Q28567: Reference Page 75 Example 9 Causes Error C2147

Article: Q28567
Product(s): See article
Version(s): 5.00 5.10 | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | docerr | mspl13_c
Last Modified: 12-OCT-1988

Using example nine on Page 75 of the "Microsoft C 5.1 Optimizing
Compiler Language Reference" manual causes the compiler error
"C2147: array: unknown size" on the line "(int *) p++".

The example is incorrect. Instead of using example nine, use one of
the two following examples:

1. ((int*) p)++;
2. p = (void *) ((char *) p + sizeof(int));

Note that the expression "((int *) p)++" is a non-standard extension
to the language, and the compiler will issue a warning.

See Pages 99-100 of the user's guide for a discussion of enabling
and disabling language extensions with the /Ze and /Za switches.
There is an example at the bottom of Page 99, which is almost
identical.

Example nine on Page 75 is as follows:

        int i;
        void *p;

        p = &i;
        (int *) p++;

The expression "(int *) p++" generates the C2147 error because the
"++" operation is preformed before the "(int *)" cast. Thus, the
expression is trying to increment a pointer that points to a void.
Because void has no size, the compiler does not know how many bytes
"p" should be incremented by.

By using the extra parentheses in the expression "((int *) p)++", you
are forcing the "(int *)" to convert "p" from a pointer-to-void to a
pointer-to-int, which can be incremented.

The second workaround, "p = (void *) ((char *) p + sizeof(int))",
works correctly because you are telling the compiler how many bytes to
increment the pointer.

The second workaround is standard ANSI C and should compile on any
standard ANSI C compiler.

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.