DEC C supports several VAX C extensions to ANSI-standard C. These extensions are recognized only when the VAX C compatibility option is used on the compiler command line. The VAX C extensions allow you to use the DEC C compiler to compile code originally written for the VAX C compiler.

The following sections describe the VAX C extensions available with the VAX C compatibility option. Extensions to the ANSI-standard C language are divided into two categories:

• Extensions compatible with ANSI C programs that produce diagnostic messages when compiled without the VAX C compatibility option
• Extensions incompatible with ANSI C programs, which could produce different compiler behavior when used without the VAX C compatibility option

• VAX C specific pragmas are recognized.
• Relaxed pointer and pointer/integer compatibility is allowed. That is, all pointer and integer types are compatible, and pointer types are compatible with each other regardless of the type of object they point to. Therefore, under the VAX C option, a pointer to float is compatible with a pointer to int.
• The #module directive is allowed. (On Digital UNIX systems this directive produces a warning message and is ignored.)
• The #dictionary directive is allowed. (On Digital UNIX systems this directive produces a warning message and is ignored.)
• The module form of #include is allowed. (On Digital UNIX systems the module form of this directive produces an error.)
• Specifying int for the type of a bit field is equivalent to specifying signed int in VAX C mode.
• Built-in functions are recognized.
• The main_program option may be used to identify a particular function as the main function for a given program.
  When compiling in VAX C mode, another way to specify the main function in a program is to include the following option in the function definition:

  main_program

  
  This option is not a keyword, and it can be written uppercase or lowercase. The main_program option is useful for allowing a name other than main for the main program.
  In a prototype-style function definition, include main_program between the function declaration part and the left brace, as in the following example:

  char lower(int c_up) main_program { . . . }

  
  In an old-style function definition, include main_program in the same place as in the prototype style, but before any parameter declarations, as in the following example:

  char lower(c_up) main_program int c_up; { . . . }

  
  Both examples establish the function lower as the main function; execution begins there, regardless of the order in which the functions are linked.

• Bit-field data types may include enum, short, char, and long. The ANSI C standard allows only int, unsigned int, or signed int.
• The last member of a structure may be an array with no size specified.
• Two struct types or two union types are considered the same type if their sizes are the same.
• Block-level declarations of functions with static storage class are allowed.
• The address of a constant may be passed to a function.
• Taking the address of a variable with register storage class is allowed.
• A third argument to the function main(), namely char*envp[], is allowed.
  When DEC C is run in VAX C compatibility mode, the main function can accept a third parameter, the environment array envp. This array contains process information such as the user name and controlling information, and has no bearing on passing command-line arguments. Its primary use is during exec and getenv library function calls. ¹
  See your platform-specific DEC C documentation for more information about invoking the main function within your host environment.
• long float is recognized as a synonym for double.
• Character constants containing multiple characters are packed in little endian order. For example, 'AB' is treated as 'B' << 8 + 'A' instead of 'A' << 8 + 'B'.
• Trailing (extra) commas are allowed in enumeration lists.
• The element type of an array may be incomplete.
• Carriage returns are accepted and treated as white space.

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); }