E.2 Extensions Incompatible with ANSI C

• Unsigned preserving rules apply. (unsigned char and unsigned short promote to unsigned int .)

• VAX C-specific predefined macros are recognized.

• VAX C-specific keywords are recognized.

• Macro parameters are recognized and are replaced as string or character constants in the macro definition.

• Comments are converted to no spaces instead of a single space to allow token concatenation. (The compiler attempts to concatenate the two adjacent tokens.)

• Comments in macro replacement lists behave like ## operators when a valid token results after concatenation, except that adjoining white space is not deleted. If the resulting token is not valid, the comment in a macro replacement is deleted.

• Trigraphs are not recognized or replaced.

• Variant structures and unions are allowed.

  Variant structure and union declarations allow reference to members of nested aggregates without having to refer to intermediate structure or union identifiers. When a variant structure or union declaration is nested within another structure or union declaration, the enclosed variant aggregate ceases to exist as a separate aggregate, and DEC C copies its members to the enclosing aggregate.

  Variant structures and unions are declared using the variant_struct and variant_union keywords. The format of these declarations is the same as that for regular structures or unions, with the following exceptions:

  • Variant aggregates must be nested within other valid structure or union declarations.

  • A tag cannot be used in a variant aggregate declaration.

  • At least one member must be declared in the variant aggregate declaration, and it may not be declared as a pointer or an array.

  Initialization of a variant structure or union is the same as that of a normal structure or union.

  With the VAX C compatibility option, two structures or unions in an assignment operation need only have the same size, rather than requiring the same members and member types.

  The following example shows the format of a variable structure declaration, and how to reference members of a variant structure:

  #include <stdio.h>
  enum packet_type {TEXT, INTEGER};
  
  /* This structure can contain either a text_packet  or an integer value.
     It can only contain one of these at a time, since they share the same
     storage. */
  
  struct packet
  {
      enum packet_type type;
      variant_union
      {
          variant_struct
          {
              int str_size;
              char *text;
          } text_packet;
          variant_struct
          {
            int value;
          } value_packet;
  
       } text_or_int;
  } packet = {TEXT, 24 ,"I love the color purple"};
  
  main()
  {
  
      if (packet.type == TEXT)
          printf(" %s. \n",packet.text);
      else
          printf(" %d \n", packet.value);
  
      packet.type = INTEGER;
      packet.value = 42;
  
      printf(" The meaning of life, the universe, and everything is: %d. \n",
  packet.value);
  
  }