Enums in Bitfields [Greenhills PowerPC Compiler for C]

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Mark R
Mark R 2017-1-19
评论： Mark R 2017-1-30
I have an enum
typedef enum
{
YES = 0u,
NO = 1u
} AnswerType;
Which is used as a bit field element in a struct
typedef struct
{
uint8_t messageField;
uint8_t padding : 7;
AnswerType answer : 1;
} MessageType;
When attempting to access this field, Polyspace is confusing the enum to be a signed 1 bit bitfield with a range of [-1..0], when in fact it's unsigned [0..1]. This of course causes a chain of errors due to incorrect range values.
Forcing the "auto-unsigned-first" option in Polyspace for "-enum-type-definition" will treat this enum as a u8, however it's actually compiled as u32, thus causing problems in other areas of the analysis (where we rely on it being a u32).
Is there a way to override the range values for enums and struct elements?

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采纳的回答

Alexandre De Barros
Hello,
Polyspace is not confused but the actual type of an enum (and its signedness) is implementation-dependent.
See this link for a reference to the C standard on this topic.
Some C coding standards like MISRA-C:2012 highlight this problem, with the rule 6.1 "Bit-fields shall only be declared with an appropriate type".
Now, you can have unsigned bitfields by choosing a gnu compiler in your Polyspace project.
But a more portable solution is not to rely on the representation of enums with bitfields.
For example, you could use constants instead:
typedef unsigned int AnswerType;
const AnswerType YES = 0u;
const AnswerType NO = 1u;
Regards,
Alexandre
  1 个评论
Mark R
Mark R 2017-1-30
Firstly thanks for the response.
I have never used typedef constants like that before, nice solution. I will consider this in future.
Unfortunately portability was not a goal of my project and so I developed the software relying on the behaviour of the compiler.
I'm toward the end of my project and as such I can't justify a large change to simply work around an analysis tool.
Luckily - I used the gnu dialect option as you suggested and that has fixed the errors I'm seeing. Thanks again for your help.

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