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Date:      Sun, 28 Jan 2007 20:36:58 +0200
From:      Giorgos Keramidas <keramida@ceid.upatras.gr>
To:        usleepless@gmail.com
Cc:        freebsd-questions@freebsd.org
Subject:   Re: structure alignment problems
Message-ID:  <20070128183657.GA88428@kobe.laptop>
In-Reply-To: <c39ec84c0701280324u4819b9ech51627c44cc17dbef@mail.gmail.com>
References:  <c39ec84c0701280324u4819b9ech51627c44cc17dbef@mail.gmail.com>

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On 2007-01-28 12:24, usleepless@gmail.com wrote:
> Hi,
> i am working on getting mythtv-0.20 talking to the pvrxxx driver.
>
> the problem appears when i want to do:
>  v4l2_capabilities vcap;
>  ioctl(videofd, VIDIOC_QUERYCAP, &vcap);
>
> the driver and mythtv have the struct differently aligned, and i get
> an "inappropiate ioctl for device". this is what the struct + IOR look
> like:

Welcome to the world of structure alignment issues :)

> struct v4l2_capability
> {
>        __u8    driver[16];     /* i.e. "bttv" */
>        __u8    card[32];       /* i.e. "Hauppauge WinTV" */
>        __u8    bus_info[32];   /* "PCI:" + pci_name(pci_dev) */
>        __u32   version;        /* should use KERNEL_VERSION() */
>        __u32   capabilities;   /* Device capabilities */
>        __u32   reserved[4];
> };
> #define VIDIOC_QUERYCAP         _IOR  ('V',  0, struct v4l2_capability)
>
> the driver is compiled like this:
>
> cc -O -pipe -Werror -D_KERNEL -DKLD_MODULE -nostdinc -I-  -I../../..
> -I. -I@ -I@/contrib/altq -I@/../include -finline-limit=8000
> -fno-common  -fno-omit-frame-pointer -mcmodel=kernel -mno-red-zone
> -mfpmath=387 -mno-sse -mno-sse2 -mno-mmx -mno-3dnow  -msoft-float
> -fno-asynchronous-unwind-tables -ffreestanding -Wall -Wredundant-decls
> -Wnested-externs -Wstrict-prototypes  -Wmissing-prototypes
> -Wpointer-arith -Winline -Wcast-qual  -fformat-extensions -std=c99 -c
> /usr/ports/multimedia/pvrxxx/work/modules/cxm/cxm/../../../dev/cxm/cxm.c
>
> it yields a struct-size of 104 bytes.
>
> mythtv is compiled like this:
>
> g++ -c -pipe -I/usr/local/include/artsc -D_REENTRANT
> -I/usr/local/include/glib-2.0 -I/usr/local/lib/glib-2.0/include -g
> -Wall -W -O2 -pthread -Wall -Wno-switch -fomit-frame-pointer
> -I/usr/local/include/freetype2 -I/usr/local/include -DPIC -fPIC
> -DUSING_FREEBOX -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64
> -DPREFIX=\"/usr/local\" -DLIBDIR=\"/usr/local/lib\"
> -D_LARGEFILE_SOURCE -DUSING_OSS -DUSING_H264TOOLS -DUSING_XV
> -DUSING_FRONTEND -DUSING_CRC_IP_NETWORK_REC -DUSING_V4L -DUSING_DBOX2
> -DUSING_FREEBOX -DUSING_HDHOMERUN -DUSING_IVTV -DUSING_BACKEND
> -DQT_NO_DEBUG -DQT_THREAD_SUPPORT -DQT_SHARED -DQT_TABLET_SUPPORT
> -I/usr/local/share/qt/mkspecs/freebsd-g++ -I.
> -I../../../../../../../local/include
> -I../../../../../../../X11R6/include -I../.. -I.. -I../libmyth
> -I../libavcodec -I../libavutil -I../libmythmpeg2 -Idvbdev -Impeg
> -I../libmythlivemedia/BasicUsageEnvironment/include
> -I../libmythlivemedia/groupsock/include
> -I../libmythlivemedia/liveMedia/include
> -I../libmythlivemedia/UsageEnvironment/include
> -I../../../../../../../local/include -I/usr/X11R6/include -o
> cardutil.o cardutil.cpp
>
> and it yields a struct-size of 128.
>
> is it the -std=c99? is it because this is on amd64?

Most probably it's the -O2 flag, because -O2 enables -fstrict-aliasing
on top of the usual -O optimizations.  You can probably disable this
per-structure, by using the special __packed macro for this structure
when building both the kernel *and* userland structures.




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