warning when using bitfield with unsigned char

Question

This is my bitfield

struct {
    unsigned char v64 : 1;
    unsigned char leg : 7;
} valid;

Then I get the warning:

main.c:17:3: warning: type of bit-field ‘v64’ is a GCC extension [-pedantic]
main.c:18:3: warning: type of bit-field ‘leg’ is a GCC extension [-pedantic]

If I change to int there is no warning. But I want a bitfield of a byte (unsigned char).

How?

`int` has a special meaning inside of bit fields. What is `sizeof(struct valid)`? — Pubby, Jun 05 '12 at 23:10
This is a very common compiler extension. Maybe you ought to ignore the warning to have your cake and eat it. — Hans Passant, Jun 05 '12 at 23:15

ouah · Answer 1 · 2012-06-05T23:26:06.317

Remove the gcc -pedantic option if you don't want to get the warning.

In C99, gcc issues a warning with -pedantic but it is permitted to have an implementation defined type for the bit-field (like unsigned char).

(C99, 6.7.2.1p4) "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 C90, only int, unsigned int and signed int are permitted.

(C90, 6.5.2.1) "A bit-field shall have a type that is a qualified or unqualified version of one of int, unsigned int, or signed int"

Actually in both C90 and C99 the warning is not required by C (it is undefined behavior in C90 only but C doesn't not require a warning for undefined behavior). The warning is added by gcc with -pedantic for information only.

@avivgood2 *"In C90, only int, unsigned int and signed int are permitted."* So no, `unsigned long int` is not permitted. — ouah, Jul 07 '20 at 10:06

warning when using bitfield with unsigned char

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