Reading and writing to serial port in C on Linux

Question

I m trying to send receive data over an USB Port using FTDI  so I need to handle serial communication using C C    I m working on Linux  Ubuntu    Basically  I am connected to a device which is listening for incoming commands  I need to send those commands and read device s response  Both commands and response are ASCII characters   Everything works fine using GtkTerm but  when I switch to C programming  I encounter problems   Here s my code    include  lt stdio h gt          standard input   output functions  include  lt stdlib h gt   include  lt string h gt         string function definitions  include  lt unistd h gt         UNIX standard function definitions  include  lt fcntl h gt          File control definitions  include  lt errno h gt          Error number definitions  include  lt termios h gt        POSIX terminal control definitions     Open File Descriptor    int USB   open    dev ttyUSB0   O RDWR  O NONBLOCK   O NDELAY        Error Handling    if   USB  lt  0     cout  lt  lt   Error    lt  lt  errno  lt  lt    opening    lt  lt    dev ttyUSB0   lt  lt        lt  lt  strerror  errno   lt  lt  endl            Configure Port        struct termios tty  memset   amp tty  0  sizeof tty       Error Handling    if   tcgetattr   USB   amp tty      0     cout  lt  lt   Error    lt  lt  errno  lt  lt    from tcgetattr     lt  lt  strerror errno   lt  lt  endl        Set Baud Rate    cfsetospeed   amp tty  B9600   cfsetispeed   amp tty  B9600       Setting other Port Stuff    tty c cflag      amp     PARENB            Make 8n1 tty c cflag      amp     CSTOPB  tty c cflag      amp     CSIZE  tty c cflag         CS8  tty c cflag      amp     CRTSCTS           no flow control tty c lflag         0              no signaling chars  no echo  no canonical processing tty c oflag         0                      no remapping  no delays tty c cc VMIN           0                      read doesn t block tty c cc VTIME          5                      0 5 seconds read timeout  tty c cflag         CREAD   CLOCAL         turn on READ  amp  ignore ctrl lines tty c iflag      amp      IXON   IXOFF   IXANY     turn off s w flow ctrl tty c lflag      amp      ICANON   ECHO   ECHOE   ISIG      make raw tty c oflag      amp     OPOST                  make raw     Flush Port  then applies attributes    tcflush  USB  TCIFLUSH     if   tcsetattr   USB  TCSANOW   amp tty      0    cout  lt  lt   Error    lt  lt  errno  lt  lt    from tcsetattr   lt  lt  endl            WRITE         unsigned char cmd       I    N    I    T          r     0    int n written   write  USB  cmd  sizeof cmd  -1        Allocate memory for read buffer    char buf  256   memset   amp buf    0   sizeof buf           READ        int n   read  USB   amp buf   sizeof buf        Error Handling    if  n  lt  0         cout  lt  lt   Error reading     lt  lt  strerror errno   lt  lt  endl        Print what I read       cout  lt  lt   Read     lt  lt  buf  lt  lt  endl   close USB     What happens is that read   returns 0  no bytes read at all  or block until timeout  VTIME   I m assuming this happens because write   does not send anything  In that case  device wouldn t receive command and I cannot receive response  In fact  turning off the device while my program is blocked on reading actually succeded in getting a response  device sends something while shutting down    Strange thing is that adding this   cout  lt  lt   I ve written     lt  lt  n written  lt  lt   bytes   lt  lt  endl     right after write   call  I receive   I ve written 6 bytes   which is exactly what I expect  Only my program doesn t work as it should  like my device cannot receive what I m actually writing on port   I ve tried different things and solution  also regarding data types  I ve tried using std  string  such as cmd    INIT  r  or const char  but nothing really worked   Can someone tell me where I m wrong   Thank you in advance   EDIT  Previously version of this code used   unsigned char cmd      INIT  n   and also cmd      INIT  r n   I changed it because command sintax for my device is reported as    lt command gt  lt SPACE gt  lt CR gt     I ve also tried avoiding the O NONBLOCK flag on reading  but then I only block until forever  I ve tried using select   but nothing happens  Just for a try  I ve created a waiting loop until data is avaliable  but my code never exit the loop  Btw  waiting or usleep   is something I need to avoid  Reported one is only an excerpt of my code  Complete code needs to work in a real-time environment  specifically OROCOS  so I don t really want sleep-like function

User · Answer

1  I d add a  n after init   i e  write  USB   init n   5    2  Double check the serial port configuration   Odds are something is incorrect in there   Just because you don t use  Q  S or hardware flow control doesn t mean the other side isn t expecting it   3  Most likely   Add a  usleep 100000   after the write     The file-descriptor is set not to block or wait  right   How long does it take to get a response back before you can call read    It has to be received and buffered by the kernel  through system hardware interrupts  before you can read   it    Have you considered using select   to wait for something to read     Perhaps with a timeout   Edited to Add   Do you need the DTR RTS lines   Hardware flow control that tells the other side to send the computer data   e g   int tmp  serialLines   cout  lt  lt   Dropping Reading DTR and RTS n   ioctl   readFd  TIOCMGET   amp  serialLines    serialLines  amp    TIOCM DTR  serialLines  amp    TIOCM RTS  ioctl   readFd  TIOCMSET   amp  serialLines    usleep 100000   ioctl   readFd  TIOCMGET   amp  tmp    cout  lt  lt   Reading DTR status     lt  lt   tmp  amp  TIOCM DTR   lt  lt  endl  sleep  2    cout  lt  lt   Setting Reading DTR and RTS n   serialLines    TIOCM DTR  serialLines    TIOCM RTS  ioctl   readFd  TIOCMSET   amp  serialLines    ioctl   readFd  TIOCMGET   amp  tmp    cout  lt  lt   Reading DTR status     lt  lt   tmp  amp  TIOCM DTR   lt  lt  endl

User · Answer

I ve solved my problems  so I post here the correct code in case someone needs similar stuff   Open Port  int USB   open    dev ttyUSB0   O RDWR  O NOCTTY      Set parameters  struct termios tty  struct termios tty old  memset   amp tty  0  sizeof tty       Error Handling    if   tcgetattr   USB   amp tty      0        std  cout  lt  lt   Error    lt  lt  errno  lt  lt    from tcgetattr     lt  lt  strerror errno   lt  lt  std  endl        Save old tty parameters    tty old   tty      Set Baud Rate    cfsetospeed   amp tty   speed t B9600   cfsetispeed   amp tty   speed t B9600       Setting other Port Stuff    tty c cflag      amp     PARENB                Make 8n1 tty c cflag      amp     CSTOPB  tty c cflag      amp     CSIZE  tty c cflag         CS8   tty c cflag      amp     CRTSCTS               no flow control tty c cc VMIN       1                      read doesn t block tty c cc VTIME      5                      0 5 seconds read timeout tty c cflag         CREAD   CLOCAL         turn on READ  amp  ignore ctrl lines     Make raw    cfmakeraw  amp tty       Flush Port  then applies attributes    tcflush  USB  TCIFLUSH    if   tcsetattr   USB  TCSANOW   amp tty      0       std  cout  lt  lt   Error    lt  lt  errno  lt  lt    from tcsetattr   lt  lt  std  endl      Write  unsigned char cmd      INIT  r   int n written   0      spot   0   do       n written   write  USB   amp cmd spot   1        spot    n written    while  cmd spot-1       r   amp  amp  n written  gt  0     It was definitely not necessary to write byte per byte  also int n written   write  USB  cmd  sizeof cmd  -1  worked fine   At last  read   int n   0      spot   0  char buf     0       Whole response   char response 1024   memset response    0   sizeof response    do       n   read  USB   amp buf  1        sprintf   amp response spot     c   buf        spot    n    while  buf      r   amp  amp  n  gt  0    if  n  lt  0        std  cout  lt  lt   Error reading     lt  lt  strerror errno   lt  lt  std  endl    else if  n    0        std  cout  lt  lt   Read nothing    lt  lt  std  endl    else       std  cout  lt  lt   Response     lt  lt  response  lt  lt  std  endl      This one worked for me  Thank you all

User · Answer

Some receivers expect EOL sequence  which is typically two characters  r n  so try in your code replace the line  unsigned char cmd       I    N    I    T          r     0      with  unsigned char cmd      INIT r n     BTW  the above way is probably more efficient  There is no need to quote every character

[c] Reading and writing to serial port in C on Linux

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