poor struct definition in ipfw (bitfield with enum base type) ?

Julian Elischer julian at elischer.org
Mon Nov 30 01:11:14 UTC 2009

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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