In the last episode (Oct 10), Li, Qing said: > > In the last episode (Oct 08), Li, Qing said: > > > The bit fields "th_x2" and "th_off" in "struct tcphdr", even > > > though defined as "u_int", actually occupies 1 byte. > > > > u_int th_x2:4, /* (unused) */ > > th_off:4; /* data offset */ > > > > The :4 after each variable means 4 bits long, so both fields > > together take up 8 bits = 1 byte. That's the whole purpose of > > bitfields :) > > D'oh > > I didn't ask the right question. > > It seems u_int specifies the packing and alignment size > for the bit fields, is that correct ?
I don't think so. C99 only allows bitfields to be of type Bool, signed _int, or unsigned int, so that seems to prevent the use of char/short/int/long to dictate padding or alignment. There must be something in the FreeBSD ABI that says structs must be padded so they are 4-byte aligned, even if none of the members require it. Try putting your 4 structs into a program and compiling them with gcc -Wpadding: > struct { > u_int a:4, > b:4; > }; is 4 bytes in size. a.c:7: warning: padding struct size to alignment boundary > struct { > u_int a:4, > b:4; > short c; > }; is 4 bytes in size. a.c:13: warning: padding struct to align 'c' (1 byte of padding added just before c) > struct { > u_int a:4, > b:4; > short c; > u_char d; > }; is 8 bytes in size; a.c:19: warning: padding struct to align 'c' (1 byte padding just before c, and 3 bytes just after d). I think it should have printed a "padding struct size to alignment boundary" warning also, since if it didn't, the padding after d would have been 1 byte, and struct would have been 6 bytes total. > But > > struct { > u_int a:4, > b:4; > u_char d; > short c; > }; is 4 bytes in size; > a.c:21: warning: padding struct size to alignment boundary This last warning I don't understand, since 1+1+2 is 4 all by itself. No padding is needed or used. -- Dan Nelson [EMAIL PROTECTED] _______________________________________________ [EMAIL PROTECTED] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "[EMAIL PROTECTED]"