How to send and receive a response from the com port

There is a problem, I understand how to send data to the com port, but I do not know how to get an answer. This is what I send: int sendData (unsigned char * buff, int len)

{ int n = write(F_ID, buff, len); if(n == -1) { char *errmsg = strerror(errno); printf("%s\n",errmsg); } return n; }