Re: KVM Block Device Driver

[Paolo Bonzini <pbonzini@redhat.com>]

Il 14/08/2013 20:44, Spensky, Chad - 0559 - MITLL ha scritto:
> Wolfgang,
> 
>   Thanks so much for the response.  It turns out that wasn't handling the
> QEMUIOVector properly.  When I first implemented it, I saw that the iovec
> was a pointer and assumed that there would only ever be one.  Given the
> lack of documentation and my lack of understanding this went undetected
> for a while.  everything now seems to work just fine.  :-)  See below for
> the portion of code that threw me off.  Thanks again!
> 
>  - Chad
> 
> 
> Proposed change to qemu-common.h (:
> 
> typedef struct QEMUIOVector {
>     struct iovec *iov;
>     int niov;
>     int nalloc;
>     size_t size;
> } QEMUIOVector;
> 
> changed to:
> 
> // Array of I/O vectors
> 
> typedef struct QEMUIOVector {
> struct iovec iov[];
> int niov;
> int nalloc;
> size_t size;
> } QEMUIOVector;

This wouldn't work.  As you wrote it, it wouldn't even compile.

Embedding an array is possible if you place it at the end of the struct,
but then it is not possible to resize the array (because when you
reallocate it, the containing QEMUIOVector could move in memory and any
pointers to the containing QEMUIOVector would become invalid).

In C, a dynamically-allocated array is represented by a pointer to the
first item.  The size of the array and the size of the
dynamically-allocated memory block are stored together with the pointer
(in this case, in the niov and nalloc members).

In fact, whenever you see something like "iovec iov[]" in Java, what's
being stored in memory is exactly a pointer to the first item, the size
of the array and the size of the dynamically-allocated memory block.  C
just makes that explicit.

Paolo