static analysis - How to properly use C++ union on embedded software - Stack Overflow

admin管理员组

文章数量:1348221

I'm an embedded software (firmware) developer (C++) and sometimes I need to convert data between types, for example

• an array of 4 bytes, need to be written to the hardware in a single 32bit register
• a float variable needs to be transmitted in a message as bytes

In order to do the conversion properly, I am using a union, to store the data, so I can read/write it in both ways, for example

union {
    uint8_t asByte[lengthInBytes]{};         // interprete the data as 16 bytes
    uint32_t asUint32[lengthInWords];        // interprete the data as 4 32bit words
} state;

Now I am running static code checking on my code (using SonarCloud) and this is flagging an issue on this construct :

Add a discriminant to this wrapped undiscriminated union or replace it with an "std::variant"

I don't want to use the std:: library containers because I need to avoid using dynamic memory allocation, and often it's also an overkill...

So how can I resolve this 'codeSmell', or should I just ignore/accept it in my use case ?

I'm an embedded software (firmware) developer (C++) and sometimes I need to convert data between types, for example

• an array of 4 bytes, need to be written to the hardware in a single 32bit register
• a float variable needs to be transmitted in a message as bytes

In order to do the conversion properly, I am using a union, to store the data, so I can read/write it in both ways, for example

union {
    uint8_t asByte[lengthInBytes]{};         // interprete the data as 16 bytes
    uint32_t asUint32[lengthInWords];        // interprete the data as 4 32bit words
} state;

Now I am running static code checking on my code (using SonarCloud) and this is flagging an issue on this construct :

Add a discriminant to this wrapped undiscriminated union or replace it with an "std::variant"

I don't want to use the std:: library containers because I need to avoid using dynamic memory allocation, and often it's also an overkill...

So how can I resolve this 'codeSmell', or should I just ignore/accept it in my use case ?

• c++
• static-analysis

Share Improve this question Follow asked Apr 2 at 8:59 StrooomStrooom 10377 bronze badges 18

• 14 std::variant doesn't use any dynamic memory allocation, but you have a bigger problem - type punning via unions is Undefined Behaviour in C++. – Yksisarvinen Commented Apr 2 at 9:01
• 3 See for example here for more info: tttapa.github.io/Pages/Programming/Cpp/Practices/… – Chronial Commented Apr 2 at 9:05
• 4 @PepijnKramer: OP wants std::bitcast not std::variant. – Jarod42 Commented Apr 2 at 9:30
• 3 @PepijnKramer: "I need to convert data between types". std::variant doesn't do that. – Jarod42 Commented Apr 2 at 9:36
• 4 No unittest can really catch 100% of the possible UBs. It's better to avoid a clear case that invokes UB. – wohlstad Commented Apr 2 at 9:51

 |  Show 13 more comments

1 Answer 1

Sorted by: Reset to default 6

Thanks for all the useful remarks,

Seems like std::bitcast is the right way to do this.
It requires C++20 but I think I'm ok with that.

本文标签： static analysisHow to properly use C union on embedded softwareStack Overflow