PM123 Decoder Plug-in Developer's Guide

Decoder plug-ins

Decoder plug-ins must implement and export the functions defined in decoder_plug.h.

int _System decoder_init(void **w) BOOL _System decoder_uninit(void *w)

w : Allocate any chunk of memory necessary for the decoder's function. This pointer will be passed to the other functions.
return value : -1 means the decoder was not initialized successfully.

Init function is called when pm123 needs the specified decoder to play the stream demanded by the user. So only one decoder plug-in is active at any given time. It should initialize the necessary semaphores and threads. decoder_uninit is called when another decoder than yours is needed, and should destroy the decoder's thread, semaphores, other opened handles and free allocated memory for w.

ULONG _System decoder_command(void *w, ULONG msg, DECODER_PARAMS *params)

msg: one of the following:
DECODER_PLAY
DECODER_STOP
DECODER_FFWD
DECODER_REW
DECODER_JUMPTO
DECODER_SETUP
DECODER_EQ
DECODER_BUFFER
params: structure that contains the parameters needed by the preceding commands
return value:
0 -> ok
1 -> command unsupported
1xx -> msg specific

There is a lot of commands to implement for this function. Parameters needed for each of the are described in the definition of the structure in the .h file.

DECODER_PLAY start playing the given filename, URL or others.
DECODER_STOP stop playing.
DECODER_FFWD engage in ffwd mode (ie.: play faster or skip when playing)
DECODER_REW engage rewind mode
DECODER_JUMPTO jump and start decoding at the given time position
DECODER_SETUP setup various parameters such as the output plugin functions, hwnd where to send messages and an optional play semaphore.
DECODER_EQ usually only useful for MPEG decoding where frequency equalization is cheap.
DECODER_BUFFER reports the reader buffer status.

The decoder MUST WinPostMsg() the following messages to hwnd:

WM_PLAYSTOP when the stream has finished decoding.
WM_PLAYERROR when a playback error occured so that pm123 should know to stop immediately.
WM_SEEKSTOP when a JUMPTO operation is completed (ie.: when no buffers from before the seek is sent to the output plugin anymore).

ULONG _System decoder_status(void *w)

return value - One of the following:
DECODER_STOPPED
DECODER_PLAYING
DECODER_STARTING

ULONG _System decoder_length(void *w)

return value - number of milliseconds the stream lasts (you can report -1 if unknown)

The call to this function must be valid even if DECODER_STARTING or DECODER_STOPPED is reported (when the stream plays too fast for example).

ULONG _System decoder_fileinfo(char *filename, DECODER_INFO *info) ULONG _System decoder_trackinfo(char *drive, int track, DECODER_INFO *info)

filename : full path or URL to the desired stream.
drive : ie.: "X:"
track : CD tracks start from 1.
info : this structure must be filled with the required information.
return value:
0 = everything's perfect, structure is set
100 = error reading file (too small?)
200 = decoder can't play that
1001 = http error occured
other values = errno

decoder_fileinfo and decoder_trackinfo (for CD decoders) must be independant from the current status of the decoder. It should always be functionnaly and give consistent results in any conditions.

ULONG _System decoder_cdinfo(char *drive, DECODER_CDINFO *info)

drive: ...
info: required info on the number of tracks available from the decoder.

This is used by pm123 before calling decoder_trackinfo.

ULONG _System decoder_support(char *fileext[], int *size)

fileext: fill this array with all the wildcard expression matching filenames this decoder can play (each element is a 8 bytes char array).
size I/O: number of wildcard expressions returned (if this value is not big enough, ie.: 0, return the appropriate size without filling fileext)
return value: return what kind of stream can the decoder play:
DECODER_FILENAME
DECODER_URL
DECODER_TRACK
DECODER_OTHER

This is used by pm123 to suggest to the user what he can play with the decoder. Extentions can be for example "*.MOD".