From: Arvind Sankar > Sent: 18 September 2020 23:40 .. > Ouch, offsetof() and sizeof() will give different results in the > presence of alignment padding. > > https://godbolt.org/z/rqnxTK > > I think, grepping at random, that at least struct scsi_vpd is like this, > size is 24 but data[] starts at offset 20. > > struct scsi_vpd { > struct rcu_head rcu; > int len; > unsigned char data[]; > }; For another standards 'brain-fart' consider: x = malloc(offsetof(struct scsi_vpd, data[count])); Since offsetof() is defined to return a compile-time constant (hi Microsoft) this is illegal unless 'count' is also a compile-time constant. (It ought to be defined to be constant if the field is constant.) If count < 4 then *x = *y will also write past the end of x. Such structure assignments should be compile-time errors. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
