Rule #15: When evaluating the equality or inequality of a variable with a constant value, always place the constant value on the left side of the comparison operator.

Example (do):

if (0 == x){    /* Do this only if x is zero. */}

Reasoning: It is always desirable to detect possible typos and as many other bugs as possible at compile-time; run-time discovery may be dangerous to the user of the product and require significant effort to locate. By following this rule, the compiler can detect erroneous attempts to assign (i.e., = instead of ==) a new value to a constant.²

Try adding some or all of the fifteen practical embedded C coding rules from this and last month's column to your coding standard to keep bugs out of your embedded system.

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Michael Barr is the author of two books and more than forty articles about embedded systems design, as well as a former editor-in-chief of this magazine. Michael is also a popular speaker at the Embedded Systems Conference and the CTO of Netrino. E-mail him at mbarr@netrino.com or read more of what he has to say at www.embeddedgurus.net/barr-code.

Endnotes:
1. Full disclosure: Netrino is my employer and our Embedded C Coding Standard is something we developed first for internal use but now also sell as a book.

2. But remember, floating-point values should never be compared to constants. A number like 0.1x10^-15 is not strictly equal to zero.