Date: Mon, 30 Nov 2009 02:16:43 +0100 From: Luigi Rizzo <rizzo@iet.unipi.it> To: current@freebsd.org Subject: poor struct definition in ipfw (bitfield with enum base type) ? Message-ID: <20091130011643.GA29020@onelab2.iet.unipi.it>
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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 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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