与USB转串口适配器通信时，为什么此串行通信代码会挂起？

I am writing an application that is supposed to control a projector from the desktop of a Mac Mini. I am writing the application in Objective-C using Xcode and termios.h to talk to the serial port. I am connecting from a USB-to-Serial adapter and I know the drivers are installed correctly because I can send HEX commands to the projector to control its functions. I am writing an application that is supposed to