Steve McIntyre <[EMAIL PROTECTED]> wrote:
> From the forst text from you, I asume that the problem is that the size
> >of char[4] is not 4 but 16.
>
> No, it's not.
>
> >So where is the real problem?
>
> Quick demonstration:
>
> 1. gcc on an i386 box:
>
> smcintyre:~$ gcc -v
> Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.5/specs
> Configured with: ../src/configure -v
> --enable-languages=c,c++,java,f77,pascal,objc,ada,treelang
> --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info
> --with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared
> --enable-__cxa_atexit --with-system-zlib --enable-nls
> --without-included-gettext --enable-clocale=gnu --enable-debug
> --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc
> i486-linux
> Thread model: posix
> gcc version 3.3.5 (Debian 1:3.3.5-13)
> smcintyre:~$ uname -a
> Linux smcintyre 2.6.13 #5 SMP Mon Sep 26 16:04:46 BST 2005 i686
> GNU/Linux
> smcintyre:~$ ./wibble
> sizeof(test1) is 10
> sizeof(test2) is 10
>
> 2. gcc on an arm box:
>
> [EMAIL PROTECTED]:~$ gcc -v
> Reading specs from /usr/lib/gcc-lib/arm-linux/3.3.5/specs
> Configured with: ../src/configure -v
> --enable-languages=c,c++,java,f77,pascal,objc,treelang --prefix=/usr
> --mandir=/usr/share/man --infodir=/usr/share/info
> --with-gxx-include-dir=/usr/include/c++/3.3 --enable-shared
> --enable-__cxa_atexit --with-system-zlib --enable-nls
> --without-included-gettext --enable-clocale=gnu --enable-debug
> --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc
> arm-linux
> Thread model: posix
> gcc version 3.3.5 (Debian 1:3.3.5-13)
> [EMAIL PROTECTED]:~$ uname -a
> Linux grieg 2.4.27-netwinder #1 Sun Dec 4 21:39:55 UTC 2005 armv4l
> GNU/Linux
> [EMAIL PROTECTED]:~$ ./wibble
> sizeof(test1) is 10
> sizeof(test2) is 12
>
> Both are using the following source:
>
> #include <stdio.h>
>
> struct test1
> {
> char blerg[1];
> char type[4];
> char flibble[3];
> char more[2];
> } __attribute__((packed));
>
> #pragma pack(1)
>
> struct test2
> {
> char blerg[1];
> char type[4];
> char flibble[3];
> char more[2];
> };
>
> int main (int argc, char **argv)
> {
> printf("sizeof(test1) is %d\n", sizeof(struct test1));
> printf("sizeof(test2) is %d\n", sizeof(struct test2));
>
> return 0;
> }
>
> As the link that I posted up-thread said, #pragma pack(1) does not
> force alignment within a structure on arm, as that would clash with
> the arm ABI. If you _must_ do things that way, you have to use
> __attribute__((packed)). Better/more portable would be to have a
> simple byte array rather than a packed structure.
I would call this default behavior of GCC a bug.
I'll try to verify this with the C99 standard, but in case GCC is able
to create working code at all (using __attribute__((packed))), it seems like
the default behavior does not make sense.
Note that countless C-programs depend on sizeof(struct something) returning
useful values.
Jörg
--
EMail:[EMAIL PROTECTED] (home) Jörg Schilling D-13353 Berlin
[EMAIL PROTECTED] (uni)
[EMAIL PROTECTED] (work) Blog: http://schily.blogspot.com/
URL: http://cdrecord.berlios.de/old/private/ ftp://ftp.berlios.de/pub/schily
