How to apply a separate file function to a blockproc?

hello , i'm doing my senior design project in image processing ,, but i have an issue that I want to add up pixels in each block after using blockproc function, I created a separate function file and I want to use sum function but I don't know what to write in the argument! is it block_struct? and if there is any better way to add up pixels values to find where is the maximum intensity please mention it. Thanks in advanced