4.2 ANSI-Compliant Method of Controlling External Objects

Sections 4.3 and 4.4 describe the following external linkage storage-class specifiers and modifiers that are specific to DEC C for OpenVMS Systems:

These keywords are supported by the DEC C compiler for compatibility purposes, and are available only in VAX C mode (/STANDARD=VAXC) and relaxed ANSI mode (/STANDARD=RELAXED_ANSI89)

However, the DEC C compiler also provides an alternative, ANSI- compliant method of controlling objects that have external linkage. To take advantage of this method, use the #pragma extern_model preprocessor directive and the /EXTERN_MODEL and /[NO]SHARE_GLOBALS command-line qualifiers.

The pragma and command-line qualifiers replace the VAX C mode storage-class specifiers (globaldef, globalref, globalvalue) and storage-class modifiers (noshare and readonly). They allow you to select the implementation model of external data and control the psect usage of your programs. The _align macro is still used to ensure object alignment.

The pragma and command-line qualifier approach also has these advantages:

• Since the VAX C mode keywords do not follow ANSI C spelling rules, they cannot be provided in strict ANSI C mode. The pragma and qualifiers, however, can be used in any mode of the DEC C compiler.

• The pragma and qualifiers allow extern on OpenVMS systems to function in a manner more similar to other systems.

• The pragma and qualifiers make it easier for you to write OpenVMS shareable images with DEC C. Previously, that task required you to add an additional keyword to every declaration of external data.

For a description of the #pragma extern_model preprocessor directive and its relationship to the external storage classes it replaces, see Section 5.4.4.

For a description of the _align storage-class modifier, see Section 4.4.3.

For a description of the /EXTERN_MODEL and /[NO]SHARE_GLOBALS command-line qualifiers, see Section 1.3.4.