Re: Copying a struct to a larger struct?

hermes_917@xxxxxxxxx wrote: 
I want to use memcpy to copy the contents of one struct to another
which is a superset of the original struct (the second struct has extra
members at the end).  I wrote a small program to test this, and it
seems to work fine, but are there any cases where doing something like
this could cause any problems?

Here's the small program I wrote to test this:

#include <stdio.h>


int main()
{
    struct simplestruct
    {
        int i;
        char str[8];
    };

    struct extendedstruct
    {
        int i;
        char str[8];
        int j;
    };

    struct simplestruct foostruct;
    struct extendedstruct barstruct;

    /* Populate the members of the simplestruct instance */
    foostruct.i=7;
    strcpy(foostruct.str, "Test");

    /* Copy the contents of the simplestruct instance to the
     extendedstruct instance */
    memcpy(&barstruct, &foostruct, sizeof(foostruct));

    /* Populate remaining member of the extendedstruct instance */
    barstruct.j=13;

    /* Print values of members of the extendedstruct instance */
    printf("i\t%d\nstr\t%s\nj\t%d\n", barstruct.i, barstruct.str,
           barstruct.j);
}

Thanks in advance for any advice. 

It is possible that sizeof foostruct == sizeof barstruct;
take another example:

   struct simplestruct
   {
      int i;
      char str[7];
   };

   struct extendedstruct
   {
     int i;
     char str[7];
     unsigned char j;
   };

On typical implementations with 8bit char and 32bit or 16bit int and
minimal padding, it is now probable that sizeof foostruct == sizeof barstruct, so your padding in simplestruct overwrites your useful
information in extendedstruct.
So, either you memcpy() (offsetof(struct simplestruct, str) + sizeof foostruct.str) bytes (the offsetof macro comes from <stddef.h>) or
you follow the hint given in another reply, namely
struct extendedstruct
{
struct simplestruct mySimple;
unsigned char j;
}
Then, you do not even need memcpy():
barstruct.mySimple = foostruct;
suffices.
(In addition, you can abuse the address of barstruct as a
struct simplestruct*, if necessary -- but I have not said that ;-))


Cheers
 Michael
--
E-Mail: Mine is an   /at/ gmx /dot/ de   address.
.

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