PRB: Misuse of ASSERT Causes Problems for MFC Apps (115086)


The information in this article applies to:

  • The Microsoft Foundation Classes (MFC), when used with:
    • Microsoft C/C++ for MS-DOS 7.0
    • Microsoft Visual C++ for Windows, 16-bit edition 1.0
    • Microsoft Visual C++ for Windows, 16-bit edition 1.5
    • Microsoft Visual C++ for Windows, 16-bit edition 1.51
    • Microsoft Visual C++ for Windows, 16-bit edition 1.52
    • Microsoft Visual C++, 32-bit Editions 1.0
    • Microsoft Visual C++, 32-bit Editions 2.0
    • Microsoft Visual C++, 32-bit Editions 2.1
    • Microsoft Visual C++, 32-bit Editions 4.0
This article was previously published under Q115086
The information in this article is included in the documentation starting with Visual C++ 5.0. Look there for future revisions.

SYMPTOMS

When you run the debug version of your MFC application, there are no problems. However, the release version of the same application crashes, returns incorrect results, and/or exhibits some other abnormal behavior.

CAUSE

This problem can be caused when you place important code in an ASSERT statement to verify that it performs correctly. Because ASSERT statements are commented out in a release build of an MFC program, the code does not run in a release build.

RESOLUTION

If you are using ASSERT to confirm that a function call succeeded, consider using VERIFY instead. The VERIFY macro evaluates its own arguments in both debug and release builds of the application.

Another preferred technique is to assign the function's return value to a temporary variable and then test the variable in an ASSERT statement.

Both of these techniques are demonstrated below in the "MORE INFORMATION" section of this article.

MORE INFORMATION

Examine the following code fragment: 
      char *buf;
      ASSERT(buf = (char *) calloc( 20, sizeof(char) ));
      strcpy( buf, "Hello, World" );
      free( buf );
This code runs perfectly in a debug version of an MFC application. If the call to calloc() fails, a diagnostic message is displayed, which includes the file and line number. However, in a retail build of an MFC application:

  • The call to "calloc()" never occurs, leaving "buf" uninitialized. -or-

  • "strcpy()" copies "Hello, World" into a random piece of memory, possibly crashing the application or hanging the system. -or-

  • "free()" attempts to free memory that was never allocated.
To use ASSERT correctly, the code sample should be changed to the following: 
      char *buf;
      buf = (char *) calloc( 20, sizeof(char) );
      ASSERT( buf != NULL );
      strcpy( buf, "Hello, World" );
      free( buf );
Or, you can use VERIFY instead: 
      char *buf;
      VERIFY(buf = (char *) calloc(20, sizeof(char) ));
      strcpy( buf, "Hello, World" );
      free( buf );
Modification Type:MajorLast Reviewed:12/2/2003
Keywords:kbprb KB115086
©2003 Microsoft Corporation. All rights reserved.