Technical Q&A QTMTB37

QuickTime CFM Error -2804

What's causing this error, and what can I do to eliminate it?

The following is a note relating to weak linking and QuickTime. It includes code that shows how to query for the availability of the library from within the application itself.

QuickTime CFM PowerPlug Libraries, Availability, Weak Links

The Code Fragment Manager supports the concept of "soft" or "weak" linking. If a library is soft-linked, the Process Manager runs your application, even if the library is missing. As a result, the application continues to function when a particular library it calls is not installed. The application can disable areas of functionality based on the current availability of libraries.

MPW PowerPC tools includes MakePEF, a tool with an additional flag that specifies when exported symbols are weak. In this case, the runtime architecture tries to resolve the CFM library, but it doesn't fail if it can't. You can specify that the library has weak linking by adding a tilde (~) character at the end of the - l option to MakePEF. For example, to soft-link to QuickTimeLib, do this:

    MakePEF -
l QuickTimeLib.xcoff=QuickTimeLib~ ...

You can also mark CFM libraries that have weak linking in the Metrowerks PowerPC environment.

Your client program can test for the presence of the CFM library and disable all functionality that depends on it. (for example, if no QuickTime CFM libraries are present, you can disable "Play Movies").

This technique has one drawback -- it does not prevent a problem when a user moves the library after it registers with Gestalt, since the library registers only once, and there is no way to de-register it. There is no direct solution to this problem, but there is an alternative way to determine if the library is loaded -- check the address of one of the library's functions before calling it. In Inside Macintosh, this technique is illustrated in the following example:

extern int printf (char *, ...);
// ...
if (printf == kUnresolvedSymbolAddress)
        DebugStr("\printf is not available.");
else
    printf("Hello, world!\n");

QuickTime also has a new Gestalt selector to determine whether
it's safe to call a weak- linked library (gestaltQuickTimeFeatures).
This function shows how to initialize QuickTime
for 68K and PowerPC Macs:

Boolean    InitQuickTime(void)
{
    long         qtVersion;
    OSErr         anErr;
#ifdef powerc
    long         qtFeatures;
#endif

    anErr = Gestalt(gestaltQuickTime, &qtVersion);
    if (anErr != noErr)
        return false;        // no QT present

#ifdef powerc

// Test if the library is registered.
    anErr = Gestalt(gestaltQuickTimeFeatures, &qtFeatures);
    if ( !( (anErr == noErr)  &&  (qtFeatures &
    (1 << gestaltPPCQuickTimeLibPresent))       )) // not true
          return false;
#endif

    anErr = EnterMovies();
    if ( anErr == noErr)
        return true;
    else
        return false;        // problems initializing QuickTime
}

Both QuickTime Gestalt selectors are tested for in the PowerPC case. You need to test for the presence of both QuickTime and the QuickTime library. If the application is unable to load the CFM library due to lack of space, the application might mysteriously die later, so it's important to always check that the libraries are loaded and available.

[May 01 1995]