software of the same age, the combined code and data size |
| overhead for enabling exception handling is around 7%. Of course, |
| if code size is of singular concern than using the appropriate |
| optimizer setting with exception handling enabled |
| (ie, <code class="literal">-Os -fexceptions</code>) may save up to twice |
| that, and preserve error checking. |
| </p><p> |
| So. Hell bent, we race down the slippery track, knowing the brakes |
| are a little soft and that the right front wheel has a tendency to |
| wobble at speed. Go on: detail the standard library support |
| for <code class="literal">-fno-exceptions</code>. |
| </p><p> |
| In sum, valid C++ code with exception handling is transformed into |
| a dialect without exception handling. In detailed steps: all use |
| of the C++ |
| keywords <code class="literal">try</code>, <code class="literal">catch</code>, |
| and <code class="literal">throw</code> in the standard library have been |
| permanently replaced with the pre-processor controlled equivalents |
| spelled <code class="literal">__try</code>, <code class="literal">__catch</code>, |
| and <code class="literal">__throw_exception_again</code>. They are defined |
| as follows. |
| </p><pre class="programlisting"> |
| #if __cpp_exceptions |
| # define __try try |
| # define __catch(X) catch(X) |
| # define __throw_exception_again throw |
| #else |
| # define __try if (true) |
| # define __catch(X) if (false) |
| # define __throw_exception_again |
| #endif |
| </pre><p> |
| In addition, for most of the classes derived from |
| class <code class="classname">exception</code>, there exists a corresponding |
| function with C language linkage. An example: |
| </p><pre class="programlisting"> |
| #if __cpp_exceptions |
| void __throw_bad_exception() |
| { throw bad_exception(); } |
| #else |
| void __throw_bad_exception() |
| { abort(); } |
| #endif |
| </pre><p> |
| The last language feature needing to be transformed |
| by <code class="literal">-fno-exceptions</code> is treatment of exception |
| specifications on member functions. Fortunately, the compiler deals |
| with this by ignoring exception specifications and so no alternate |
| source markup is needed. |
| </p><p> |
| By using this combination of language re-specification by the |
| compiler, and the pre-processor tricks and the functional |
| indirection layer for thrown exception objects by the library, |
| libstdc++ files can be compiled |
| with <code class="literal">-fno-exceptions</code>. |
| </p><p> |
| User code that uses C++ keywords |
| like <code class="literal">throw</code>, <code class="literal">try</code>, |
| and <code class="literal">catch</code> will produce errors even if the user |
| code has included libstdc++ headers and is using constructs |
| like <code class="classname">basic_iostream</code>. Even though the standard |
| library has been transformed, user code may need modification. User |
| code that attempts or expects to do error checking on standard |
| library components compiled with exception handling disabled should |
| be evaluated and potentially made conditional. |
| </p><p> |
| Some issues remain with this approach (see bugzilla entry |
| 25191). Code paths are not equivalent, in |
| particular <code class="literal">catch</code> blocks are not evaluated. Also |
| problematic are <code class="literal">throw</code> expressions expecting a |
| user-defined throw handler. Known problem areas in the standard |
| library include using an instance |
| of <code class="classname">basic_istream</code> |
| with <code class="function">exceptions</code> set to specific |
| <span class="type">ios_base::iostate</span> conditions, or |
| cascading <code class="literal">catch</code> blocks that dispatch error |
| handling or recovery efforts based on the type of exception object |
| thrown. |
| </p><p> |
| Oh, and by the way: none of this hackery is at all |
| special. (Although perhaps well-deserving of a raised eyebrow.) |
| Support continues to evolve and may change in the future. Similar |
| and even additional techniques are used in other C++ libraries and |
| compilers. |
| </p><p> |
| C++ hackers with a bent for language and control-flow purity have |
| been successfully consoled by grizzled C veterans lamenting the |
| substitution of the C language keyword |
| <code class="literal">const</code> with the uglified |
| doppelganger <code class="literal">__const</code>. |
| </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="intro.using.exception.compat"></a>Compatibility</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.c"></a>With <code class="literal">C</code></h4></div></div></div><p> |
| C language code that is expecting to interoperate with C++ should be |
| compiled with <code class="literal">-fexceptions</code>. This will make |
| debugging a C language function called as part of C++-induced stack |
| unwinding possible. |
| </p><p> |
| In particular, unwinding into a frame with no exception handling |
| data will cause a runtime abort. If the unwinder runs out of unwind |
| info before it finds a handler, <code class="function">std::terminate()</code> |
| is called. |
| </p><p> |
| Please note that most development environments should take care of |
| getting these details right. For GNU systems, all appropriate parts |
| of the GNU C library are already compiled |
| with <code class="literal">-fexceptions</code>. |
| </p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="using.exception.compat.posix"></a>With <code class="literal">POSIX</code> thread cancellation</h4></div></div></div><p> |
| GNU systems re-use some of the exception handling mechanisms to |
| track control flow for <code class="literal">POSIX</code> thread cancellation. |
| </p><p> |
| Cancellation points are functions defined by POSIX as worthy of |
| special treatment. The standard library may use some of these |
| functions to implement parts of the ISO C++ standard or depend on |
| them for extensions. |
| </p><p> |
| Of note: |
| </p><p> |
| <code class="function">nanosleep</code>, |
| <code class="function">read</code>, <code class="function">write</code>, <code class="function">open</code>, <code class="function">close</code>, |
| and <code class="function">wait</code>. |
| </p><p> |
| The parts of libstdc++ that use C library functions marked as |
| cancellation points should take pains to be exception neutral. |
| Failing this, <code class="literal">catch</code> blocks have been augmented to |
| show that the POSIX cancellation object is in flight. |
| </p><p> |
| This augmentation adds a <code class="literal">catch</code> block |
| for <code class="classname">__cxxabiv1::__forced_unwind</code>, which is the |
| object representing the POSIX cancellation object. Like so: |
| </p><pre class="programlisting"> |
| catch(const __cxxabiv1::__forced_unwind&) |
| { |
| this->_M_setstate(ios_base::badbit); |
| throw; |
| } |
| catch(...) |
| { this->_M_setstate(ios_base::badbit); } |
| </pre></div></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="using.exceptions.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.2"></a><p><span class="title"><em> |
| <a class="link" href="http://www.opengroup.org/austin/" target="_top"> |
| System Interface Definitions, Issue 7 (IEEE Std. 1003.1-2008) |
| </a> |
| </em>. </span><span class="pagenums"> |
| 2.9.5 Thread Cancellation |
| . </span><span class="copyright">Copyright © 2008 |
| The Open Group/The Institute of Electrical and Electronics |
| Engineers, Inc. |
| . </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.3"></a><p><span class="title"><em> |
| <a class="link" href="https://www.boost.org/community/error_handling.html" target="_top"> |
| Error and Exception Handling |
| </a> |
| </em>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams </span>. </span><span class="publisher"><span class="publishername"> |
| Boost |
| . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.4"></a><p><span class="title"><em> |
| <a class="link" href="https://www.boost.org/community/exception_safety.html" target="_top"> |
| Exception-Safety in Generic Components |
| </a> |
| </em>. </span><span class="author"><span class="firstname">David</span> <span class="surname">Abrahams</span>. </span><span class="publisher"><span class="publishername"> |
| Boost |
| . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.5"></a><p><span class="title"><em> |
| <a class="link" href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/1997/N1077.pdf" target="_top"> |
| Standard Library Exception Policy |
| </a> |
| </em>. </span><span class="author"><span class="firstname">Matt</span> <span class="surname">Austern</span>. </span><span class="publisher"><span class="publishername"> |
| WG21 N1077 |
| . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.6"></a><p><span class="title"><em> |
| <a class="link" href="http://gcc.gnu.org/ml/gcc-patches/2001-03/msg00661.html" target="_top"> |
| ia64 c++ abi exception handling |
| </a> |
| </em>. </span><span class="author"><span class="firstname">Richard</span> <span class="surname">Henderson</span>. </span><span class="publisher"><span class="publishername"> |
| GNU |
| . </span></span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.7"></a><p><span class="title"><em> |
| <a class="link" href="https://www.stroustrup.com/3rd_safe.pdf" target="_top"> |
| Appendix E: Standard-Library Exception Safety |
| </a> |
| </em>. </span><span class="author"><span class="firstname">Bjarne</span> <span class="surname">Stroustrup</span>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.8"></a><p><span class="citetitle"><em class="citetitle"> |
| Exceptional C++ |
| </em>. </span><span class="pagenums"> |
| Exception-Safety Issues and Techniques |
| . </span><span class="author"><span class="firstname">Herb</span> <span class="surname">Sutter</span>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.9"></a><p><span class="title"><em> |
| <a class="link" href="http://gcc.gnu.org/PR25191" target="_top"> |
| GCC Bug 25191: exception_defines.h #defines try/catch |
| </a> |
| </em>. </span></p></div><div class="biblioentry"><a id="id-1.3.3.4.9.10.10"></a><p><span class="title"><em> |
| <a class="link" href="https://www.gnu.org/software/libc/manual/html_node/Tunables.html" target="_top"> |
| Tunables, The GNU C Library |
| </a> |
| </em>. </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_concurrency.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="debug.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Concurrency </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Debugging Support</td></tr></table></div></body></html> |
