Q61975: Odd Size Structures > 10 Bytes Passed Incorrectly If Packed

Article: Q61975
Product(s): See article
Version(s): 6.00    | 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 11-JUL-1990

If you declare a structure of odd length greater than 10 and pass that
structure by value, the Microsoft C version 6.00 compiler will pass a
correct size for the structure, but will pad the size of the structure
by an extra byte in the receiving function. The next byte on the stack
after the structure will be overwritten with a null padding character.

The best workaround for this problem is to manually pad the structure
to an even size.

The sample code shown below declares an 11-byte structure and passes
that as well as a character to a function. Note that the character
following the structure is overwritten with a null character.

Remove the comment from the struct ODDSTRUCT definition to add in a
char pad and recompile to demonstrate the workaround for the problem.

Sample Code
-----------

// Compile with /Zp

#include <stdio.h>
#include <string.h>

struct ODDSTRUCT
   {
   char string[11];
  // char pad ;          /* Remove comments to pad structure. */
   }   oddstruct;

void catchastruct ( struct ODDSTRUCT , char ) ;

void main ( void )
{
   strcpy ( oddstruct.string, "0123456789" ) ;
   printf ("Msg = %s\n", oddstruct.string ) ;
   catchastruct ( oddstruct , '!' );
}

void catchastruct ( struct ODDSTRUCT oddstruct, char trashme )
{
   printf ( "Msg = %s\ntrashme=%c(char) or %x(hex)\n",
            oddstruct.string , trashme, trashme ) ;
}

The following is the output of this code:

Msg = 0123456789
Msg = 0123456789
trashme= (char) or 0(hex)

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