Microsoft KB Archive/117203

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FIX: Compiler Generates Bad Char Pointer Code with /Ot

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Q117203

6.00 6.00a 6.00ax 7.00 | 6.00 6.00a | 1.00 MS-DOS | OS/2 | WINDOWS kbtool kbfixlist kbbuglist -- The information in this article applies to: - The Microsoft C/C++ Compiler (CL.EXE), included with: - Microsoft C for MS-DOS, versions 6.0, 6.0a, and 6.0ax - Microsoft C for OS/2, versions 6.0 and 6.0a - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, version 1.0 -- SYMPTOMS ======== The compiler may treat an unsigned character pointer as an integer pointer when compiling using the /Ot option. RESOLUTION ========== There are several ways to work around this problem: - In the sample code in the "MORE INFORMATION" section, below, remove the register modifier on the integer variable or remove the unsigned modifier on the character-string variable. -or- - Disable optimization by either compiling using the /Od option or by using #pragma optimize to disable optimization on a per-function basis. -or- - If you are using the C/C++ Compiler, version 7.0 or later, using the /f option (fast compile) causes the compiler to generate correct code. STATUS ====== Microsoft has confirmed this to be a problem with the C Compiler for MS- DOS, versions 6.0, 6.0a, 6.0ax, 7.0, and 8.0. This problem was corrected in the C/C++ compiler for MS-DOS, version 8.0c, included with Visual C++ for MS-DOS, version 1.5. This problem does not occur with the 32-bit C/C++ Compiler, version 8.0, included with Visual C++ 32-bit Edition, version 1.0. MORE INFORMATION ================ Using the /Fc option, you can generate an assembly listing of the source code. This is an excerpt from the assembly listing of the sample code below, which shows where the error occurs: ;|*** while(ch==*str); ; Line 8 $FC180: *** 00000f 8b 5e fe mov bx,WORD PTR [bp-2] ;str *** 000012 8b 37 mov si,WORD PTR [bx] *** 000014 81 e6 ff 00 and si,255 ;00ffH *** 000018 75 f5 jne $FC180 Notice the code uses WORD PTR, when it should be using BYTE PTR. The following is the code generated when one of the workarounds is used: ; while(ch==*str); ; Line 8 L00180: *** 000010 8b 5e fc mov bx,WORD PTR -4[bp] *** 000013 8a 07 mov al,BYTE PTR [bx] *** 000015 25 ff 00 and ax,OFFSET 255 *** 000018 8b f0 mov si,ax *** 00001a 8b c6 mov ax,si *** 00001c 3d 00 00 cmp ax,OFFSET 0 *** 00001f 75 03 e9 00 00 je L00181 *** 000024 e9 e9 ff jmp L00180 Sample Code --- /* Compile options needed: /Ot (/Fc to generate assembly listing) */ #include void main(void) { register int ch; unsigned char *str="abc"; while(ch==*str); } Additional reference words: 6.00 6.00a 6.00ax 7.00 8.00 1.00 KBCategory: kbtool kbfixlist kbbuglist KBSubcategory: CodeGen

Keywords : kb16bitonly kbCodeGen

Issue type :

Technology : kbVCsearch kbAudDeveloper kbCVCComp