PRB: C2118: Negative Subscript May Be Caused by int Expression

• The Microsoft C/C++ Compiler (CL.EXE), included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 6.0, 6.0a - 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, 5.0

SYMPTOMS

The compiler error:

   C2118: negative subscript

      -or-

   C2118: negative subscript or subscript is too large

CAUSE

This is correct and expected behavior for the Microsoft C Compiler because the evaluation of integer expressions is done using integer math. In some cases, integer math produces an incorrect result because the value of the expression is larger than an integer.

RESOLUTION

16-bit: To work around this situation, add an uppercase (or lowercase) "L" to one of the terms of the expression. This will force the compiler to evaluate the expression using long math which is less likely to overflow.

32-bit (or 16-bit expressions greater than LONG_MAX): Unfortunately, the above resolution will not work as INT_MAX equals LONG_MAX. In these cases, we recommend that the array be broken into several smaller arrays, each of whose size will not exceed INT_MAX.

MORE INFORMATION

If the sample code below is compiled with C/C++ version 7.0 or Visual C++ for Windows, the following error message will be generated:

   error C2118: negative subscript

   warning C4307: '*' : integral constant overflow; result truncated

   char mystring[1000L * 33];

Sample Code

   /* Compile options needed: <none>; for warning, /W2 or greater
   */

   char mystring[1000 * 33];