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How to code a circular buffer in an external.

May 23 2006 | 5:17 pm

Hey everybody, thanks for the help with my float inlet yesterday!
I am now trying to add a circular buffer to my external. The buffer will need to hold something in the order of 2048 samples.
Now Im thinking I need to create this buffer outside the perform routine as this routine only works in chunks of samples as per the Signal Vector size, right? So where do I create the array to store these samples, in the structure defn or in the new instance routine, or where?
As far as I understand it the array/buffer will need to outside the perform routine so it can store samples from previous Signal Vectors. So i guess this needs to be a global variable that can be accessed from the perform routine, but i guess Im not sure where is the best place to do this code in an MSP external.
Your help is greatly appreciated,

May 23 2006 | 5:51 pm

I think you should allocate the mem in your dsp function.

Put a float* into your main object, like so:

#define kNumSamplesInCircularBuffer 2048;

typedef struct _MyObjName {
t_pxobject xObj; // requred for msp
int curReadIndex;
int curWriteIndex;

float* circularBuffer;
} MyObjName;

and then malloc a buffer and give it to your object like so:

void MyObjName_dsp(MyObjName* inObj, t_signal **sp, short* count) {

//allocate the buffer
circularBuffer = (float*)malloc(sizeof(float) *

//initialize the read and write index
curReadIndex = 0;
curWriteIndex = 0;

//initialize the buffer with zeros
bzero(circularBuffer, sizeof(float) * kNumSamplesInCircularBuffer);

and then free the buffer when you’re done.

void MyObjName_free(MyObjName* inObj) {
dsp_free((t_pxobject*) xObj);


//for paranoia’s sake I always zero out my structs after I free any
malloc’d members
bzero(inObj, sizeof(MyObjName));

then inside of your process function, just make sure you don’t access
outside of the bounds of your buffer.


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