Sending a CSV File over ble Gatt connection

Question

Hi, 
 I need to send data from a CSV file on the nordic board(Via fatfs library). I have presently gotten a Gatt service up and running. 
 I have a mobile application which, can see the gatt service and write to the characteristics. From a specific write command from the mobile device I want the nordic board to send the CSV file over. 
 I can presently read a single line of the CSV file formatted like this: (Time stamp, data1, data 2, data3, data 4, data 5). 
 What I need to implement is: 
 
 Reading the whole file but, one or multiple lines at a time due to the size of the CSV 
 Changing the read pointer of the SD card (So I know where to continue to read from) 
 Send this data to the characteristic 
 Read this characteristic data. 
 
 My code is based upon : https://github.com/bjornspockeli/custom_ble_service_example 
 My CSV file size depends on the length of time I have been storing data from. It could be upto around 1-2mbytes. I beleive I need to set up some kind of notification service. Where I read a line(or several) of the SD card, set the GATT characteristic value to this. Then change the SD card pointer. Wait till the data is received by the mobile application. Then repeat until the file is read full.y 
 Note I am using SDK 16 NRF52840-dk PCA 10056 S140. 
 This post is similar and mentions using two characteristics one from commication from the mobile app and, one for receiving the data. 
 https://devzone.nordicsemi.com/f/nordic-q-a/53987/nrf52840-file-transfer-via-ble

Edvin · Accepted Answer

Thomas said: When I try to send 1800 it gets stuck and when I cancel the connection it crashes. I am not sure why. 
 I assume it is an APP_ERROR_CHECK(err_code) that receives an err_code != 0. Which one is it? Does your log say something like "fatal error"? If you define DEBUG in your preprocessor defines, the log should say where it is. So where does it point to? 
 When you find out where it points to, see if you can figure out why the function call that returned the err_code returned that err_code. 
 You need to check this yourself. If I just figure out each and every case for you, I basically have to develop the entire application for you. It is better (and faster) if you learn how to deal with the error handler yourself.