retc = retc & lt& lt 1; //Variable moves to the left

if(SDA = = 1)retc = retc+ 1； //This sentence is actually to receive one bit, because the previous sentence is shifted to the left, so this sentence is about to receive the next bit, which is shifted to the left, and the lowest bit is 0. If the receiving pin is 0, it is 0. If it is 1, the lowest bit of the rec variable should be changed to 1. In the program, it is actually changed to 1.