Thank you very much for your reply!
I have found where my problem is and now I have got it over.As you said,I checked my “ldc_init() “ function and “readValue” function and I finally found I misused Wire.endTranmission(),I put an extra parameter into it so the I2C is closed after I write the register ‘s address.Now I can read the data normally!
For it’s my first time using this forum so I’m very sorry to trouble you with my awful code, next time I’ll use the method(Although I still did’t find the button on iPad website, maybe I can try another time on PC).