FIFO的C语言实现。buffer指向存放数据的缓冲区，size是缓冲区的大小，in是写指针下标，out是读指针下标,在len和(fifo->size - fifo->in + fifo->out)之间取一个较小的值赋给len。注意，当(fifo->in == fifo->out+fifo->size)时，表示缓冲区已满，此时得到的较小值一定是0，后面实际写入的字节数也全为0。-FIFO the C language. buffer point to the storage of data buffer size is the size of the buffer, in a write pointer subscr ipt, out the subscr ipt of the read pointer, len and (fifo->　size- fifo->　in+ fifo->　out) take a smaller value assigned to len. Note when (fifo->　in == fifo->　out+fifo->　size), indicating that the buffer is full, the smaller value obtained must be behind the actual number of bytes written all.