struct nbd_reply not packed => trouble?

Sebastian Pipping sebastian at
Thu Jan 10 11:09:33 EST 2013

Hello kernel hackers,

I noticed that in [linux]/include/uapi/linux/nbd.h the structure

   struct nbd_request {
           __be32 magic;
           __be32 type;    /* == READ || == WRITE */
           char handle[8];
           __be64 from;
           __be32 len;
   } __attribute__((packed));

is packed but its reply counter part

   struct nbd_reply {
           __be32 magic;
           __be32 error;    /* 0 = ok, else error */
	  char handle[8];  /* handle you got from request */

is not.  Since Linux seems to read sizeof(nbd_reply) bytes from the 
network (see [1]), the number of bytes read varies with the number of 
bytes of the structure.

So my understanding is that if the size of struct nbd_reply varies from 
platform/compiler to another that means trouble.  I wonder:

  - Is there anything about struct nbd_reply that would keep its size
    equal everywhere or a reason why variance in size is no problem here?

  - Any ideas for a platform where you would expect struct nbd_reply to
    be other than 4 + 4 + 8 = 16 bytes in size?




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