Re: Making gcc less pedantic

[Guy Harris <guy () alum mit edu>]

On Feb 13, 2015, at 12:54 PM, Gerald Combs <gerald () wireshark org> wrote:

> Would it make sense to make gcc's "-pedantic" warnings a bit less
> pedantic, e.g. with "-std=c99"?

C90 says

        A bit-field shall have a type that is a qualified or unqualified version of one of int, unsigned int, or signed 
int. Whether the high-order bit position of a (possibly qualified) “plain” int bit-field is treated as a sign bit is 
implementation-defined. A bit-field is interpreted as an integral type consisting of the specified number of bits.

in 6.5.2.1 "Structure and union specifiers".

C99 says

        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.

in 6.7.2.1 "Structure and union specifiers".

At least as I read C99's 6.2.5 "Types", an enumeration is not any of the specified types, so it would be 
implementation-defined whether an enumeration is a valid type for a bit-field.

So -std=c99 combined with -pedantic does *not* suppress the warning:

        $ gcc -pedantic -std=c99 enum.c
        enum.c:4:7: warning: type of bit-field ‘twobits’ is a GCC extension [-Wpedantic]
          enum frobozz twobits:2;

when compiling

        enum frobozz { red, green, blue };

        struct foo {
                enum frobozz twobits:2;
                int hello;
        };

        int
        main(void)
        {
                return 0;
        }

-std=cXX, by itself, does *not* cause warnings when features not in cXX are used; to quote the man page:

           ...  When a base standard is specified, the compiler accepts
           all programs following that standard plus those using GNU
           extensions that do not contradict it.  For example, -std=c90 turns
           off certain features of GCC that are incompatible with ISO C90,
           such as the "asm" and "typeof" keywords, but not other GNU
           extensions that do not have a meaning in ISO C90, such as omitting 
           the middle term of a "?:" expression.

so even -std=c89 prints no "a bit-field is an enum" warning when used without -pedantic.

> In this particular case having a "packet_char_enc" encoding instead of
> an "unsigned int" encoding is useful, otherwise you have to cast back to
> packet_char_enc further down the line. It also compiles under Clang,
> Visual C++, and Solaris Studio without warning.

In practice, I suspect few if any C compilers would refuse to allow enumerations as bit-fields.
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