Gert Doering <g...@greenie.muc.de> writes:
> Hi,
>
> On Sat, Jun 10, 2017 at 07:58:13PM +0200, Jeremie Courreges-Anglas wrote:
>> OpenBSD:
>>
>> struct ip {
>> #if _BYTE_ORDER == _LITTLE_ENDIAN
>> u_int ip_hl:4, /* header length */
>> ip_v:4; /* version */
>> #endif
>> #if _BYTE_ORDER == _BIG_ENDIAN
>> u_int ip_v:4, /* version */
>> ip_hl:4; /* header length */
>> #endif
>> ...
>>
>> u_char -> u_int. Dunno if it matters to the compiler, I can take a look
>> later.
>
> I'm fairly sure it does matter. An int is likely to be at least 16 bit,
> maybe 32 bit, so the compiler can assume "word aligned" - and *boom*.
It doesn't seem to matter that much. Which aligns well with the
"bit fields are nasty" experience people seem to have. From a quick
glance at the c99 standard, I couldn't find language about this.
"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."
Here's a snippet:
--8<--
#include <sys/types.h>
#include <netinet/in.h>
#include <endian.h>
#include <stdlib.h>
struct ip {
#if _BYTE_ORDER == _LITTLE_ENDIAN
u_int ip_hl:4, /* header length */
ip_v:4; /* version */
#endif
#if _BYTE_ORDER == _BIG_ENDIAN
u_int ip_v:4, /* version */
ip_hl:4; /* header length */
#endif
u_int8_t ip_tos; /* type of service */
u_int16_t ip_len; /* total length */
u_int16_t ip_id; /* identification */
u_int16_t ip_off; /* fragment offset field */
#define IP_RF 0x8000 /* reserved fragment flag */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_int8_t ip_ttl; /* time to live */
u_int8_t ip_p; /* protocol */
u_int16_t ip_sum; /* checksum */
struct in_addr ip_src, ip_dst; /* source and dest address */
};
struct ip_freebsd {
#if _BYTE_ORDER == _LITTLE_ENDIAN
u_char ip_hl:4, /* header length */
ip_v:4; /* version */
#endif
#if _BYTE_ORDER == _BIG_ENDIAN
u_char ip_v:4, /* version */
ip_hl:4; /* header length */
#endif
u_int8_t ip_tos; /* type of service */
u_int16_t ip_len; /* total length */
u_int16_t ip_id; /* identification */
u_int16_t ip_off; /* fragment offset field */
#define IP_RF 0x8000 /* reserved fragment flag */
#define IP_DF 0x4000 /* dont fragment flag */
#define IP_MF 0x2000 /* more fragments flag */
#define IP_OFFMASK 0x1fff /* mask for fragmenting bits */
u_int8_t ip_ttl; /* time to live */
u_int8_t ip_p; /* protocol */
u_int16_t ip_sum; /* checksum */
struct in_addr ip_src, ip_dst; /* source and dest address */
};
void
f(void *p)
{
struct ip *iph;
iph = (struct ip *)p;
if (iph->ip_v == 6)
abort();
}
void
g(void *p)
{
struct ip_freebsd *iph;
iph = (struct ip_freebsd *)p;
if (iph->ip_v == 6)
abort();
}
--8<--
gcc-4.2.1 (system compiler on OpenBSD)
(gdb) disas f
Dump of assembler code for function f:
0x0000000000000040 <f+0>: save %sp, -208, %sp
0x0000000000000044 <f+4>: ld [ %i0 ], %g1
0x0000000000000048 <f+8>: sethi %hi(0xf0000000), %g2
0x000000000000004c <f+12>: sethi %hi(0x60000000), %g3
0x0000000000000050 <f+16>: and %g1, %g2, %g1
0x0000000000000054 <f+20>: cmp %g1, %g3
0x0000000000000058 <f+24>: be,pn %icc, 0x68 <f+40>
0x000000000000005c <f+28>: nop
0x0000000000000060 <f+32>: rett %i7 + 8
0x0000000000000064 <f+36>: nop
0x0000000000000068 <f+40>: call 0x68 <f+40>
0x000000000000006c <f+44>: nop
0x0000000000000070 <f+48>: nop
End of assembler dump.
(gdb) disas g
Dump of assembler code for function g:
0x0000000000000000 <g+0>: save %sp, -208, %sp
0x0000000000000004 <g+4>: ld [ %i0 ], %g1
0x0000000000000008 <g+8>: sethi %hi(0xf0000000), %g2
0x000000000000000c <g+12>: sethi %hi(0x60000000), %g3
0x0000000000000010 <g+16>: and %g1, %g2, %g1
0x0000000000000014 <g+20>: cmp %g1, %g3
0x0000000000000018 <g+24>: be,pn %icc, 0x28 <g+40>
0x000000000000001c <g+28>: nop
0x0000000000000020 <g+32>: rett %i7 + 8
0x0000000000000024 <g+36>: nop
0x0000000000000028 <g+40>: call 0x28 <g+40>
0x000000000000002c <g+44>: nop
0x0000000000000030 <g+48>: nop
End of assembler dump.
gcc 4.9.4:
(gdb) disas f
Dump of assembler code for function f:
0x0000000000000000 <f+0>: save %sp, -176, %sp
0x0000000000000004 <f+4>: ld [ %i0 ], %g2
0x0000000000000008 <f+8>: sethi %hi(0xf0000000), %g1
0x000000000000000c <f+12>: and %g2, %g1, %g1
0x0000000000000010 <f+16>: sethi %hi(0x60000000), %g2
0x0000000000000014 <f+20>: cmp %g1, %g2
0x0000000000000018 <f+24>: be,pn %icc, 0x28 <f+40>
0x000000000000001c <f+28>: nop
0x0000000000000020 <f+32>: rett %i7 + 8
0x0000000000000024 <f+36>: nop
0x0000000000000028 <f+40>: call 0x28 <f+40>
0x000000000000002c <f+44>: nop
0x0000000000000030 <f+48>: nop
End of assembler dump.
(gdb) disas g
Dump of assembler code for function g:
0x0000000000000040 <g+0>: save %sp, -176, %sp
0x0000000000000044 <g+4>: ld [ %i0 ], %g2
0x0000000000000048 <g+8>: sethi %hi(0xf0000000), %g1
0x000000000000004c <g+12>: and %g2, %g1, %g1
0x0000000000000050 <g+16>: sethi %hi(0x60000000), %g2
0x0000000000000054 <g+20>: cmp %g1, %g2
0x0000000000000058 <g+24>: be,pn %icc, 0x68 <g+40>
0x000000000000005c <g+28>: nop
0x0000000000000060 <g+32>: rett %i7 + 8
0x0000000000000064 <g+36>: nop
0x0000000000000068 <g+40>: call 0x68 <g+40>
0x000000000000006c <g+44>: nop
0x0000000000000070 <g+48>: nop
End of assembler dump.
So, no difference in the generated code between the OpenBSD and FreeBSD
structures.
--
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