PRB: Undesired Side Effects from toascii, tolower, toupper

• The C Run-time (CRT) included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 4.1,

       4.2, 5.0
  

SYMPTOMS

In an application compiled with Microsoft C/C++, using the toascii, tolower, or toupper macros with a pointer value produces incorrect results, such as corrupted strings or a GP fault, depending on which compiler you use.

CAUSE

Because the tolower and toupper macros evaluate the input argument twice, the compiler performs any pointer arithmetic specified in a macro argument twice. This problem occurs most often with macros that affect an individual character because character pointers are common arguments for these macros.

RESOLUTION

The text below presents two methods to address the behavior the side effects cause.

1. Modify the code to remove the pointer arithmetic expression from the macro argument. In the code fragment below, because the argument to the toupper macro does not change, the side effect does not occur when the macro evaluates the argument a second time.

        while (*x != '\0')
           {
           *x = (char) toupper((int) *x);
           x++;
           }
   
   

2. The compiler provides more than one implementation for some functions. For example, to use a different implementation of the tolower or toupper function, perform the following three steps:

   a. Specify the /Za option (ANSI compatibility) on the compiler

         command line.
   

   b. Use the _tolower() or _toupper() functions in place of the

         tolower or toupper macros.
   

   c. After including the <ctype.h> file in your application source

         code, use the #undef tolower or #undef toupper statement to
         remove the macro definition from the code.
   

MORE INFORMATION

Sample Code

   /*
    * Compile options needed: none
    */

   #include <stdio.h>
   #include <ctype.h>

   char string[20]="this is a string";

   void main(void)
   {
      char *x;

      x = string;
      while (*x != '\0')
         *x = (char)toupper((int) *(x++));
      printf("%s\n", string);
   }

   thHssiI    ttiIgg

Keywords          : CRTIss
Version           : 6.0 6.0a 6.0ax 7.0 1.0 1.5 1.51 2.0 2.1 4.0 4.1 4.2 5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbprb