Technical Q&A QA1411

Sequence Grabber preallocates large file when recording

Q: I'm using the sequence grabber to record video to a movie. I call SGSetDataOutput with the seqGrabToDisk flag to specify the recorded video be written to a movie file on disk. However, when I begin recording by calling either SGPrepare or SGStartRecord QuickTime will preallocate a large file that fills up much of my disk. How can I prevent this?

A: QuickTime 7 for Mac OS X version 10.4 introduced the seqGrabDontPreAllocateFileSize flag which you can use to prevent the sequence grabber from preallocating a large file when recording to a movie file on disk.

The following Objective-C code listing demonstrates how to specify the destination data reference for a record operation using the seqGrabDontPreAllocateFileSize flag, taking into account the state of the seqGrabAppendToFile flag as well:

Listing 1: Configuring the destination data reference for a sequence grabber record operation.

- (OSStatus)setCapturePath:(NSString*)filePath flags:(long)flags
{
    OSStatus err = noErr;
    Handle dataRef = NULL;
    OSType dataRefType = 0;

    if (filePath)
    {
        require(QTNewDataReferenceFromFullPathCFString(
                        (CFStringRef)filePath,
                        (UInt32)kQTNativeDefaultPathStyle,
                        0, &dataRef, &dataRefType) == noErr, bail);
    }

    if ( !(flags & seqGrabDontMakeMovie) )
    {
        if ( (flags & seqGrabDontPreAllocateFileSize) &&
            !(flags & seqGrabAppendToFile) )
        {
            // Sequence Grabber will still preallocate the file on disk unless
            // seqGrabAppendToFile is also set.  So if you wish to keep
            // Sequence Grabber from preallocating a large file on disk, but
            // you don't want to append to an existing file, you need to open
            // up the desired file and truncate it first, then set the
            // seqGrabAppendToFile flag.

            // truncate the file on disk first
            fclose(fopen([filePath cString], "w"));
            flags |= seqGrabAppendToFile;
        }
    }

    err = SGSetDataRef(mSeqGrab, dataRef, dataRefType, flags);

bail:
    DisposeHandle(dataRef);
    return err;
}

For applications which must run pre-QuickTime 7, you can specify a maximum offset for data written to a given sequence grabber output using the SGSetOutputMaximumOffset function. If an attempt is made to write data beyond the maximum offset, the sequence grabber switches to the next output as designated by SGSetOutputNextOutput. If no more outputs are available, an end-of-file error is returned and recording ends.

To monitor the space used by a sequence grabber output while recording, use SGGetStorageSpaceRemaining or SGGetStorageSpaceRemaining64. This will return a value indicating the amount of space remaining for the current record operation. If you are recording to memory, this value contains information about the amount of memory remaining. If you are recording to a movie file, this value contains information about the amount of storage space available on the device that holds the file.

If you are using multiple sequence grabber outputs, use SGGetDataOutputStorageSpaceRemaining or SGGetDataOutputStorageSpaceRemaining64 to monitor the available space while recording.

References

• SGSetDataOutput

• SGStartRecord

• SGSetOutputMaximumOffset

• SGSetOutputNextOutput

• SGGetStorageSpaceRemaining

• SGGetStorageSpaceRemaining64

• SGGetDataOutputStorageSpaceRemaining

• SGGetDataOutputStorageSpaceRemaining64

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Posted: 2006-02-06