Date: Sun, 29 Nov 2009 17:11:17 -0800 From: Julian Elischer <julian@elischer.org> To: Luigi Rizzo <rizzo@iet.unipi.it> Cc: current@freebsd.org Subject: Re: poor struct definition in ipfw (bitfield with enum base type) ? Message-ID: <4B131BB5.7010106@elischer.org> In-Reply-To: <20091130011643.GA29020@onelab2.iet.unipi.it> References: <20091130011643.GA29020@onelab2.iet.unipi.it>
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Luigi Rizzo wrote:
> While trying to compile ipfw (userland) with tcc, i hit an issue
> on the following struct in ip_fw.h (I think I wrote it):
>
> typedef struct _ipfw_insn { /* template for instructions */
> enum ipfw_opcodes opcode:8;
> u_int8_t len; /* number of 32-bit words */
> u_int16_t arg1;
> } ipfw_insn;
>
> gcc correctly packs the structure in 4 bytes, however tcc
> fails to realize that the enum fits in 8 bits, and uses
> the base type (int, which is 32 bit) which results in
> the structure using 8 bytes. You can imagine the results.
I think from memory enums are only defined to be ints
and an 8 bit enum is a gcc extension.
>
> I wonder if the difference is a bug in 'tcc' or it is one of
> those things that are 'implementation-defined' by the C99 standard
> (googling around seems to suggest the latter).
>
> grepping through the entire /usr/src shows only one instance of
> the above construct (a bitfield using an enum as base type).
>
> To be safe, I'd be inclined to change 'opcode' to uint8_t,
> which is non ambiguous and generates the same code on gcc and tcc
>
> cheers
> luigi
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