Problem to read on a serial port

hi

i need to read from a  serial port (c programming)

we use a sixnet Mini-VersaTRAK mIPm
 http://www.**--****.com/ 

under bash:
~ # stty -a </dev/ttyS2
speed 9600 baud; rows 0; columns 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>;
eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z;
rprnt = ^R; werase = ^W; lnext = ^V; flush = ^U; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon
-ixoff -iuclc -ixany -imaxbel
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0
vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop
-echoprt echoctl echoke


i send a command via serial port to a led display (alpha sign
communications)

void ledDisplayExist()
{
    char msg[]={'\0', '\0', '\0', '\0', '\0', '\1', 'Z', '0', '0',
'\2', 'H', 'A', '\4'  };
    int length = sizeof(msg);
    writeopen(msg, length);
    readopen();
}

i write msg with this function:

int writeopen(char msg[], int size)
{
    printf("write\n");
    int fd1;
    int wr;
    fd1 = open(ledisplay, O_RDWR | O_NOCTTY | O_NDELAY );
    if (fd1 == -1)
        fprintf(stderr, " %s open_port: Unable to open %s\n",
strerror(errno), ledisplay);
    else
    {
        fcntl(fd1, F_SETFL, 0);
        wr=write(fd1, msg, size);
        if (wr < 0)
            fputs("write() of n bytes failed!\n", stderr);
        close(fd1);
    }
    return (fd1);
}

i try to read with this function:

int readopen()
{
    printf("read\n");
    int fd1;
    int rd;
    char buff[255];
    fd1 = open(ledisplay, O_RDWR | O_NOCTTY | FNDELAY );
    if (fd1 == -1)
        fprintf(stderr, "%s open_port: Unable to open %s\n",
strerror(errno), ledisplay);
    else
    {
        fcntl(fd1, F_SETFL, FNDELAY);
        printf(" Port 1 has been sucessfully opened and %d is the file
descriptor\n",fd1);
        rd=read(fd1, buff, 20);
        printf(" Bytes recieved are %d \n",rd);
        close(fd1);
    }
    return (fd1);
}

that return always return me:  Bytes recieved are -1 (i write without
problem, the problem is the read)

any idea?

thanks

Unfortunately neither of those are particularly good examples of
serial port coding.  There are several problems with the second
one in particular, which seems to be the one you have used as an
example to base your code on.

Read those for the information they provide, but don't use the code!

In the first one there is a reference to the Linux HOWTO...  which
should be used in the same way as those two, for information only
and not for coding examples.

However, the other reference given is *really* *good*:

   http://www.**--****.com/ ~mike/serial/serial.html

I don't know of a single error in that entire document.  It is *the*
URL to pay attention to.  Unfortunately it doesn't have code examples
that will help you much.

I've put several "terminal programs" as serial port programming
examples on my web site.  These are more or less a match for the
discussion in the Linux serial port programming HOWTO, without
(I hope) all of the coding errors and bugs.

     http://www.**--****.com/ 

These programs are configured to talk to a modem, but changing
them to talk to virtually any serial device is relatively easy.
There are five different versions of the same program, using
different methods for accessing the serial port:

 term_async.c:   Single process using asynchronous interrupts.
 term_select.c:  Single process using select().
 term_canon.c:   Forked processes using canonical mode input.
 term_fork.c:    Forked processes using raw mode input.
 term_thread.c:  Pthreads and raw mode input.

I would suggest that you *not* attempt to use either the async or
threaded examples.  The easiest to use is probably the one calling
select().

It would be interesting to see your entire program.  It's
probably too big to post the whole thing, but perhaps you could
put it on a web site?

-- 
Floyd L. Davidson           < http://www.**--****.com/ ;
Ukpeagvik (Barrow, Alaska)                          XXXX@XXXXX.COM 

What do you think about the

  fcntl(fd, F_SETFL, 0);

Wouldn't that also clear out other flags than O_NDELAY, which
might have unintended effects (at least I don't know if there's
a consensus between systems which flags exist at all and which
get influenced by fcntl())? So, shouldn't one use e.g.

fcntl(fd, F_SETFL, fcntl( fd, F_GET_FL ) & ~ O_NDELAY );

to explicitely only reset the non-delay flag? Also, I didn't find
'FNDELAY' in POSIX, so wouldn't it be better to use O_NDELAY? Or
is there some magic involved with the use of 'FNDELAY'?

Another thing I am not sure about is what read() is supposed to
return in non-blocking mode when no data are available. Is it
really true that read() should return 0 in that case and not -1
with errno being set to EAGAIN? Is this something specific to
the serial port (I wouldn't expect a 0 return value when e.g.
reading from a normal file or a pipe in that case because it
would be impossible to distinguish it from a closed pipe)?

                                  Regards, Jens
-- 
  \   Jens Thoms Toerring  ___   XXXX@XXXXX.COM 
   \__________________________   http://www.**--****.com/ 

 XXXX@XXXXX.COM  wrote, on Thu, 31 Mar 2005:


In the general case the get-and-set method should always be used.
According to POSIX fcntl(fd, F_SETFL, 0) would turn off O_APPEND,
O_NONBLOCK, O_SYNC, O_DSYNC and O_RSYNC if any of them were set.
Implementations could also have additional non-standard flags for
use with fcntl F_[GS]ETFL (such as O_NDELAY) which would be
turned off if they were set.

However, if you know for sure that other flags were not set when
the fd was opened, and have not been set since, then
fcntl(fd, F_SETFL, 0) should work fine to turn off only the
flags you know are set.


Neither FNDELAY nor O_NDELAY are in POSIX.  It is best to use
O_NONBLOCK.


The zero return is the historical behaviour with O_NDELAY, and the
fact that it is broken-by-design (because it can't be distinguished
from EOF) is the reason POSIX invented O_NONBLOCK to replace O_NDELAY.
When O_NONBLOCK is used read() must indicate that no data are
available by returning -1 and setting errno to EAGAIN.

-- 
Geoff Clare < XXXX@XXXXX.COM >

Thanks for the clarification! I actually always was wondering WTF
the difference between O_NDELAY and O_NONBLOCK is supposed to be
and started to think of them as the same thing. So you may have
spared me some future trouble;-)

                                   Regards, Jens
-- 
  \   Jens Thoms Toerring  ___   XXXX@XXXXX.COM 
   \__________________________   http://www.**--****.com/ 

ok i will read it

ya no problem, you can get the file:
 http://www.**--****.com/ 

...


FNDELAY is BSD, while O_NDELAY came from SYSV. Because there was
no consistant standard for implementation, and variations existed
on different platforms, POSIX adopted O_NONBLOCK to provide a well
defined portable standard.


On Linux, all of O_NDELAY, FNDELAY and O_NONBLOCK are exactly the
same.  Grepping in /usr/include turns up these two defines in
/usr/include/bits/fcntl.h,

    #define O_NDELAY     O_NONBLOCK
    #define FNDELAY      O_NDELAY

Which can indeed be a problem if the code is ported to various
other flavors of unix.  On some (Solaris being one example I've
seen referenced) they are slightly different.  And of course how
they differ changes between implementations too.

-- 
Floyd L. Davidson           < http://www.**--****.com/ ;
Ukpeagvik (Barrow, Alaska)                          XXXX@XXXXX.COM 

Having used Linux exclussively when dealing with the serial port may
have helped my confusion;-) (and I always opted to only use O_NONBLOCK
since it seemed like the right thing...) While I read the man pages
for other systems I have access to, things always remained a bit hazy,
and especially the business with having read() return 0 when in non-
blocking mode when no data are available was rather counterintuitive.
But I guess I now got the complete picture. Thanks, Floyd and Goeff!

                                  Regards, Jens
-- 
  \   Jens Thoms Toerring  ___   XXXX@XXXXX.COM 
   \__________________________   http://www.**--****.com/