1. Introduction

    System Information

  2. Inter-Process Communication
  3. Signals

    Programming in Various Environments

  4. Script Programming
    1. Script Variables
    2. Test Conditions
    3. Control and Iteration
    4. Commonly used Programs
    5. Shell Capabilities
    6. Example looping script
    7. Example using Variables
    8. Example working with files
    9. Example install script
  5. C and C++ Programming
    1. POSIX System Capabilities
    2. More POSIX
    3. Threads
    4. Mutexes
    5. An example viewmod program
    6. An example serial program
  6. X Programming
  7. Debugging
  8. Credits

A Linux serial port test program

Below is a Linux serial test program which requires the command parameters to be entered on the command line when the program is started. This program will send output typed on the computer keyboard after the program is started, through the serial port specified. This program can be downloaded using this link: com.c. I recommend that you right click on the link to download it rather than viewing it in your browser and saving it so you do not get carriage returns and line feeds in the text which may cause the compilation to fail.

#include <termios.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/signal.h>
#include <sys/types.h>

#define BAUDRATE B38400
#define MODEMDEVICE "/dev/ttyS1"
#define _POSIX_SOURCE 1         //POSIX compliant source
#define FALSE 0
#define TRUE 1

volatile int STOP=FALSE;

void signal_handler_IO (int status);    //definition of signal handler
int wait_flag=TRUE;                     //TRUE while no signal received
char devicename[80];
long Baud_Rate = 38400;         // default Baud Rate (110 through 38400)
long BAUD;                      // derived baud rate from command line
long DATABITS;
long STOPBITS;
long PARITYON;
long PARITY;
int Data_Bits = 8;              // Number of data bits
int Stop_Bits = 1;              // Number of stop bits
int Parity = 0;                 // Parity as follows:
                  // 00 = NONE, 01 = Odd, 02 = Even, 03 = Mark, 04 = Space
int Format = 4;
FILE *input;
FILE *output;
int status;

main(int Parm_Count, char *Parms[])
{
   char version[80] = "       POSIX compliant Communications test program version 1.00 4-25-1999\r\n";
   char version1[80] = "          Copyright(C) Mark Zehner/Peter Baumann 1999\r\n";
   char version2[80] = " This code is based on a DOS based test program by Mark Zehner and a Serial\r\n";
   char version3[80] = " Programming POSIX howto by Peter Baumann, integrated by Mark Zehner\r\n";  
   char version4[80] = " This program allows you to send characters out the specified port by typing\r\n";
   char version5[80] = " on the keyboard.  Characters typed will be echoed to the console, and \r\n";
   char version6[80] = " characters received will be echoed to the console.\r\n";
   char version7[80] = " The setup parameters for the device name, receive data format, baud rate\r\n";
   char version8[80] = " and other serial port parameters must be entered on the command line \r\n";
   char version9[80] = " To see how to do this, just type the name of this program. \r\n";
   char version10[80] = " This program is free software; you can redistribute it and/or modify it\r\n";
   char version11[80] = " under the terms of the GNU General Public License as published by the \r\n";
   char version12[80] = " Free Software Foundation, version 2.\r\n";
   char version13[80] = " This program comes with ABSOLUTELY NO WARRANTY.\r\n";
   char instr[100] ="\r\nOn the command you must include six items in the following order, they are:\r\n";
   char instr1[80] ="   1.  The device name      Ex: ttyS0 for com1, ttyS1 for com2, etc\r\n";
   char instr2[80] ="   2.  Baud Rate            Ex: 38400 \r\n";
   char instr3[80] ="   3.  Number of Data Bits  Ex: 8 \r\n";
   char instr4[80] ="   4.  Number of Stop Bits  Ex: 0 or 1\r\n";
   char instr5[80] ="   5.  Parity               Ex: 0=none, 1=odd, 2=even\r\n";
   char instr6[80] ="   6.  Format of data received:  1=hex, 2=dec, 3=hex/asc, 4=dec/asc, 5=asc\r\n";
   char instr7[80] =" Example command line:  com ttyS0 38400 8 0 0 4 \r\n";
   char Param_strings[7][80];
   char message[90];

   int fd, tty, c, res, i, error;
   char In1, Key;
   struct termios oldtio, newtio;       //place for old and new port settings for serial port
   struct termios oldkey, newkey;       //place tor old and new port settings for keyboard teletype
   struct sigaction saio;               //definition of signal action
   char buf[255];                       //buffer for where data is put
   
   input = fopen("/dev/tty", "r");      //open the terminal keyboard
   output = fopen("/dev/tty", "w");     //open the terminal screen

   if (!input || !output)
   {
      fprintf(stderr, "Unable to open /dev/tty\n");
      exit(1);
   }

   error=0;
   fputs(version,output);               //display the program introduction
   fputs(version1,output);
   fputs(version2,output);
   fputs(version3,output);
   fputs(version4,output);
   fputs(version5,output);
   fputs(version6,output);
   fputs(version7,output);
   fputs(version8,output);
   fputs(version9,output);
   fputs(version10,output);
   fputs(version11,output); 
   fputs(version12,output);
   fputs(version13,output);
   //read the parameters from the command line
   if (Parm_Count==7)  //if there are the right number of parameters on the command line
   {
      for (i=1; i&#60Parm_Count; i++)  // for all wild search parameters
      {
         strcpy(Param_strings[i-1],Parms[i]);
      }
      i=sscanf(Param_strings[0],"%s",devicename);
      if (i != 1) error=1;
      i=sscanf(Param_strings[1],"%li",&Baud_Rate);
      if (i != 1) error=1;
      i=sscanf(Param_strings[2],"%i",&Data_Bits);
      if (i != 1) error=1;
      i=sscanf(Param_strings[3],"%i",&Stop_Bits);
      if (i != 1) error=1;
      i=sscanf(Param_strings[4],"%i",&Parity);
      if (i != 1) error=1;
      i=sscanf(Param_strings[5],"%i",&Format);
      if (i != 1) error=1;
      sprintf(message,"Device=%s, Baud=%li\r\n",devicename, Baud_Rate); //output the received setup parameters
      fputs(message,output);
      sprintf(message,"Data Bits=%i  Stop Bits=%i  Parity=%i  Format=%i\r\n",Data_Bits, Stop_Bits, Parity, Format);
      fputs(message,output);
   }  //end of if param_count==7
   if ((Parm_Count==7) && (error==0))  //if the command line entries were correct
   {                                    //run the program
      tty = open("/dev/tty", O_RDWR | O_NOCTTY | O_NONBLOCK); //set the user console port up
      tcgetattr(tty,&oldkey); // save current port settings   //so commands are interpreted right for this program
      // set new port settings for non-canonical input processing  //must be NOCTTY
      newkey.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
      newkey.c_iflag = IGNPAR;
      newkey.c_oflag = 0;
      newkey.c_lflag = 0;       //ICANON;
      newkey.c_cc[VMIN]=1;
      newkey.c_cc[VTIME]=0;
      tcflush(tty, TCIFLUSH);
      tcsetattr(tty,TCSANOW,&newkey);

      switch (Baud_Rate)
      {
         case 38400:
         default:
            BAUD = B38400;
            break;
         case 19200:
            BAUD  = B19200;
            break;
         case 9600:
            BAUD  = B9600;
            break;
         case 4800:
            BAUD  = B4800;
            break;
         case 2400:
            BAUD  = B2400;
            break;
         case 1800:
            BAUD  = B1800;
            break;
         case 1200:
            BAUD  = B1200;
            break;
         case 600:
            BAUD  = B600;
            break;
         case 300:
            BAUD  = B300;
            break;
         case 200:
            BAUD  = B200;
            break;
         case 150:
            BAUD  = B150;
            break;
         case 134:
            BAUD  = B134;
            break;
         case 110:
            BAUD  = B110;
            break;
         case 75:
            BAUD  = B75;
            break;
         case 50:
            BAUD  = B50;
            break;
      }  //end of switch baud_rate
      switch (Data_Bits)
      {
         case 8:
         default:
            DATABITS = CS8;
            break;
         case 7:
            DATABITS = CS7;
            break;
         case 6:
            DATABITS = CS6;
            break;
         case 5:
            DATABITS = CS5;
            break;
      }  //end of switch data_bits
      switch (Stop_Bits)
      {
         case 1:
         default:
            STOPBITS = 0;
            break;
         case 2:
            STOPBITS = CSTOPB;
            break;
      }  //end of switch stop bits
      switch (Parity)
      {
         case 0:
         default:                       //none
            PARITYON = 0;
            PARITY = 0;
            break;
         case 1:                        //odd
            PARITYON = PARENB;
            PARITY = PARODD;
            break;
         case 2:                        //even
            PARITYON = PARENB;
            PARITY = 0;
            break;
      }  //end of switch parity
       
      //open the device(com port) to be non-blocking (read will return immediately)
      fd = open(devicename, O_RDWR | O_NOCTTY | O_NONBLOCK);
      if (fd < 0)
      {
         perror(devicename);
         exit(-1);
      }

      //install the serial handler before making the device asynchronous
      saio.sa_handler = signal_handler_IO;
      sigemptyset(&saio.sa_mask);   //saio.sa_mask = 0;
      saio.sa_flags = 0;
      saio.sa_restorer = NULL;
      sigaction(SIGIO,&saio,NULL);

      // allow the process to receive SIGIO
      fcntl(fd, F_SETOWN, getpid());
      // Make the file descriptor asynchronous (the manual page says only
      // O_APPEND and O_NONBLOCK, will work with F_SETFL...)
      fcntl(fd, F_SETFL, FASYNC);

      tcgetattr(fd,&oldtio); // save current port settings 
      // set new port settings for canonical input processing 
      newtio.c_cflag = BAUD | CRTSCTS | DATABITS | STOPBITS | PARITYON | PARITY | CLOCAL | CREAD;
      newtio.c_iflag = IGNPAR;
      newtio.c_oflag = 0;
      newtio.c_lflag = 0;       //ICANON;
      newtio.c_cc[VMIN]=1;
      newtio.c_cc[VTIME]=0;
      tcflush(fd, TCIFLUSH);
      tcsetattr(fd,TCSANOW,&newtio);

      // loop while waiting for input. normally we would do something useful here
      while (STOP==FALSE)
      {
         status = fread(&Key,1,1,input);
         if (status==1)  //if a key was hit
         {
            switch (Key)
            { /* branch to appropiate key handler */
               case 0x1b: /* Esc */
                  STOP=TRUE;
                  break;
               default:
                  fputc((int) Key,output);
//                  sprintf(message,"%x ",Key);  //debug
//                  fputs(message,output);
                  write(fd,&Key,1);          //write 1 byte to the port
                  break;
            }  //end of switch key
         }  //end if a key was hit
         // after receiving SIGIO, wait_flag = FALSE, input is available and can be read
         if (wait_flag==FALSE)  //if input is available
         {
            res = read(fd,buf,255);
            if (resɬ)
            {
               for (i=0; i<res; i++)  //for all chars in string
               {
                  In1 = buf[i];
                  switch (Format)
                  {
                     case 1:         //hex
                        sprintf(message,"%x ",In1);
                        fputs(message,output);
                        break;
                     case 2:         //decimal
                        sprintf(message,"%d ",In1);
                        fputs(message,output);
                        break;
                     case 3:         //hex and asc
                        if ((In1ថ) || (In1))
                        {
                           sprintf(message,"%x",In1);
                           fputs(message,output);
                        }
                        else fputc ((int) In1, output);
                        break;
                     case 4:         //decimal and asc
                     default:
                        if ((In1ថ) || (In1))
                        {
                           sprintf(message,"%d",In1);
                           fputs(message,output);
                        }
                        else fputc ((int) In1, output);
                        break;
                     case 5:         //asc
                        fputc ((int) In1, output);
                        break;
                  }  //end of switch format
               }  //end of for all chars in string
            }  //end if resɘ
//            buf[res]=0;
//            printf(":%s:%d\n", buf, res);
//            if (res==1) STOP=TRUE; /* stop loop if only a CR was input */
            wait_flag = TRUE;      /* wait for new input */
         }  //end if wait flag == FALSE

      }  //while stop==FALSE
      // restore old port settings
      tcsetattr(fd,TCSANOW,&oldtio);
      tcsetattr(tty,TCSANOW,&oldkey);
      close(tty);
      close(fd);        //close the com port
   }  //end if command line entrys were correct
   else  //give instructions on how to use the command line
   {
      fputs(instr,output);
      fputs(instr1,output);
      fputs(instr2,output);
      fputs(instr3,output);
      fputs(instr4,output);
      fputs(instr5,output);
      fputs(instr6,output);
      fputs(instr7,output);
   }
   fclose(input);
   fclose(output);
}  //end of main

/***************************************************************************
* signal handler. sets wait_flag to FALSE, to indicate above loop that     *
* characters have been received.                                           *
***************************************************************************/

void signal_handler_IO (int status)
{
//    printf("received SIGIO signal.\n");
   wait_flag = FALSE;
}

An example invocation of this program

This program will allow a user to communicate from one computer to another using two serial lines. It is intended only as a test for the serial connection allowing any possible serial setting. This program is named com when compiled. To use it, the following is an example command line:

com /dev/ttyS0 38400 8 1 0 4

This line works for com1 at 38400 baud, 8 data bits. I make it a file, call it "fcom1" and put it in my system path and invoke it as a script program.