Reading from Serial Port in linux using C language
I am new to serail programing in Linux (Fedora 12) using C Language. I have a simple device, it is supposed to get 3 bytes command in hex (wirte) like <0x02,0x03,0x0D>and return 4 bytes response.
First I wrote a simple java program on windows, and I get the correct response.. as and when,I switch to linux, I can’t read from serial com port neither using java nor C language.
I tried using libraries like rs232 .. but still the problem remains. I can open «/dev/ttyS0», and write on it .. (none of them returns any Error), but read is not possible ..
If I use canonical mode, the program blocks on reading until i kill the program.. If use non-canonical mode, with VMIN=0 and VTIME=5, read function returns whith -107725432 bytes for example .. (I have tried reading and writing byte by byte or all at the same time .. no difference ..)
I am really confused .. I tried almost every samples on the Internet .. An
y Idea Please?! I have traced the program, using strace ..
mmap2(NULL,4096,PROT_READ|PROT_WRITE, MAP_PRIVATE| MAP_ANONYMOUS,-1,0) = 0Xb78b4000
How to open, read, and write from serial port in C?
I am a little bit confused about reading and writing to a serial port. I have a USB device in Linux that uses the FTDI USB serial device converter driver. When I plug it in, it creates: /dev/ttyUSB1.
I thought itd be simple to open and read/write from it in C. I know the baud rate and parity information, but it seems like there is no standard for this?
Am I missing something, or can someone point me in the right direction?
2 Answers 2
I wrote this a long time ago (from years 1985-1992, with just a few tweaks since then), and just copy and paste the bits needed into each project.
You must call cfmakeraw on a tty obtained from tcgetattr . You cannot zero-out a struct termios , configure it, and then set the tty with tcsetattr . If you use the zero-out method, then you will experience unexplained intermittent failures, especially on the BSDs and OS X. «Unexplained intermittent failures» include hanging in read(3) .
The values for speed are B115200 , B230400 , B9600 , B19200 , B38400 , B57600 , B1200 , B2400 , B4800 , etc. The values for parity are 0 (meaning no parity), PARENB|PARODD (enable parity and use odd), PARENB (enable parity and use even), PARENB|PARODD|CMSPAR (mark parity), and PARENB|CMSPAR (space parity).
«Blocking» sets whether a read() on the port waits for the specified number of characters to arrive. Setting no blocking means that a read() returns however many characters are available without waiting for more, up to the buffer limit.
CMSPAR is needed only for choosing mark and space parity, which is uncommon. For most applications, it can be omitted. My header file /usr/include/bits/termios.h enables definition of CMSPAR only if the preprocessor symbol __USE_MISC is defined. That definition occurs (in features.h ) with
How to read from serial device in C++
I’m trying to figure out how I should read/write to my Arduino using serial communication in Linux C++. Currently I’m trying to read a response from my Arudino that I «trigger» with
I’ve tried looking at the response from my Arduino in my terminal, by using the following command:
This is working as it should. Now I want to do the same thing in a C++ application. So I made this test
After a few «messages», the transmission gets a little messed up. My test application writes the response characters in random order and something’s not right. I tried configuring the /dev/ttyACM0 device by this command:
No dice. Can someone help me understand how to communicate with my Arduino in C++?
1 Answer 1
The shown code opens /dev/ttyACM0 , attempts to seek to the end of this «file», and based on the resulting file position allocates an old-fashioned, C-style memory buffer.
The problem with this approach is that you can only seek through regular, plain, garden-variety files. /dev/ttyACM0 is not a regular file. It’s a device. Although some devices are seekable, this one isn’t. Which, according to the comments, you’ve discovered independently.
Serial port devices are readable and writable. They are not seekable. There’s no such thing as «seek»ing on a serial port. That makes no sense.
To read from the serial port you just read it, that’s all. The operating system does maintain an internal buffer of some size, so if some characters were already received over the serial port, the initial read will return them all (provided that the read() buffer size is sufficiently large). If you pass a 1024 character buffer, for example, and five characters were already read from the serial port read() will return 5, to indicate that accordingly.
If no characters have been read, and you opened the serial port as a blocking device, read() will block at least until one character has been read from the serial port, and then return.
So, in order to read from the serial port all you have to do is read from it, until you’ve decided that you’ve read all there is to read from it. How do you decide that? That’s up to you. You may decide that you want to read only until reading a newline character. Or you may decide that you want to read only until a fixed #n number of characters have been read. That’s entirely up to you.
And, of course, if the hardware is suitably arranged, and you make the necessary arrangements with the serial port device to respect the serial port control pins, and, depending on your configuration, the DCD and/or DSR pins are signaled to indicate that the serial port device is no longer available, your read() will immediately return 0, to indicate a pseudo-end of file condition on the serial port device. That’s also something that you will need to implement the necessary logic to handle.
Read from serial port linux
I’m trying to read from serial port, but always get 0 (zero) characters back. Already read the «Serial Programming Guide for POSIX Operating Systems», but can’t find out why the program not waiting (blocking). The code:
2 Answers 2
This information was originally from the Serial Programming Guide 1 .
The reason you are getting a 0 return value is because of this line:
If you want a normal blocking read, unset that flag.
You are using O_NDELAY
O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking mode. Neither the open() nor any subsequent operations on the file descriptor which is returned will cause the calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discussion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).
EDIT: You’re doing the same thing in your fcntl() call, as well.
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Reading and writing to serial port in C on Linux
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.
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
right after write() call, I receive:
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
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.
How do I read data from serial port in Linux using C?
I am new to serial programming in Linux using C. I have found a small piece of code to write data on serial port which I am sharing here. After running this code I may assume that data has written on a specific port. Now I would like to open another terminal and using separate code want to read the data written on that specific port — how do I do that?
The code above will write the data on a specific port.
2 Answers 2
In theory, all you have to do is open the relevant port for reading, and use read() to get the data.
There are differences; notably, the read needs a buffer to put the data in. The code shown discards the first message read. Note that a short read simply indicates that there was less data available than requested at the time when the read completed. It does not automatically indicate an error. Think of a command line; some commands might be one or two characters ( ls ) where others might be quite complex ( find /some/where -name ‘*.pdf’ -mtime -3 -print ). The fact that the same buffer is used to read both isn’t a problem; one read gives 3 characters (newline is included), the other 47 or so.