Question

This is a followup to this question: How to wait for input from the serial port in the middle of a program

I am writing a program to control an Iridium modem that needs to wait for a response from the serial port in the middle of the program in order to verify that the correct response was given. In order to accomplish this, a user recommended I use the select() command to wait for this input.

However, I have run into some difficulty with this approach. Initially, select() would return the value indicated a timeout on the response every time (even though the modem was sending back the correct responses, which I verified with another program running at the same time). Now, the program stops after one iteration, even with the correct response sent back from the modem.

 //setting the file descriptor to the port
int fd = open(portName.c_str(), O_RDWR | O_NOCTTY | O_NDELAY);

if (fd == -1)
{
  /*
   * Could not open the port.
   */

  perror("open_port: Unable to open /dev/ttyS0 - ");
}
else
 fcntl(fd, F_SETFL, 0);

FILE *out = fopen(portName.c_str(), "w");//sets the serial port
FILE *in = fopen(portName.c_str(), "r");


fd_set fds;
FD_ZERO(&fds);
FD_SET(fd, &fds);
struct timeval timeout = { 10, 0 }; /* 10 seconds */
//int ret = select(fd+1, &fds, NULL, NULL, &timeout);
/* ret == 0 means timeout, ret == 1 means descriptor is ready for reading,
 ret == -1 means error (check errno) */

char buf[100];

int i =0; 
while(i<(sizeof(messageArray)/sizeof(messageArray[0])))
{
  //creates a string with the AT command that writes to the module
  std::string line1("AT+SBDWT=");
  line1+=convertInt( messageArray[i].numChar);
  line1+=" ";
  line1+=convertInt(messageArray[i].packetNumber);
  line1+=" ";
  line1+=messageArray[i].data;
  line1+=std::string("\r\n");

  //creates a string with the AT command that initiates the SBD session
  std::string line2("AT+SBDI");
  line2+=std::string("\r\n");

  fputs(line1.c_str(), out); //sends to serial port

  //usleep(7000000);     
  int ret =select(fd+1, &fds, NULL, NULL, &timeout);
  /* ret == 0 means timeout, ret == 1 means descriptor is ready for reading,
 ret == -1 means error (check errno) */

  if (ret ==1){
fgets (buf ,sizeof(buf), in);
//add code to check if response is correct
  }
  else if(ret == 0) {
perror("timeout error ");
  }
  else if (ret ==-1) {
perror("some other error");
  }

  fputs(line2.c_str(), out); //sends to serial port

  //usleep(7000000); //Pauses  between the addition of each packet.
  int ret2 = select(fd+1, &fds, NULL, NULL, &timeout);
  /* ret == 0 means timeout, ret == 1 means descriptor is ready for reading,
 ret == -1 means error (check errno) */

  if(ret2 == 0) {
perror("timeout error ");
  }
  else if (ret2 ==-1) {
perror("some other error");
  }

  i++;
}

Answer 1

You aren’t using the same file handle for read/write/select, which is somewhat strange.

You are not resetting your fd_sets, which are modified by select and would have all of your fds unset in the case of a timeout, making the next call timeout by default (as you are asking for no fds).

you are also using buffered IO, which is bound to create headaches in this case. eg. fgets waits for either EOF (which won’t occur), or a newline, reading all the while. It will block until it gets its newline, so may keep you hanging indefinitely if that never occurs.
It may also read more than it needs into the buffer, messing up your select read signal (you have data in the buffer, but select will time out, since there’s nothing to read on the filehandle).

Bottom line is this:

• use FD_SET in the loop to set/reset your fd sets, also reset your timeout, as select may modify it.
• use a single handle for read/write/select, instead of multiple handles, eg. open file with fopen(..., "w+") or open(..., O_RDWR)
• if still using fopen, try disabling buffering using setvbuf with the _IONBF buffering option.
• otherwise, use open/read/write instead of fopen etc.

I will note that part of this was mentioned in this answer to your previous question.