24-bit Integer in GCC?

[P . Douglas Reeder]

Is there a way to declare a field of a struct as a 24-bit integer in 
GCC?  Barring that, do I have any guarantees about the order and 
packing of bitfields, as in the following:

typedef struct {
        .
        .
        .
        UInt32 streamType : 6;
        UInt32 upStream : 1;
        UInt32 reserved : 1;   // always 1
        UInt32 bufferSize : 24;
} EsdsFixed;

I need to match a binary structure (the DecoderConfigDescriptor in the 
Elementary Stream descriptor of a .mp4 file, as you might have 
guessed).  FWIW, integers in an .mp4 file are big-endian, because it 
derives from Apple's Quicktime, written originally for Motorola 
processors.
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