Compilation modes

The compilation system has four compilation modes that correspond to degrees of compliance with ANSI C. The modes are:

-Xa

ANSI C mode. Specifies standards conformance except that some required warnings are omitted and the name space is expanded to include names that are not specified by the standards. All C constructions behave as specified in the standards. All implemented language and library extensions beyond the standards are also available. This is the default.

-Xb

Backward compatibility mode. Like -Xa, except that restrict and inline are not taken as C99 keywords.

-Xc

Conformance mode. Specifies strict standards conformance. Since the name space of the language and headers are reduced from that of -Xa, certain extensions, such as the asm keyword, and some commonly expected header file declarations are not available.

-Xt

Transition mode. Specifies standards conformance except where the semantics differ from classic C. Under this option, the compiler provides new ANSI C features and supports all extensions that were provided in C Issue 4 (CI4), which was the release made available
with System V Release 3. Where the interpretation of a construct differs between CI4 and the Standard, the compiler issues a warning and follows the CI4 behavior.