Microsoft C conforms to the standard for the C language as set forth in the 9899:1990 edition of the ANSI C standard.

Microsoft extensions to the ANSI C standard are noted in the text and syntax of this book as well as in the online reference. Because the extensions are not a part of the ANSI C standard, their use may restrict portability of programs between systems. By default, the Microsoft extensions are enabled. To disable the extensions, specify the /Za compiler option. With /Za, all non-ANSI code generates errors or warnings.

Organization of the C Language Reference