6.15 Switch Statements
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I've a question concerning the following example:
Code: Select all
case 0: /* valid */
case 255: /* valid? */
case 1L: /* invalid, implicit cast from long to int */
I would expect any good static checker to report that x can never equal 255 here. But is 'case 255:' actually a MISRA 2004 rule violation? The C standard says that the switch expression x gets promoted to 'int' in this case, so the type of the case label 255 does match the type of the promoted switch expression. I can't find any MISRA rule that says that we should consider the underlying type (instead of the promoted type) of the switch variable when deciding whether a case label is allowed or not. If "case 255:" was the only case label on some code, then rule 14.1 (no unreachable code) would apply; but in this example, there is no unreachable code.
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The underlying type of the controlling expression of the switch statement is signed 8-bit.
Conversion of 1L from underlying long int to signed 8-bit violates Rule 10.1.
Conversion of 255 from underlying signed 16-bit to signed 8-bit also violates Rule 10.1.
This conversion is mentioned in Section 6.10.2 under the subsection Assigning Conversions. See also the Exemplar Suite file mc2_1001.c.
Posted by and on behalf of
the MISRA C Working Group