try :
typedef struct mystruct {
.....
} __attribute__(packed);
This will pack your structure !
"Wayne E. Van Loon Sr." wrote:
> Stephen:
>
> I have been using memcpy() to copy stucts (sometimes several of them) into a
> buffer. I then call write() on the buffer and then the opposite
> at the other end. I think this is the same thing as calling write() on a
> structure. I have been doing something like:
>
> unsigned char *pu8to = &buffer;
> unsigned char *pu8from = &struct;
>
> for(int i=0; i<sizeof(struct); i++){
> *pu8to++ = *pu8from++
> }
>
> write(fd, buffer, sizeof(struct));
>
> If I understand you, this means that my code might fail when both ends of a fifo
> were not compiled by the same gcc version or when both ends of a network are not
> compiled by the same gcc version or are differing platforms.
>
> I am trying to imagine some reasonable way to unpack / pack structures across a
> network that does not depend upon some structure being organized in the same
> manner at both ends. This is the only thing that I could come up with:
>
> struct USED_BOTH_IN_RTL_AND_LINUX_1
> {
> type1 member1;
> type2 member2;
> type3 member2;
> };
>
> struct USED_BOTH_IN_RTL_AND_LINUX_2
> {
> type4 member1;
> type5 member2;
> type6 member3;
> USED_BOTH_IN_RTL_AND_LINUX_1 member4
> ...
> };
>
> unsigned char buffer[sizeof(USED_BOTH_IN_RTL_AND_LINUX_2)];
> USED_BOTH_IN_RTL_AND_LINUX_2 object1;
>
> unsigned char *pu8s = (unsigned char *) &object1.member1;
> unsigned char *pu8d = (unsigned char *) &buffer;
> unsigned n;
>
> n = sizeof(object1.member1);
> memcpy(pu8d, pu8s, n);
> pu8d += n;
>
> pu8s = (unsigned char *) &object1.member2;
> n = sizeof(object1.member2);
> memcpy(pu8d, pu8s, n);
> pu8d += n;
>
> pu8s = (unsigned char *) &object1.member3;
> n = sizeof(object1.member3);
> memcpy(pu8d, pu8s, n);
> pu8d += n;
>
> pu8s = (unsigned char *) &object1.member4.member1;
> n = sizeof(object1.member4.member1);
> memcpy(pu8d, pu8s, n);
> pu8d += n;
>
> pu8s = (unsigned char *) &object1.member4.member2;
> n = sizeof(object1.member4.member2);
> memcpy(pu8d, pu8s, n);
> pu8d += n;
>
> ....
>
> This does depend upon the size of the types involved being the same size at both
> ends and I think that it requires the bytes in the types, (int for example) to
> be stored in the same order at both ends.
>
> How do you do it?
>
> Wayne
>
> > Mr. Calianu Wrote:
> > > More specifically, I plan on writing structs into the fifo and reading
> > > them in userland.
> >
>
> "Stephen D. Cohen" wrote:
>
> > Then you are already dooming yourself to a great deal of pain.
> > Avoid simply using rtf_put on a struct and expecting the same thing in user
> > space. That is, of course, unless you are willing to sort through every
> > compiler option and optimization setting to make absolutely certain that the
> > structs will be aligned exactly the same way in RT space and user space. It
> > would be much better to simply define your own format for the data in the
> > FIFO and copy out of the struct to a buffer, send the buffer, then copy out
> > of the buffer into your struct. This also assures that your code is
> > maximally portable across platforms, etc.
> >
> > You will be amazed to find that it is *much* easier to textualize /
> > bufferize it yourself and disassemble the buffers at the other end, rather
> > than simply writing structs. You will find this the first time the darned
> > compiler bites you in the rear end and you spend two days tracking down why
> > something that obviously works doesn't. You may not find this until your
> > version of gcc changes or some such. By then you won't remember that you
> > might have a problem. In that case, expect to spend the whole week tracking
> > this down. Creating the buffers properly, on the other hand, will only take
> > an hour of your life and you will have the pleasure of the knowledge of a
> > job done properly. Your choice. :-)
> >
> > Regards,
> >
> > Steve Cohen
> >
> > --------------------------
> > Stephen D. Cohen
> > Xybion Sensor Positioning Systems
> > 11528 53rd Street North
> > Clearwater, FL 33760
> > Voice: (727) 299-0150
> > Fax: (727) 299-0804
> > [EMAIL PROTECTED]
> > www.xybion.com
> >
> > -- [rtl] ---
> > To unsubscribe:
> > echo "unsubscribe rtl" | mail [EMAIL PROTECTED] OR
> > echo "unsubscribe rtl <Your_email>" | mail [EMAIL PROTECTED]
> > --
> > For more information on Real-Time Linux see:
> > http://www.rtlinux.org/
>
> -- [rtl] ---
> To unsubscribe:
> echo "unsubscribe rtl" | mail [EMAIL PROTECTED] OR
> echo "unsubscribe rtl <Your_email>" | mail [EMAIL PROTECTED]
> --
> For more information on Real-Time Linux see:
> http://www.rtlinux.org/
-- [rtl] ---
To unsubscribe:
echo "unsubscribe rtl" | mail [EMAIL PROTECTED] OR
echo "unsubscribe rtl <Your_email>" | mail [EMAIL PROTECTED]
--
For more information on Real-Time Linux see:
http://www.rtlinux.org/
