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This appendix documents many features that distinguish Compaq C for OpenVMS Systems from VAX C Version 3.2.
This appendix was written for the first release of Compaq C as a guide for installations migrating from VAX C to Compaq C. It is not intended to be a complete compendium of new features for all Compaq C versions. For a summary of new features for the current version of the compiler, see the release notes and the New and Changed Features section in the Preface of this and the other Compaq C manuals. For additional help migrating from VAX C to Compaq C, see the DEC C Migration Guide for OpenVMS VAX Systems (VAX ONLY).
The major focus of Compaq C for OpenVMS Systems is to bring it into full conformance with the ANSI C Standard. The language described by the ANSI C Standard differs in many ways from the language originally implemented by VAX C. These differences include additional language features and constructs, the removal of obsolete features and usages, and a number of other changes that generally involve a tightening up of semantic rules.
Some of the new ANSI C Standard features have already been implemented in previous versions of VAX C. Some of these are: support for function prototypes, the const and volatile type qualifiers, and the void type specifier.
Although every attempt has been made to maintain compatibility with earlier versions of the VAX C compiler, many of the changes required to bring the compiler into conformance with the ANSI C Standard would introduce unavoidable incompatibilities with these earlier versions. For example, VAX C supports a number of language and semantic extensions that are not ANSI-conformant.
Therefore, to provide compatibility with previous versions of the compiler, Compaq C for OpenVMS Systems supports several modes of operation:
Note that some of the language changes dictated by the ANSI C Standard
are present in VAX C mode. Some of these changes are quiet
changes; that is, they cannot be detected as such by the compiler, so
no diagnostic messages are issued. Also note that some extensions are
permitted in the strict ANSI C mode. These extensions are diagnosed,
but with no greater severity than Warning. Both types of changes are
included in the following sections that describe all new and changed
features.
A.1 Features Affecting the Compiler
This section describes Compaq C compiler features.
( Section A.2 describes features that affect the Compaq C
run-time library and include files.)
A.1.1 Compaq C Qualifiers
Qualifiers new to Compaq C:
VAX C treats a comment in a macro definition as if the comment were replaced with no characters. This allows comments to paste tokens together, as in the following example:
#define PASTE(X) X/* */1 int PASTE(VAR); |
This example declares the variable VAR1 . ANSI C requires that comments be treated as if they were replaced by a single space. In Compaq C, therefore, comments cannot be used to concatenate tokens when /STANDARD=ANSI89 or /STANDARD=RELAXED_ANSI89 is specified. (The new operator ## is provided to allow token concatenation in macros.)
Compaq C for OpenVMS Systems continues to replace comments with no characters when /STANDARD=VAXC or /STANDARD=COMMON is specified; and /WARN=ENABLE=CHECK provides a diagnostic to flag comments that are used to concatenate tokens.
For /STANDARD=COMMON and /STANDARD=RELAXED_ANSI89, C++ style comments
(//) are supported.
A.1.3 String Literal Concatenation
Compaq C introduces a new ANSI-compliant feature that allows convenient continuation of string literals. If string literals are separated only by white space, the string literals are concatenated to form one string literal. For example:
fputs("This is really "
"one string literal", stderr);
|
String literal concatenation works for both normal string literals and
wide string literals.
A.1.4 Recursive main() Function
In VAX C, main , or any function using the VAX C main_program option, is not recursively reentrant.
As required by ANSI C, the main function in Compaq C can now be
called recursively.
A.1.5 Trigraph Sequences
ANSI C defines an additional representation of some of the special characters in the C language source abstract character set. These additional representations are sequences of three characters called trigraphs. Table A-1 lists the trigraphs and the character each is mapped to.
| Trigraph | Replacement |
|---|---|
| ??= | # |
| ??( | [ |
| ??/ | \ |
| ??) | ] |
| ??' | ^ |
| ??< | { |
| ??! | | |
| ??> | } |
| ??- | ~ |
Conceptually, every trigraph is removed from the file and its replacement is substituted. Each ? that does not begin one of the trigraphs is not changed. Trigraph processing occurs before tokenization takes place. Thus, even trigraphs in string constants have their replacements substituted.
Trigraph support has the potential to change the meaning of existing C code that unintentionally contains a trigraph in a string literal. However, since such conflicts will be quite rare, there is no facility for disabling trigraph support.
Trigraph support is available in strict and relaxed ANSI C mode.
A.1.6 Alert Escape Sequence
As specified by ANSI C, Compaq C defines a new escape sequence
for the alert character. The escape sequence
\a
represents the ASCII BEL (Ctrl/G) character.
A.1.7 Hexadecimal Escape Sequence
VAX C limits hexadecimal escape sequences to at most 3 hex digits, but ANSI C allows an unlimited number of digits. Compaq C removes the limit imposed by VAX C.
This can cause some programs to behave differently. The string
"\x0012"
is currently interpreted by VAX C as a string with two
characters in it: a Ctrl/A followed by the character "2".
Under ANSI C rules, the string consists of a single character whose
character code is hexadecimal 12 (Ctrl/R). However, this problem is
unlikely to occur in practice.
A.1.8 Invalid Escape Sequences
Compaq C issues a warning message if it encounters an invalid
escape sequence. VAX C did not diagnose such usage.
A.1.9 $ in Macro Names
The dollar sign ($) is not an element of the minimum basic character
set allowed by the ANSI C standard. By a systemwide convention, the
dollar sign identifies DIGITAL reserved identifiers. Compaq C for OpenVMS Systems
supplies header files containing many macros with dollar signs in their
names, and the VAX C compiler predefines some macros with
dollar signs in their names. In strict ANSI C mode, such macros trigger
a warning.
A.1.10 Null Arguments to Macros
In Compaq C, null arguments to a macro produce a BUGCHECK.
VAX C allowed macro arguments to be null.
A.1.11 ANSI C Name Space Conformance
ANSI C strictly controls the name space of C programs, and prohibits compilers or their standard-specified header files from intruding on the name space reserved for user programs. Specifically, the ANSI C Standard requires that compiler extensions begin with an underscore followed by an uppercase letter or another underscore.
This affects VAX C extensions involving additional keywords and predefined macros. It also affects the freedom of Compaq C to add additional macros, variables, and functions to the standard-specified header files, such as <stdio.h> .
The following sections describe how Compaq C solves the reserved
name space problem for extensions involving keywords, predefined
macros, and header file contents.
A.1.11.1 Non-ANSI Keywords
VAX C has several keywords that intrude into the user name space. The Compaq C compiler in strict ANSI C mode (/STANDARD=ANSI89) does not recognize keywords that are VAX C-specific extensions to the language. They are recognized instead as identifier names. As a result, programs that use these extensions as keywords cannot be compiled in strict ANSI C mode without eliciting syntax errors.
Similarly, the Compaq C compiler in VAX C mode and relaxed ANSI C mode does recognize keywords that are VAX C-specific extensions to the language. Therefore, programs that use these names as identifiers cannot be compiled in VAX C or relaxed ANSI C mode without eliciting syntax errors. In relaxed ANSI C mode, the compiler generates a warning for these keywords. When the /STANDARD=ANSI89 qualifier is used, the compiler strictly follows the ANSI C rules about the name space, and does not recognize the old spellings as keywords.
Table A-2 shows the traditional spelling and the new spelling of the keywords affected, as well as their corresponding ANSI-compliant pragmas.
| Keyword | Corresponding ANSI-Compliant Pragma |
|---|---|
| globaldef | #pragma extern_model |
| globalref | #pragma extern_model |
| globalvalue | #pragma extern_model |
| noshare | #pragma extern_model |
| readonly | #pragma extern_model |
Alternate spellings that follow ANSI C rules are added to Compaq C for all VAX C predefined macros. For compatibility, both the old spellings of the predefined macros and the new spellings are recognized by the compiler. However, when the /STANDARD=ANSI89 qualifier is used, the compiler strictly follows the ANSI C rules about the name space, and does not recognize the old spellings as predefined macros. You are encouraged to use the new ANSI C conformant spelling of the macros.
Table A-3 shows the traditional spelling and the new spelling of the predefined macros affected
| Traditional Spelling | New Spelling |
|---|---|
| vax | __ vax |
| vax11c | __ vax11c |
| vaxc | __ vaxc |
| VAX | __ VAX |
| VAX11C | __ VAX11C |
| VAXC | __ VAXC |
| vms | __ vms |
| VMS | __ VMS |
| vms_version | __ vms_version |
| VMS_VERSION | __ VMS_VERSION |
The ANSI C standard specifies exactly what identifiers in the normal name space are declared by the standard header files. A compiler is not free to declare additional identifiers in a header file unless the identifiers follow defined rules (the identifier must begin with an underscore followed by an uppercase letter or another underscore).
When running the Compaq C compiler on OpenVMS systems in strict ANSI C mode (/STANDARD=ANSI89), versions of the standard header files are included that hide many identifiers that do not follow the rules. The <stdio.h> header file, for example, hides the definition of the macro TRUE . The compiler accomplishes this by predefining the macro __ HIDE_FORBIDDEN_NAMES in strict ANSI C mode.
You can use the command-line qualifier /UNDEFINE="__HIDE_FORBIDDEN_NAMES" to prevent the compiler from predefining this macro, thus including macro definitions of the forbidden names.
The header files are modified to only define additional VAX C names if __ HIDE_FORBIDDEN_NAMES is undefined. For example, <stdio.h> might contain the following:
#ifndef __HIDE_FORBIDDEN_NAMES #define TRUE 1 #endif |
Compaq C for OpenVMS Systems supports the following new system-identification macros:
__ DECC
__ alpha
__ ALPHA
__ Alpha_AXP
__ 32BITS
__ mia
__ STDC __
The following sections describe changes to the data types supported by
Compaq C.
A.1.13.1 signed Reserved Word
Compaq C supports the new reserved word signed to complement unsigned . The signed keyword may be used with the char , short , int , and long keywords to specify the types signed char , signed short , signed int , and signed long . (These types are already supported by VAX C.) The signed keyword can also be used when declaring bit fields to specify explicitly that the bit field is signed.
ANSI C specifies that signed short , signed int , and signed long are the same types as short , int , and long , respectively. However, signed char is not the same type as char , even though Compaq C uses the same representation for both of them. This does not affect normal mixing of the two types, but it does mean that in Compaq C a pointer to signed char is not compatible with a pointer to char . Note that programs that previously used signed as an identifier will now be in error, even in VAX C mode. The /[NO]UNSIGNED_CHAR qualifier can be used to specify whether char is signed or unsigned.
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