INFO: wsprintf() %s Parameters Not Cast to LPSTR

ID: Q64759

3.00 3.10 WINDOWS kbprg

The information in this article applies to:

Microsoft Windows Software Development Kit (SDK) 3.1

SUMMARY

Unrecoverable application errors (UAEs) can result from improperly using the wsprintf() function. Any parameter passed to wsprintf() that corresponds to an %s format string MUST be cast to a LPSTR.

The documentation for wsprintf() in the "Microsoft Windows Software Development Kit Reference Volume 1" states:


   Sequence        Meaning
   ----------------------------------------------------------------
   s               Insert a string argument referenced by a long
                   pointer. The argument corresponding to this
                   sequence MUST be passed as a long pointer (LPSTR).

Wsprintf() is a function with a variable number of parameters. Therefore, it must be prototyped using the following C calling convention for a variable number of arguments:


   int FAR cdecl wsprintf(LPSTR, LPSTR,...);

Because the only type information in the prototype describes the output buffer and the format string, the C compiler cannot perform implicit casts on the other parameters at compile time. Normally, when a near pointer (char *) is used as an argument to a function requiring a LPSTR, the compiler will implicitly cast the (char *) to LPSTR, or (char far *).

Because the compiler cannot cast any of the additional parameters, in small and medium model programs, any string pointer that is not explicitly cast FAR will be passed to wsprintf() as a near pointer. wsprintf() attempts to retrieve a far pointer from the stack, which results in an invalid pointer and an unrecoverable application error.

The following two code fragments show incorrect and correct usage of %s fields within wsprintf():


  //INCORRECT use of a near pointer. Assume small or medium model.

   {
   char        sz[30];      //sz is a NEAR pointer.
   char        szOut[50];   //szOut is also NEAR
   LPSTR       szFar = sz;  //szFar is FAR
       .
       .
       .
   /*
    * Because it is the output buffer, szOut is implicitly cast to a
    * LPSTR. However, sz is pushed on the stack as a NEAR pointer,
    * which wsprintf will pop as a FAR pointer.
    * This call will cause a UAE.
    */ 
    wsprintf(szOut, "sz=%s", sz);

    /*
     * This call will succeed since szFar is already a LPSTR.
     */ 
    wsprintf(szOut, "sz=%s", szFar);
        .
        .
        .
    }


   //CORRECT--

    {
    char        sz[30];      //sz is a NEAR pointer.
    char        szOut[50];   //szOut is also NEAR
    LPSTR       szFar = sz;  //szFar is FAR
        .
        .
        .
    /*
     * Because it is the output buffer, szOut is implicitly cast to a
     * LPSTR. Due to the explicit cast, sz is pushed on the stack
     * as a FAR pointer.  This call will succeed.
     */ 
    wsprintf(szOut, "sz=%s", (LPSTR)sz);

    /*
     * This call will succeed since szFar is already an LPSTR.
     * The cast is redundant, but it's free insurance.
     */ 
    wsprintf(szOut, "sz=%s", (LPSTR)szFar);
        .
        .
        .
    }

Additional query words: 3.00 3.10

Keywords : kb16bitonly kbSDKWin16
Version : WINDOWS:3.1
Platform : WINDOWS
Issue type : kbinfo