an open-source digital signal processing and sound synthesis language
about · links · contact


INSTRUMENT design -- reposition input pointer for arbitrary file-reading

The rtinrepos() function is used in RTcmix instrument design to set the input point for reading. This is done in preparation for a subsequent call to rtgetin. Designed for use by instruments that want to reposition the input file arbitrarily (like REVMIX, which reads a file backwards).

Typically it is used in the INSTRUMENT::run() member function of an instrument. It only works on file input, as arbitrary movement forwards and backwards in real-time isn't quite possible (yet).

It replaces the older inrepos function used in disk-only cmix.


    int rtinrepos(Instrument *inst, int nframes, int whence)

    *inst is a pointer to the INSTRUMENT object being scheduled. Usually this is the token this (i.e. a pointer to the INSTRUMENT calling the rtinrepos() function).
    nframes is the number of samples to move the input pointer forwards or backwards from a point set by whence. nframes can take a negative value.
    whence works the same way that the unix lseek() function does:

    • If whence is SEEK_SET, then the input read pointer will be set nframes from the beginning of the soundfile.
    • If whence is SEEK_CUR, the the input read pointer will be set nframes from the current read position (positive or negative values).

      [note: The values of SEEK_SET and SEEK_CUR are defined in the system header file "unistd.h". SEEK_END is not defined for use by rtinrepos().]

    The rtinrepos() function returns 0 if it is successful, it will exit if there is an error.


    #include <Instrument.h>
    #include <unistd.h>
    float in[];
    int MYINSTRUMENT::run()
    	int rsamps;
    	int somenumber;
    	if (in == NULL) // first time, we need to allocate the buffer memory
    		in = new float[RTBUFSAMPS*inputChannels()]
    	rsamps = framesToRun() * inputChannels();
    	rtinrepos(this, somenumber, SEEK_CUR);
    	rtgetin(in, this, rsamps);

See Also