Date: Sun, 6 Apr 2003 10:36:06 +0200 From: Stefan Farfeleder <stefan@fafoe.dyndns.org> To: Bruce Evans <bde@zeta.org.au> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/dev/fxp if_fxpreg.h Message-ID: <20030406083608.754DB3FC4@fafoe.dyndns.org> In-Reply-To: <20030406134416.A3578@gamplex.bde.org> References: <200304052346.h35Nkwoi037742@repoman.freebsd.org> <20030406134416.A3578@gamplex.bde.org>
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On Sun, Apr 06, 2003 at 02:16:07PM +1000, Bruce Evans wrote: > On Sat, 5 Apr 2003, Maxime Henrion wrote: > > > mux 2003/04/05 15:46:58 PST > > > > FreeBSD src repository > > > > Modified files: > > sys/dev/fxp if_fxpreg.h > > Log: > > ... > > - Change some u_int to u_int8_t which make more sense here since > > we're really defining bytes. That produces the same code due to > > how bitfields work. > > This gives undefined behaviour and thus produces random code if it is > compiled by a C compiler (unless Bool_t happens to be u_int8_t). From > n869.txt: > > [#8] A bit-field shall have a type that is a qualified or > unqualified version of _Bool, signed int, or unsigned int. > > I fixed this bug in many places, including in rev.1.13 of if_fxpreg.h. > but it keeps getting reintroduced :-(. > > Bit-fields of other integer types are an unportable gcc extension. > They affect the struct layout in unportable apparently-undocumented > ways. IIRC, they don't affect internal padding but they do affect the > size and alignment the struct -- a struct that has only uint8_t > bit-fields in it has only the size and alignment requirements of > uint8_t, while a struct with only u_int bit-fields in it has the size > and alignment requirements of u_int. This may be controlled to some > extent using other unportable gcc extensions. FYI, the final standard says 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. and moved it from the Semantics to the Constraints section, so a C compiler not supporting u_int8_t has to issue at least a diagnostic before producing random code :) Regards, Stefan Farfeleder
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