Bool wxWriteMediaGlobalHeader (wxMediaStreamOut& out)
Writes data to out to initialize for writing buffers to the stream. The return value is TRUE if the write succeeds, or FALSE otherwise. One or more buffers can be written to the stream by calling the buffer's WriteToFile method. When all buffers are written, call wxWriteMediaGlobalFooter. Calls to wxWriteMediaGlobalHeader and wxWriteMediaGlobalFooter must bracket any call to WriteToFile, and only one stream at a time can be written using these methods or read using wxReadMediaGlobalHeader and wxReadMediaGlobalFooter.
See also section 
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