Technical Q&A QA1676

AudioFileOpenWithCallbacks - Avoiding Permissions Error With MPEG-4 File Types

Q: We're using `AudioFileOpenWithCallbacks` for playback of audio data stored in memory and provide the appropriate procedures for reading, writing and so on. However, we encounter a `kAudioFilePermissionsError` when working with MPEG-4 file containers. What's wrong?

A: We're using AudioFileOpenWithCallbacks for playback of audio data stored in memory and provide the appropriate procedures for reading, writing and so on. However, we encounter a kAudioFilePermissionsError when working with MPEG-4 file containers. What's wrong?

AudioFileOpenWithCallbacks will open an audio file (represented by the AudioFileID opaque data type) using callbacks you provide for reading data, writing data and getting/setting size information. These callbacks may be used to provide audio file data from a source other than a physical file (for example memory) and lets you take advantage of the standard AudioFile APIs such as AudioFileReadPackets to work with this audio data.

Listing 1 AudioFileOpenWithCallbacks

OSStatus AudioFileOpenWithCallbacks(void *inClientData,
                                    AudioFile_ReadProc inReadFunc,
                                    AudioFile_WriteProc inWriteFunc,
                                    AudioFile_GetSizeProc inGetSizeFunc,
                                    AudioFile_SetSizeProc inSetSizeFunc,
                                    AudioFileTypeID inFileTypeHint,
                                    AudioFileID *outAudioFile);
Parameters:

inClientData - A pointer to a constant passed to your callbacks. The constant should contain any information you use to manage the state for reading data from the file.

inReadFunc - A callback function invoked when the audio file object wants to read data.

inWriteFunc - A callback function called when the audio file object wants to write data.

inGetSizeFunc - A callback function called when the audio file object wants to know the file size.

inSetSizeFunc - A callback function called when the audio file object wants to set the file size.

inFileTypeHint - A hint about the type of the designated file. For example, kAudioFileAAC_ADTSType, kAudioFileMP3Type and so on.

outAudioFile - On output, a pointer to the newly opened file ID.

Fixing the Permissions Error

There is however a limitation when working with MPEG-4 file types which include; .mp4 (kAudioFileMPEG4Type), .m4a (kAudioFileM4AType), .3g2 (kAudioFile3GP2Type) and .3gp (kAudioFile3GPType).

AudioFile does not currently support writing to existing files of these types.

Therefore, if you provide the inWriteFunc procedure it is an indication to AudioFileOpenWithCallbacks that the file is being opened for writing, which in turn results in the kAudioFilePermissionsError being returned.

Since you can't write to these files, the AudioFile_WriteProc is not required and should not be used. Setting the inWriteFunc to NULL solves the problem.

Listing 2 AudioFileOpenWithCallbacks with a NULL inWriteFunc and inSetSizeFunc

static OSStatus MyAudioFile_ReadProc(void *inClientData, SInt64   inPosition, UInt32 requestCount,
                                     void *buffer, UInt32 *actualCount)
{
    // this is my read proc.
    ...
}

{
    ...

    AudioFileID myAudioFileID = 0;
    OSStatus result;

    result = AudioFileOpenWithCallbacks(self, MyAudioFile_ReadProc, NULL, MyAudioFile_GetSizeProc, NULL,
                                        kAudioFileMPEG4Type, &myAudioFileID);

    ...
}

Document Revision History

Date	Notes
2009-10-27	New document that describes how to avoid a kAudioFilePermissionsError error with MPEG-4 files when using AudioFileOpenWithCallbacks.

AudioFileOpenWithCallbacks - Avoiding Permissions Error With MPEG-4 File Types

Q: We're using AudioFileOpenWithCallbacks for playback of audio data stored in memory and provide the appropriate procedures for reading, writing and so on. However, we encounter a kAudioFilePermissionsError when working with MPEG-4 file containers. What's wrong?

Fixing the Permissions Error

Document Revision History

Q: We're using `AudioFileOpenWithCallbacks` for playback of audio data stored in memory and provide the appropriate procedures for reading, writing and so on. However, we encounter a `kAudioFilePermissionsError` when working with MPEG-4 file containers. What's wrong?