On Wed, 28 Apr 2004, David Mattes wrote:
> For some reason on the ARM processor the line in libusb-0.1.8/linux.c:
> ret = read(fd, (void *)&dev->descriptor, sizeof(dev->descriptor))
> sizeof(dev->descriptor) is returning 20 instead of 18!!! I have no idea
> why this might be happening as everything checks out and the sum of all
> the sizeof(dev->descriptor.*) adds up to 18!!!
>
> Anyway the hack is to subtract 2 from the amount to be read from fd and
> everything is golden again... any ideas???
The C compiler is allowed to pad structures to satisfy the ABI's
alignment constraints.
For example, on a machine where int need to be aligned, the structure:
struct foo {int a; char b;};
will be padded with 3 bytes, otherwise you would get alignment faults
on constructs like
struct foo {int a; char b;} bar[2];
bar[1].a = 23;
And the sizeof operator returns the size including padding...
/Krister
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