Linux中的串行端口通信返回错误答案

Question

I am trying to create a linux ubuntu program that will read data from tty serial port (COM port in windows). I'm not using USB adapter but actual COM port. This is my code for communication so far:

int OpenPort(void)
{
   int fd; // file description for sp

   fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_SYNC);

   if(fd == -1) // port not opened 
   {
      printf("Error:\n%s.\n", strerror(errno));
   }
   else
   {
      fcntl(fd, F_SETFL, 0);
      printf("Success.\n");
   }

   return(fd);
} // Open sp

void Communicate(void)
{
   struct termios settings;    

   tcgetattr( fd, &settings );

   cfsetispeed(&settings, B9600);    // Set bouds
   cfsetospeed(&settings, B9600);

   settings.c_cflag &= ~PARENB;    // set no parity, stop bits, data bits
   settings.c_cflag &= ~CSTOPB;
   settings.c_cflag &= ~CSIZE;
   settings.c_cflag |= CS8;

   tcflush( fd, TCIFLUSH );

   if (tcsetattr(fd, TCSAFLUSH, &settings)!= 0)
   {
      printf("Error message");
   }   

   //Create byte array
   unsigned char send_bytes[] = { 0x1, 0x6, 0x2, 0xAA, 0x2, 0x3, 0xB8, 0x4 };

   write(fd, send_bytes, sizeof(send_bytes));  // Send data
   printf("Data sent. \n");

   char buffer[64]; // buffer to receive data

   printf("I'm reading data...\n");
   int n = read(fd, buffer, sizeof(buffer));

   if (n < 0)
     printf("Failed to read\n");

   int i; 
   printf("Showing data...\n");

   for(i=0; i<sizeof(buffer); i++)
   {
      printf("Hex: %x\n", buffer[i]);
   } 

   printf("Closing...\n");

   close(fd);

   printf("All done!\n");
}

I have several problems here:

1. After I run program once it executes correctly but when I try to run it again it stops at "I'm reading data..." and won't start even after I restart computer. After some time it allows me to execute program again.
2. After program returns data it should send me hex data like A7, 9F etc. but this gives me integer values.
3. Should I and how do I clear buffer array to free memory

Can anyone help with these problems?

Answer 1

'int n = read(fd, buffer, sizeof(buffer));' returns the number of bytes read in n, which you then ignore, (apart from checking for negative error value), and assume the buffer has been completely filled. This is a mistake because recv() does not, in general, wait for the number of bytes requested because serial ports provide a byte-stream, not 'buffers' or 'messages'.

printf("Hex: %x\\n", buffer[i]); will display integers because you specify integers with %x. Look CAREFULLY at printf format codes.