linux 下实现打开串口，连接GPRS功能， GPRS采用sim900，用AT指令通过串口对sim900进行操作来实现GPRS通信。用到来第三方库posix_qextserialport.h-linux under open serial port, connect GPRS function, GPRS using sim900, using AT commands through the serial port to operate to achieve sim900 GPRS com