ios - Buffer size of CMSampleBufferRef -

i trying size of cmsamplebufferref avfoundation call-back

- (void)captureoutput:(avcaptureoutput *)captureoutput didoutputsamplebuffer:(cmsamplebufferref)samplebuffer fromconnection:(avcaptureconnection *)connection 

according documentation https://developer.apple.com/library/mac/documentation/coremedia/reference/cmsamplebuffer/index.html#//apple_ref/c/func/cmsamplebuffergetsamplesize

size_t cmsamplebuffergettotalsamplesize ( cmsamplebufferref sbuf ); 

if understand correctly, should use method buffer size. got 0 return. , said "if there no sample sizes in cmsamplebuffer, size of 0 returned." in case, wonder if avfoundation framework not provide buffer size information or mis-understand document.

a follow question: way, wonder in case if use

cvpixelbufferref pixelbuffer = cmsamplebuffergetimagebuffer(samplebuffer);

from pixelbuffer size of samplebuffer?

how create avcapturesession?

basically cmsamplebuffer can contain multiple information.

• raw image buffer (which can extract using cmsamplebuffergetimagebuffer)
• audio buffer (which have extract using cmsamplebuffergetblockbuffer)
• corevideo encoded image data buffer (also have extract using cmsamplebuffergetblockbuffer)

so should first check if captureoutput wanted, , if , expect image, should image cvpixelbufferref check data size on image using cvpixelbuffergetdatasize