Misra C Rule 12.2 - false positive warning?

问题

My CCS 6.1 ARM compiler (for LM3Sxxxx Stellaris) throws a warning :

"MISRA Rule 12.2. The value of an expression shall be the same under any order of evaluation that the standard permits"

for following code:

typedef struct {
  ...
  uint32_t bufferCnt;
  uint8_t buffer[100];
  ...
} DIAG_INTERFACE_T;

static DIAG_INTERFACE_T diagInterfaces[1];

...
DIAG_INTERFACE_T * diag = &diagInterfaces[0];
uint8_t data = 0;
diag->bufferCnt = 0;
diag->buffer[diag->bufferCnt++] = data; // line where warning is issued
...

I don't see a problem in my code. Is it false positive or my bug?

回答1:

Although you don’t indicate it, this is MISRA-C:2004, Rule 12.2, and is now MISRA-C:2012 Rule 13.2. As oauh says, this has nothing to do with "order of evaluation”.

I highly recommend referring to MISRA-C:2012 even if you are required to be MISRA-C:2004 compliant, having MISRA-C:2012 around helps, because it has clarified many of the guidelines, including additional rationale, explanations and examples.

You should not be using a compiler to solely check for MISRA-C compliancy, its nice, but compilers #1 goal is not to warn you about all the traps and pitfalls of the language it is dedicated to take advantage of (optimization). They're not very precise either, as in this case. Also, there are many undefined behaviors across translation units, compilers cannot warn about. Its best to also use a dedicated MISRA Static analysis tool, one that is not compiler specific, but that warns about all unpredictable constructs from the ISO C standards point of view, not a particular implementation.

As oauh also said, this is a violation of MISRA-C:Rule 12.13, which is now MISRA-C:2012 Rule 13.3 which has been relaxed to permit ++ and -- to be mixed with other operators, provided that the ++ or -- is the only source of side-effects (in your case the assignment is also a side effect in C terminology).

The Rule is not critical, i.e. its well defined behavior, but the different values resulting from the prefix version and the postfix version can cause confusion, thus it is “advisory” meaning no formal deviation is required (again, a decent MISRA-C tool would allow you to suppress this particular violation).

回答2:

Put diag->bufferCnt++ in a separate statement (as it is also advised by Hans in OP comments) and the warning should not appear.

But regarding MISRA rule 12.2 I see no violation of 12.2 (there is a single sequence point in your statement and no unspecified behavior) in your program and I think it's a bug in your MISRA software.

For information there is also an advisory 12.13 rule in MISRA that says:

(MISRA-C:2004, 12.13) "The increment (++) and decrement (--) operators should not be mixed with other operators in an expression"

The problem with MISRA is their terminology use is far from perfect, for 12.3, while -> or = are C operators, in the explanation they then seem to talk only about arithmetic operators...

来源：https://stackoverflow.com/questions/32554813/misra-c-rule-12-2-false-positive-warning

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misra