Sorry, forgot to CC the following message to gcc-help: ---------- Forwarded message ---------- From: John Graham <johngavingraham@xxxxxxxxxxxxxx> Date: Wed, Jun 2, 2010 at 5:13 PM Subject: Re: g++ serious bug in Ubuntu Lucid Lynx To: "David R. Doucette" <ddoucette@xxxxxxxxxxxxxxxx> You can use the variable attributes (http://gcc.gnu.org/onlinedocs/gcc/Variable-Attributes.html), specifically `aligned' and `packed' to achieve what you want. For example, with the following code: // start #include <stdio.h> struct s { short int x __attribute__((__aligned__(2))); int y __attribute__((__packed__)); }; int main(void) { printf("offset of y: %d\n", __builtin_offsetof(struct s, y)); printf("size of struct s: %d\n", sizeof(struct s)); } // end The result is that the offset of y in struct s is 2 and the overall size is 6. Without the attributes (e.g. if you were to add #define __attribute__(x)' after the #include) the results are 4 and 8, on my machine. Hope this helps. John G On Wed, Jun 2, 2010 at 4:53 PM, David R. Doucette <ddoucette@xxxxxxxxxxxxxxxx> wrote: > We just installed Lucid Lynx a few days ago, and have hit a fatal flaw with > g++ that has forced us to stop the deployment and roll back the systems we > could. > > Structures in Lucid are now mapping differently than in the past, so we can > no longer read our own files! Further, if any files are written by code > produced by the new g++, they are corrupted. > > The problem is that doubles are now being mapped onto a boundary divisible > by 8 rather than a boundary divisible by 4, meaning that offsets change and > structs get bigger. > > I've attached a tar file showing the mapping on Jaunty, Karmic, and Lucid to > show the change. This is a greatly simplified program that shows the > problem, but it's happening in all of our C++ programs. > > HELP! > > System is Ubuntu 10.4 Lucid Lynx 64 bit. > > David Doucette >
