E.1 Extensions Compatible with ANSI C

• 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.

[1] Parameters to the function main() are only checked in strict ANSI mode.