Linux Serial Programming

[ibwhoib]

Hey all,

I caught a job where I have to write a linux daemon that communicates with a
device connected to a serial port. Here are some details.

I connect to the device in the standard fashion:

fd = open(device, O_RDWR | O_NOCTTY | O_NDELAY);

I set the port options:

options.c_cflag = B9600 | CS8 | CREAD | CLOCAL | HUPCL;
cfsetospeed(&options, B9600);
tcsetattr(fd, TCSANOW, &options);

I now want to write some data to the port. The documentation on the device
has the following information:
"The command structure consist of one start byte, one command byte, five
bytes of data, and one byte checksum"
So I need a string of bytes. I created this string by using a unsigned
character array and setting the values to a command. Like so:

char cmd[7];
cmd[0] = 42;
cmd[1] = 0;
cmd[2] = 0;
cmd[3] = 132;
cmd[4] = 177;
cmd[5] = 0;
cmd[6] = 119;
cmd[7] = 188;

I then try to write the command to the port.

int written;
written = write(fd, cmd, 7);

My first question is is this the correct way to send this command?
Unfortunately, the device doesn't seem to think it is and returns 3 bytes
that are undocumented. I would appreciate any help. If you need any further
information, please let me know.

Thanks,
ib

 

[Ben Pfaff]

I caught a job where I have to write a linux daemon that communicates with a
device connected to a serial port. Here are some details.

Your question is outside the domain of comp.lang.c, which discusses
only the standard C programming language, including the standard C
library. This is a remarkably narrow topic compared to what many
people expect.

For your convenience, the list below contains topics that are not
on-topic for comp.lang.c, and suggests newsgroups for you to explore
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[Chris Torek]

fd = open(device, O_RDWR | O_NOCTTY | O_NDELAY);

Calls to open() and ioctl() (or tcsetattr(), which in turn is
usually just an ioctl()) and write() are off-topic in comp.lang.c,
in part because the details about which options you should use
(like O_NOCTTY and/or O_NDELAY) change from one system to another.
This, however, is on topic:

So I need a string of bytes. I created this string by using a unsigned
character array and setting the values to a command. Like so:

char cmd[7];
cmd[0] = 42;
cmd[1] = 0;
cmd[2] = 0;
cmd[3] = 132;
cmd[4] = 177;
cmd[5] = 0;
cmd[6] = 119;
cmd[7] = 188;

An array of size 7 has 7 total elements, in this case named cmd[0]
through cmd[6] inclusive. You are putting eight values (go ahead,
count them, there are 8) into a seven-value-size bucket. This
causes undefined behavior, i.e., the program may behave unpredictably
(or at least in a manner that, to predict, requires details that
one would not normally even want to think about).

 

[Nick Austin]

I now want to write some data to the port. The documentation on the device
has the following information:
"The command structure consist of one start byte, one command byte, five
bytes of data, and one byte checksum"

That's 8 bytes.

So I need a string of bytes. I created this string by using a unsigned
character array and setting the values to a command. Like so:

char cmd[7];

unsigned char cmd[8];

cmd[0] = 42;
cmd[1] = 0;
cmd[2] = 0;
cmd[3] = 132;
cmd[4] = 177;
cmd[5] = 0;
cmd[6] = 119;
cmd[7] = 188;

I then try to write the command to the port.

int written;
written = write(fd, cmd, 7);

written = write(fd, cmd, 8);
or
written = write(fd, cmd, sizeof cmd);

Nick.