FIX: Compiler Allocates Struct with Nested Struct Incorrectly

• Microsoft Visual C++, 32-bit Edition, versions 4.0, 4.1, 4.2

SYMPTOMS

In the sample code below, the amount of space the compiler allocates for struct A is incorrect. It is influenced by adding a member with data type double to the nested struct in A even though A only declares a pointer to this nested struct and not an instance of the struct itself. If you uncomment the line with the double member, the code will produce different results. The size of struct A will be reported as being larger.

Changing the size of the nested struct by adding a member of any data type other than double does not affect the size of struct A.

RESOLUTION

The problem is very specific to code similar to that below, so there are no generic workarounds. It may be necessary to change the data type of the member to something other than a double.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ version 5.0.

MORE INFORMATION

Sample Code

   /* Compile options needed: none
   */

   #include <stdio.h>

   struct A
   {
       long l1;
       long l2;

       struct
       {
           int i1;
           long l3;
   //        double d1;
       } *s1;
   };

   void main()
   {
       printf( "sizeof(struct A) should be 12\n" );
       printf( "sizeof(struct A) = %d\n", sizeof(struct A) );
   }

Keywords          : CodeGen vcbuglist400 vcfixlist500 kbprg
Version           : 4.0 4.1 4.2
Platform          : NT WINDOWS
Issue type        : kbbug
Solution Type     : kbfix