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POUsing select() and fgets() to access information from a serial port
 

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  1. POUsing select() and fgets() to access information from a serial port
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    <p>This is a followup to this question: <a href="https://stackoverflow.com/questions/6656579/how-to-wait-for-input-from-the-serial-port-in-the-middle-of-a-program">How to wait for input from the serial port in the middle of a program</a></p> <p>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 <code>select()</code> command to wait for this input.</p> <p>However, I have run into some difficulty with this approach. Initially, <code>select()</code> 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.</p> <pre><code> //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(&amp;fds); FD_SET(fd, &amp;fds); struct timeval timeout = { 10, 0 }; /* 10 seconds */ //int ret = select(fd+1, &amp;fds, NULL, NULL, &amp;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&lt;(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, &amp;fds, NULL, NULL, &amp;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, &amp;fds, NULL, NULL, &amp;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++; } </code></pre>
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    1. COYou are trying this on Windows or Linux?
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    2. COWhy are you passing (fd+1) in the first argument of select function? Try passing 1 instead as the first argument of select is count of descriptors in fd array although this argument is ignored in windows
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    3. CO@Muhammad Linux
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      1. USWandering Sophist
 

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