PRB: Exception Not Caught Using CArchive or CFile

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, 2.0, 2.1, 2.2, 4.0, 4.1, 4.2

SYMPTOMS

When using CArchive or CFile, Visual C++ exceptions may not be caught if the CArchive and/or CFile are in the same try block as a read or write:

   void Test1()
   {
        try {
           //Drive A: should contain a floppy with zero bytes free
           CFile file("A:\\BadFile.tmp", CFile::modeCreate
                      | CFile::shareExclusive | CFile::modeWrite );

           try {
               CArchive archive(&file, CArchive::store);
               //Fill up disk, and cause a
               //CFileException::diskFull exception
               while(1)
                   archive << (DWORD)17;
           }
           catch ( CFileException* ex ) {
               ex->Delete();
           }
           catch (...) {
               ASSERT(FALSE);
           }
       }
       catch ( CFileException* ex ) {
           ex->Delete();
       }
       catch (...){
           ASSERT(FALSE);
       }
   }

CAUSE

In the process of handling exceptions, objects inside the try block are destructed before the first catch block. During unwinding, when a second exception is thrown by a destructor before the first exception is caught, terminate() is called.

Both the destructors for CArchive and CFile attempt to flush the buffer and close the open file. This can cause a second exception to get thrown and terminate() to get called.

RESOLUTION

Make sure that the CArchive or CFile object is not inside the same try block as any read or write on that object.

The CArchive object should also not be inside of the same try block as the CFile object it uses unless the CArchive was created in the CArchive::bNoFlushOnDelete mode. If this mode is used, the CArchive and CFile can safely be in the same try block. Care must be taken to call CArchive::Flush() before the CArchive is deleted, and in a separate try block. The second example below shows using a CArchive in this mode.

STATUS

This behavior is by design.

MORE INFORMATION

This is a behavior of Visual C++ exception handling and did not occur with earlier versions of MFC, which did not use C++ exception handling.

Sample Code

Example 1

   // This example shows placing CFile, CArchive, and writes in separate
   // try blocks

   /* Compile options needed: default AppWizard application
   */

   void Test2()
   {
       try {
           //Drive A: should contain a floppy with zero bytes free
           CFile file("A:\\BadFile.tmp", CFile::modeCreate
                      | CFile::shareExclusive | CFile::modeWrite );
           try {
               CArchive archive(&file, CArchive::store);
               try {

                   //Fill up disk, and cause a
                   // CFileException::diskFull exception
                   while(1)
                       archive << (DWORD)17;

               }
               catch ( CFileException* ex ) {
                   archive.Abort();
                   ex->Delete();
               }
               catch (...){
                   ASSERT(FALSE);
               }
           }
           catch ( CFileException* ex ) {
               file.Abort();
               ex->Delete();
           }
           catch (...){
                ASSERT(FALSE);
           }
       }
       catch ( CFileException* ex ) {
           ex->Delete();
       }
       catch (...){
           ASSERT(FALSE);

       }
   }

Example 2

   // This example uses CArchive::bNoFlushOnDelete so that CFile and
   // CArchive can be placed in the same try block.

   /* Compile options needed: default AppWizard application
   */

   void Test3()
   {
        try {
           //Drive A: should contain a floppy with zero bytes free
           CFile file("A:\\BadFile.tmp",
               CFile::modeCreate | CFile::shareExclusive
                           | CFile::modeWrite );
           CArchive archive(&file,
               CArchive::store | CArchive::bNoFlushOnDelete);
           try {

               //Fill up disk, and cause a CFileException::diskFull
               //exception
               while(1)
                   archive << (DWORD)17;

               //Manually flush archive in try-block
               archive.Flush();
           }
           catch ( CFileException* ex ) {
               archive.Abort();
               ex->Delete();
           }
           catch (...){
               ASSERT(FALSE);
           }
       }
       catch ( CFileException* ex ) {
           ex->Delete();
       }
       catch (...){
           ASSERT(FALSE);
       }
   }