PRB: Nested Nameless Structs Can Cause C2020 Error in C

The information in this article applies to:

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 4.1, 4.2,

       5.0
  

SYMPTOMS

In Microsoft C, using a nameless structure as a member of a structure causes a compiler error if the two structures both have members with the same name. C versions 6.x generate the error:

   C2030: 'varname': struct/union member redefinition.

   error C2020: 'varname' : 'struct' member redefinition

CAUSE

This is expected behavior, not a bug. When a nameless structure is used within another structure, the members of the nameless structure become members of the new structure. In the C language, the member names must be unique.

MORE INFORMATION

Sample Code

   /* compile options needed: none /Tc */

   void main(void)
    {

      struct s1
        {
          int a,b,c;
        };

      struct s2
        {
          struct s1;  /* nameless struct */
          float z;
          char a[10];  /* error occurs here */
        } *p_s2;
    }

   struct s2
     {
       int a,b,c;  /* Was nameless struct s1 */
       float z;
       char a[10];  /* Error occurs here */
     } *p_s2;