Re: fixed memory bytes
A nice kernel document regarding unaligned memory access. It may not be directly answering all the questions asked, but once gone through and understood completely, it will become easy to figure out how to write portable kernel code. http://lxr.linux.no/#linux+v2.6.36/Documentation/unaligned-memory-access.txt Rajat On Wed, Jan 5, 2011 at 4:29 AM, julie Sullivan wrote: > >> i have seen many places in kernel where the variables specially the >> structures should be of fixed size independent of the architecture. i went >> through the definitions of them but dint get clearly (or frankly say >> ...dint get them even a bit) . > > Hi Mohit > > I'm not sure whether we are interpreting your question correctly. Do you > mean > > 1. you've seen some code in the kernel which you think means the size of a > structure/ > variable (and its resulting binary footprint) is set to be the same (in > bytes), > regardless of the architecture, and you are confused about it? > > 2. you think that there should be a way of fixing the structure/variable > (binary footprint) > size to be the same (in bytes) regardless of the architecture and you are > wondering if this > is possible? > > In my (uninformed) opinion (2) is not be possible with the kernel due to > portability > issues - not only do natural word types differ (as others here are > explaining) but you > have no control over what optimization settings the kernel's user might set > in gcc, > for example. This is one of the problems with trying to maintain > closed-source drivers > and other binary code for the kernel, as I understand. > > Thanks > Julie > > ___ > Kernelnewbies mailing list > Kernelnewbies@kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
> i have seen many places in kernel where the variables specially the > structures should be of fixed size independent of the architecture. i went > through the definitions of them but dint get clearly (or frankly say > ...dint get them even a bit) . > Hi Mohit I'm not sure whether we are interpreting your question correctly. Do you mean 1. you've seen some code in the kernel which you think means the size of a structure/ variable (and its resulting binary footprint) is set to be the same (in bytes), regardless of the architecture, and you are confused about it? 2. you think that there should be a way of fixing the structure/variable (binary footprint) size to be the same (in bytes) regardless of the architecture and you are wondering if this is possible? In my (uninformed) opinion (2) is not be possible with the kernel due to portability issues - not only do natural word types differ (as others here are explaining) but you have no control over what optimization settings the kernel's user might set in gcc, for example. This is one of the problems with trying to maintain closed-source drivers and other binary code for the kernel, as I understand. Thanks Julie ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
> Linux (compiler actually) supports C99 fixed-width types such as 8, > 16, 32, 64 bits. > Just look through the include/linux/types.h > and small example here: > http://lxr.linux.no/#linux+v2.6.36/arch/powerpc/boot/types.h#L8 >> while writing portable applications always remember that "unsigned long" is the size of pointer and not necessarily unsigned int. For example on x86_64 sizeof(unsigned int) != sizeof(void *). -syed ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
Please reply all..I added back list. On Tue, Jan 4, 2011 at 1:58 PM, mohit verma wrote: > thnx john,. > but i know that this indicates to compiler to align the structure to its > nearest boundries. is that so? I am not sure of your definition of boundary, but I would say it does the opposite. It tells the compiler not to align the struct to boundaries. -- John ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
On Tue, Jan 4, 2011 at 8:40 PM, mohit verma wrote: > > hi all, > i have seen many places in kernel where the variables specially the > structures should be of fixed size independent of the architecture. i went > through the definitions of them but dint get clearly (or frankly say > ...dint get them even a bit) . Linux (compiler actually) supports C99 fixed-width types such as 8, 16, 32, 64 bits. Just look through the include/linux/types.h and small example here: http://lxr.linux.no/#linux+v2.6.36/arch/powerpc/boot/types.h#L8 > > so ,can please someone help me to understand this?? > > > thanks in advance for help > -- > > MOHIT VERMA > > ___ > Kernelnewbies mailing list > Kernelnewbies@kernelnewbies.org > http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies > > -- Regards, Denis ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
Hi.. On Wed, Jan 5, 2011 at 01:22, mohit verma wrote: > that is it mulyadi. but how the compiler or kernel forces the things to get > initialized in that fixed size only independent of the architecture?? well, AFAIK by mapping that new type into native one...for example, let's say I have "u_int", which in turn when this code is compiled for x86 32 bit, it is a typedef of "int". -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
On Tue, Jan 4, 2011 at 1:07 PM, Rajesh S R wrote: > > > On Tue, Jan 4, 2011 at 11:28 PM, Mulyadi Santosa > wrote: >> >> On Wed, Jan 5, 2011 at 00:40, mohit verma wrote: >> > >> > hi all, >> > i have seen many places in kernel where the variables specially the >> > structures should be of fixed size independent of the architecture. i >> > went >> > through the definitions of them but dint get clearly (or frankly >> > say >> > ...dint get them even a bit) . >> >> Your question isn't specific enough, so I'll just guess. Let's say >> "int". In 32 bit, AFAIK it's 4 byte, but in 64 bit (like IA 64, not >> sure if it's x64) it's 8 byte. So, if you just say "int", you will >> likely getting screwed up. >> >> By using types like u_int or something like that, you pretty much say >> "I mean 4 byte kind of integer" etc > > > Still there can be padding issues due to byte alignment, which may vary > across architecture. Am not sure if that is controllable (probably some > pragma to gcc?). Probably, OP is asking about it? I believe you are referring to __attribute__( ( packed ) ) ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
> > >By using types like u_int or something like that, you pretty much say > >"I mean 4 byte kind of integer" etc > that is it mulyadi. but how the compiler or kernel forces the things to get initialized in that fixed size only independent of the architecture?? > > -- > regards, > > Mulyadi Santosa > Freelance Linux trainer and consultant > > blog: the-hydra.blogspot.com > training: mulyaditraining.blogspot.com > -- *MOHIT VERMA* ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
Re: fixed memory bytes
On Wed, Jan 5, 2011 at 00:40, mohit verma wrote: > > hi all, > i have seen many places in kernel where the variables specially the > structures should be of fixed size independent of the architecture. i went > through the definitions of them but dint get clearly (or frankly say > ...dint get them even a bit) . Your question isn't specific enough, so I'll just guess. Let's say "int". In 32 bit, AFAIK it's 4 byte, but in 64 bit (like IA 64, not sure if it's x64) it's 8 byte. So, if you just say "int", you will likely getting screwed up. By using types like u_int or something like that, you pretty much say "I mean 4 byte kind of integer" etc -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
fixed memory bytes
hi all, i have seen many places in kernel where the variables specially the structures should be of fixed size independent of the architecture. i went through the definitions of them but dint get clearly (or frankly say ...dint get them even a bit) . so ,can please someone help me to understand this?? thanks in advance for help -- *MOHIT VERMA* ___ Kernelnewbies mailing list Kernelnewbies@kernelnewbies.org http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
