Dr. Dobb's Journal - May 2008 - (Page 50)

d05sutt_p4as 3/13/08 10:42 AM Page 50 Effective Concurrency • Joining with another thread or task. • Sleeping. Under Options 2, 3, and 4, these are the points at which a thread or task could be interrupted. This still imposes a burden on the author of the thread’s or task’s code: The code has to be ready to be interrupted at such points, and especially you have to either reestablish your invariants before you make any such calls at which you could be interrupted, or arrange for the invariants to be reestablished if you are actually inter- rupted. Let’s see what this looks like under the remaining three Options. Option 2: Peremptory, Don’t Take No For an Answer Option 2 is to follow the model of POSIX threads (Pthreads) deferred cancellation via pthread_cancel: Wait until the target thread reaches its next well-defined cancellation point, then stop it and run the chain of cancellation handlers that the program installed (if any) which serves a similar purpose as destructor/dispose functions in PC Presents Bug of the Month #571 -lint 8.0 for C/C++ A florist who knows flowers better than programming is automating his shop, but there seems to be a problem with the printout of bridal bouquets. Somehow the baby's breath is always missing. What's going on?Visit our web site at www.gimpel.com #include const char *bouquet = "DRM\xFF"; const char *flower( char ch ) { switch( (unsigned) ch ) { case 'D': return "Daisies"; case 'R': return "Roses"; case 'M': return "Mums"; case 0xFF: return "Baby's Breath"; default: return "What?"; } } int main() { const char *b, *flwr; printf( "Bridal bouquet: \n" ); for( b = bouquet; *b; b++ ) { flwr = flower( *b ); if(flwr) printf( "%s\n",flwr); else break; } return 0; } modern languages. This is much better than Option 1. The key drawback of Option 2 is that Pthreads cancellation requests cannot be ignored or caught; the target has no choice but to be stopped at its next cancellation point, and once cancellation has begun it cannot be stopped. This is a reasonable design for a language that does not have exceptions or objects with destructor/dispose functions (the cancellation handlers simulate the latter), but it is largely inappropriate for modern languages which have exception handling and know how to catch and recover from errors and continue correct execution. So Option 2 is appropriate in languages like C and Fortran if it is acceptable to force target threads to stop, but is less well suited for use with modern languages that have more sophisticated errorrecovery mechanisms or your threads or tasks may legitimately want to handle the cancellation request and continue or ignore it entirely, neither of which is permitted under Option 2. l. st ge y oo nd on sl e t ki e l uou ar an Th n tw m nti of . . co sed s of . ti ry ver to ad e his th in Option 3: Ask Politely Option 3 is to follow the interruption model common to modern languages and frameworks, including Java and .NET, which spell it as Thread.interrupt and Thread.Interrupt, respectively. Like Option 2, the target thread continues to run until it reaches its next interruption point, at which point in most systems implementing Option 3 the interruption manifests as an exception thrown from the wait/join/sleep call. Then, unlike Option 2, the target thread can catch and handle the exception like any other exception, including that it has more options: As in Pthreads, it can simply let destructors/disposers and finally clauses unwind the stack entirely and exit. Unlike Pthreads, the target thread can choose to unwind its stack partway until it finds a handler that catches and handles the exception, and then continue normal operations. Also unlike Pthreads, the target thread can immediately catch and ignore the exception entirely. This is polite interruption, the state of the art in automated interruption facilities. PC-lint for C/C++ will catch this and many other bugs. It will analyze a mixed suite of C and C++ modules to uncover bugs, glitches, quirks and inconsistencies. Not your Grandpa’s lint: PC-lint has introduced several spectacular and revolutionary innovations in the art of static program analysis. Taking clues from initializers, assignments, and conditionals, variable and member values are tracked, enabling reports on potential uses of null pointers and out-of-bounds subscripts. New with Version 8: Interfunction value tracking – Actual argument values are used to initialize parameters; return values are computed; a multipass operation (you control the number of passes) allows you to plumb the depths of function behavior to arbitrary levels. PC-lint for C/C++ Plus Our Traditional C/C++ Warnings: Uninitialized variables, inherited non-virtual destructors, strong type mismatches,ill-formed macros, inadvertent name-hiding, suspicious expressions, etc., etc. Full Language Support for ANSI/ISO C and C++. Numerous compilers/ libraries supported. Runs on Windows, MS-DOS, and OS/2. $239 FlexeLint for C/C++ The same great product for other operating systems. Runs on all UNIX systems, VMS, mainframes, etc. Distributed in shrouded C source form. Call for pricing. G i m p e l S o f t wa r e Serving the C/C++ Community for 22 Years. 30 Day Money Back Guarantee Option 4: Cooperate Finally, Option 4, which you can and should use together with Option 3, is a fully CALL TODAY (610) 584-4261 Or FAX (610) 584-4266 www.gimpel.com PC-lint and FlexeLint are trademarks of Gimpel Software 50 Dr. Dobb’s Journal l www.ddj.com l May 2008 http://www.gimpel.com http://www.gimpel.com http://www.gimpel.com http://www.ddj.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - May 2008

Dr. Dobb's Journal - May 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Software Development Goes to the Movies Cat: A Functional Stack-Based Little Language Mojax: Mobile Ajax Framework Kernel-Mode Databases Getting Better Search Results Effective Concurrency The Agile Edge

Dr. Dobb's Journal - May 2008

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