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2012-12-01T22:13:00.140
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No, it is not OK to leave a critical section with exceptions. <code>g++</code> does not complain in this case but it silently inserts an implicit <code>try/catch</code> around the block of the critical section. For example the following code: <pre><code>#pragma omp critical (my_crit) { throw 3; } </code></pre> gets lowered by the OpenMP processor of GCC 4.7 into: <pre><code>#pragma omp critical (my_crit) __builtin_GOMP_critical_name_start (&.gomp_critical_user_my_crit); try { D.20639 = __cxa_allocate_exception (4); try { MEM[(int *)D.20639] = 3; } catch { __cxa_free_exception (D.20639); } __cxa_throw (D.20639, &_ZTIi, 0B); } catch { <<<eh_must_not_throw (terminate)>>> } __builtin_GOMP_critical_name_end (&.gomp_critical_user_my_crit); </code></pre> Reaching the implicit built-in catch-all handler <code><<<eh_must_not_throw (terminate)>>></code> results in quite ungraceful termination: <pre><code>terminate called after throwing an instance of 'int' Abort trap: 6 </code></pre> The implicit <code>try/catch</code> is inserted regardless of the presence of an outer <code>try/catch</code> construct, i.e. the exception would never ever leave the <code>critical</code> section. The OpenMP standard mandates, that if an exception is thrown within most OpenMP constructs (<code>parallel</code>, <code>section</code>, <code>master</code>, <code>single</code>, <code>for</code>, <code>critical</code>, <code>task</code>, etc.), execution must resume within the same construct and that the same thread must catch the exception. Violation of this restriction leads to non-conformant OpenMP code and <code>g++</code> simply enforces the conformance by inserting <code>try/catch</code> blocks with termination handlers inside all such constructs. As for the error when a <code>return</code> statement is present, OpenMP defines a strcutured block in C/C++ as: <blockquote> For C/C++, an executable statement, possibly compound, with a single entry at the top and a single exit at the bottom, or an OpenMP construct. </blockquote> and also (for all languages): <blockquote> The point of exit cannot be a branch out of the structured block. </blockquote> Obviously <code>return</code> constitutes a branch ouf of the block, different from simply falling of the bottom of the block.
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1. POThrowing a C++ exception inside an omp critical section
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1. USHristo Iliev
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