Im writing a program which reads and writes to serial. I was wondering, is there a way to peek the next value from a tty device such as /dev/ttyS0 without removing it from the queue. I have looked for documentation or even source code for this module and have been so far, unable to find it. If anyone knows where it is, or which ioctls are available, please let me know.
Using a combination of getc and ungetc will allow you to peek a single byte.
If you need more than one byte, you'll need to buffer it manually.
Related
I'm new to ALSA and I've managed to get PCM sound played in SND_PCM_ACCESS_RW_INTERLEAVED mode. My problem is that I just can't find a way to make that mode useful for what I'm trying to do. (If someone can tell me how, I'll be glad to read). I've been reading there is this MMAP mode, but it's not as easy to find simple examples for it. I wonder if it is what I need and how I could implement it.
What I want to do is have my little game (a simple space shoot-up) to immediately play a sound when I shoot or get shot. If an enemy shoots while another sound is being played, the sounds should add up and saturate as necessary, but no sound event should be interrupted. In other words, I need to be able to edit the very byte that's about to be played.
In my useless attempts to try MMAP (without really knowing how it works in practice; just following vague theoretical instructions), I set up everything just like for SND_PCM_ACCESS_RW_INTERLEAVED, but change it to SND_PCM_ACCESS_MMAP_INTERLEAVED. Then I call snd_pcm_avail_update, which seems to work and returns a large number of available frames. After that, I call snd_pcm_mmap_begin, passing the parameters, previously filling "frames" with a reasonable number (a 10, for example). The function fails and returns an error code -77. I haven't been able to find what that means. The areas array remains unmodified.
What does that error mean? Where can I get a list of the errors? How can I overcome it? Is there a good, simple, example of how to use MMAP (or some other thing) to perform something more or less like what I'm trying to do?
I appreciate your help :)
ALSA returns negative values on error. 77 is most likely EBADFD which indicates that the device is in an invalid state (under/overrun or not running at all). In case of underrun you're probably using a too low buffersize.
In any case, there's no way to modify audio data that you've already submitted to the alsa driver (snd_pcm_mmap_commit/writei/writen). The trick to have audio sound immediately is just to use very low buffer sizes, < 10ms will do. For this you'll want to use hw: devices, other device types usually add latency.
You still have to mix sounds together manually before you pass them to alsa.
There's a nice mmap example in the comments on this question: Alsa api: how to use mmap in c?.
That being said, ALSA is a valid choice for this kind of application but you don't necessarily need to use memory mapping. Read/write access doesn't introduce additional latency, it just copies audio around a bit more.
As known, u-boot loader supports some sort of "input device system" that allows to register a set of functions (like getc, tstc, start, etc...). So I need to implement my own hardware-specific "keyboard". What should I do? Sounds simple:
1) Create at least "init", "tstc", "getc" and "start" functions in driver file.
2) In "init" call "stdio_register" to let system know about my device.
3) Call driver "init" from my custom board init code.
What the problem? Unfortunately, it doesn't work. stdio_register returns 0 (like all OK), but "start", "tstc", "getc" were never called.
I guess I'm doing something wrong, but I cannot understand: another keyboards drivers seems to not contain any additional code for keyboard registration. Can anybody tell the truth?
Or maybe I should just hack u-boot and insert polling call somewhere in main_loop? :)
It sounds like you have created a custom device driver.
struct stdio_dev kon_dev
Try something like this at U-Boot command line to switch console to the custom driver
setenv stdin kon_dev
Of course, your custom driver will have to be debugged before you get to the happy place. It may not work the first time.
Using netconsole is similar to what you're doing. With netconsole it's possible for example to keep stdout continuously on serial device, switch stdin to nc device (that is, "setenv stdin nc", enter commands through that netcat session a while, then switch stdin back to serial.
I'm writing a program to track the mouse movements in linux. I read in another post that this can be done using read() system call to read the EventX file related to the mouse. I earlier was reading the serial port file and i used the read() to read it. But, then i sent in a character array to it and got back the serial characters. But, it doesnt seem to be in the mouse's case. The lines:
struct input_event ie;
read(fd, &ie, sizeof(struct input_event)
are used to read it. Here the ie is a struct. But i used to send in a char buffer in the serial port case. So, my question is: how do I know what struct/buffer to send. I got to know the answer for the above two code lines after googling, but if I want to read some other file,how would i know what struct/buffer to send. Please help me.
Thank you.
The input subsystem in Linux uses a standarized format to deliver its messages. It is actually quite simple:
You open the relevant input file, usually /dev/input/event<n>, using the open() system call.
You read input events from that file, using the read() function, as you noted in your question.
Every event from that file has a well known structure: that is struct input_event. You don't need to know the exact layout of that structure, that is done by the compiler. Just include the relevant header file: #include <linux/input.h>.
What you do want to know are the fields of this structure that are useful, and what they mean. I recommend you to read the official documentation as well as the input.h source.
I have an assignment in my Operating Systems class to make a simple pseudo-stack Linux device driver. So for an example, if I was to write "Hello" to the device driver, it would return "olleH" when I read from it. We have to construct a tester program in C to just call upon the read/write functions of the device driver to just demonstrate that it functions in a FILO manner. I have done all of this, and my tester program, in my opinion, demonstrates the purpose of the assignment; however, out of curiosity, inside BASH I execute the following commands:
echo "Test" > /dev/driver
cat /dev/driver
where /dev/driver is the special file I created using "mknod". However, when I do this, I get a black screen full of errors. After I swap back to the GUI view using CNTRL+ALT+F7, I see that BASH has returned "Killed".
Does anyone know what could be causing this to happen? I am confused since my tester program calls open(), read(), and write() with everything functioning as it should.
If I need to show some code, just ask.
The function in your device driver that writes to the buffer you are providing it is most likely causing this issue.
To debug, you can do the following:
First, make sure the read part is fine. You can printk your internal buffer after you read from input to ensure this.
Second, in your write function, printk some information instead of actually writing anything and make sure everything is fine.
Also, make sure the writer makes it clear that the write has ended. I'm not particularly sure about device drivers, but you either need to return 0 as the number of bytes written when called a second time, or set an eof variable (if that is one of the arguments to your function)
This seems like a simple question, but it is difficult to search for. I need to interface with a device over the serial port. In the event my program (or another) does not finish writing a command to the device, how do I ensure the next run of the program can successfully send a command?
Example:
The foo program runs and begins writing "A_VERY_LONG_COMMAND"
The user terminates the program, but the program has only written, "A_VERY"
The user runs the program again, and the command is resent. Except, the device sees "A_VERYA_VERY_LONG_COMMAND," which isn't what we want.
Is there any way to make this more deterministic? Serial port programming feels very out-of-control due to issues like this.
The required method depends on the device.
Serial ports have additional control signal lines as well as the serial data line; perhaps one of them will reset the device's input. I've never done serial port programming but I think ioctl() handles this.
There may be a single byte which will reset, e.g. some control character.
There might be a timing-based signal, e.g. Hayes command set modems use “pause +++ pause”.
It might just reset after not receiving a complete command after a fixed time.
It might be useful to know whether the device was originally intended to support interactive use (serial terminal), control by a program, or both.
I would guess that if you call write("A_VERY_LONG_COMMAND"), and then the user hits Ctrl+C while the bytes are going out on the line, the driver layer should finish sending the full buffer. And if the user interrupts in the middle of the call, the driver layer will probably just ignore the whole thing.
Just in case, when you open a new COM port, it's always wise to clear the port.
Do you have control over the device end? It might make sense to implement a timeout to make the device ignore unfinished or otherwise corrupt packets.
The embedded device should be implemented such that you can either send an abort/clear/break character that will dump the contents of its command buffer and give you a clean slate on your client app startup.
Or else it should provide a software reset character which will reset the command buffer and all state.
Or else it so be designed so that you can send a command termination (perhaps a newline, etc, depending on command protocol) and possibly have an error generated on the parsing of a garbled partial command that was in its buffer, query/clear the error, and then be good to go.
It wouldn't be a bad idea upon connection of your client program to send some health/status/error query repeatedly until you get a sound response, and only then commence sending configuration or operation commands. Unless you can via a query determine that the device was left in a suitable state, you probably want to assume nothing and configure it from scratch, after a configuration reset if available.