Bit-fields of type other than int?

Question

I have a code which uses bit-fields declared as follows

typedef struct my{
    const char *name;
    uint8_t is_alpha : 1;   
    uint8_t is_hwaccel : 1; 
           


        

Answer 1

1] Is there any problems/potential issues in using bit fields of type other than int? Why the warning?

Since bit-fields are low-level, there may be issues with portability if you are using non-standard types. Hence the warning -- note it is still a warning and not an error.

2] Are other than int type bit-fileds they allowed by C99 C language specification?

From the draft of C99:

6.7.2.1 Structure and union specifiers

4 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type.

Answer 2

Why not use int? The actual implementation of bitfields varies from compiler to compiler. If you want to write portable code, use int. If you want to create a small structure, or a structure of a fixed number of bytes, or a structure where the bits are in a fixed position, don't use bitfields. Create a uint8_t member called something like flags and define macros to use as bitmasks.

Answer 3

As others have mentioned about portability issues et al, if you didn't know you can just disable the warning via the warning pragma:
https://docs.microsoft.com/en-us/cpp/preprocessor/warning?view=vs-2019

#pragma warning(push)
#pragma warning(disable: 4214) // warning C4214: nonstandard extension used: bit field types other than int
typedef struct my{
    const char *name;
    uint8_t is_alpha : 1;   
    uint8_t is_hwaccel : 1; 
    uint8_t x_chroma_shift; 
    uint8_t y_chroma_shift; 
} mystr; 
#pragma warning(pop)

Also you can disable specific warnings in your project properties but then they are project wide. This way you can control them per data type.

Then if you are ever not sure 100% what kind of binary code MSVC will generate for these either run it in the debugger and look at the "disassembly view" (break on where it gets accessed), or load up your executable (with PDB file for symbols) in a disassembler like IDA Pro, Ghidra, etc.