Embedded Systems Design - March 2008 - (Page 50)

0308esd.p49to52 2/14/08 1:02 PM Page 50 break points (www.hpl.hp.com/ complexity analysis. Think of cyclomatic complexity like the hpjournal/pdfs/ That rule has been IssuePDFs/1989canary in a mine. It’s an early warning that the interpreted in a lot 04.pdf). But it’s imfunction may be difﬁcult to maintain and error of mostly dysfuncpossible to deﬁne a tional ways and is prone. single value of it that often used to excuse separates good funcmany other sorts of tions from bad ones. constructs. It’s betThink of cyclomatic complexity like the canary in a mine. ter to say: “No function can exceed 10, except in cases It’s an early warning that the function may be difﬁcult to where the resulting code would be excessively hard to maintain and error prone. understand or maintain.” Long ago, George A. Miller (“The Magical Number Over the years, the following rules of thumb have Seven, Plus or Minus Two: Some Limits on Our Capacity been observed: for Processing Information,” Psychological Review, 63 (1956), 81-97) showed that humans can typically keep a v(G) Risk maximum of five to nine things in short-term memory. 1 to 10 A simple function That’s a pretty good place to start: why write functions without much risk that are more complex than we can understand? 11 to 20 More complex, McCabe suggested that 10 is probably a good limitmoderate risk ing value for v(G), and that number is certainly the most 21 to 50 Complex, widely used. It’s comfortingly close to Miller’s 7±2 as high risk well. McCabe pointed out an important exception: 51+ Untestable, switch statements can generate very high-complexity very high risk metrics even in cases where the logic is pretty simple, so he suggested exempting functions with switches from And: Can CMX Really Put TCP/IP On My Little Ole 8-bit Chip? Yes, Ma’am, CMX has been doing amazing things with RTOSes and TCP/IP stacks for many years now. If you haven’t visited us in a while, you are missing a lot of cool, new technology that is economical, royalty free, and comes with source code. v(G) Risk of injecting another bug when modifying the code 1 to 10 . . . . . . . . . . . . . . . . . . 5% 20 to 30 . . . . . . . . . . . . . . . . . 0% >50 . . . . . . . . . . . . . . . . . . . 40% Around 100 . . . . . . . . . . . . 60% For a variety of reasons, few companies routinely practice code inspections, which are the best tool we have for cheaply and effectively eliminating bugs. But one can use the complexity metric to flag risky routines that should get an inspection. I recommend sending, at the very least, any function that scores over 10 through an inspection. Validated Software (validatedsoftware.com) certifies firmware to various safety-critical standards. Scott Nowell there tells me that on one recent project the code had an average v(G) of 4.7, suggesting that great code is composed of simple functions. So how do typical packages stack up? Here are some results from my lab using the RSM tool (described later). Apache 2.2.8, the open source web server, is composed of 5,752 functions with an average cyclomatic complexity of 6.04. Although 61 functions exceed 50, just under 5,000 are below 10. One scored a terrifying 725! That function— a single function, mind you—is 2,700 lines long with 145 cases and 353 if statements. If a v(G) of 100 indicates an It might just put the sparkle back in your eyes! CMX RTOSes and TCP/IP Stacks Support Most 8-, 16-, 32-bit Processors and DSP’s. 12276 San Jose Blvd Jacksonville, FL 32223 Ph: 904.880.1840 Fax: 904.880.1632 email: cmx@cmx.com www.cmx.com 50 MARCH 2008 | embedded systems design | www.embedded.com http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1989-04.pdf http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1989-04.pdf http://www.validatedsoftware.com http://www.cmx.com http://cmx.com http://www.cmx.com http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - March 2008

Embedded Systems Design - March 2008 Contents #Include Programming Pointers Designing DSP-based Motor Control Using Fuzzy Logic Hardware/Software Verification Enters the Atomic Age Efficient CRC Calculation with Minimal Memory Footprint Programming Your Own Microcontroller Advertising Index Break Points Marketplace

Embedded Systems Design - March 2008

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