I am trying to write an audio application using PortAudio, but if any other audio programs (usually Firefox) are running at the time which I try to run my program, I get the following error:
PaHost_OpenStream: could not open /dev/dsp for O_WRONLY
PaHost_OpenStream: ERROR - result = -10000
An error occured while using the portaudio stream
Error number: -10000
Error message: Host error.
Obviously, this makes my program pretty useless since it won't work if another program is using sound. Is there a way to get around this or should I just not use PortAudio?
You need to choose a device named "pulse" for PortAudio to work with PulseAudio, which is the sound server used for sound card sharing on the biggest Linux distros nowadays. The error message suggests that it is trying to use the OSS /dev/dsp interface, which does not support card sharing at all.
You can use code like this for listing the devices:
for (int i = 0, end = Pa_GetDeviceCount(); i != end; ++i) {
PaDeviceInfo const* info = Pa_GetDeviceInfo(i);
if (!info) continue;
printf("%d: %s\n", i, info->name);
}
Then supply the right number to OpenStream within stream parameter.
Notice that you need PortAudio v19. The older v18 only supported OSS.
Related
I would like to get a list of the wireless networks available. Ideally this would be via some C call, but I don't mind if I have to kludge it with a system call. Even better if the required C call or program doesn't require some exotic 3rd party package.
The internet seems to suggest I use sudo iwlist <interface> scan which does seem to do the trick from the command line, but I'd rather not require root permissions. I only want to see the basics, not change anything.
It's pretty easy to do a scan in the command line. The man pages are your friend here (check out iwconfig and iwlist). But using the C interface is a little more difficult so I'll focus on that.
First of all, as other people have mentioned, definitely download out the wireless tools source code. All the documentation for the programming interface is in the .c files. As far as I can tell, there is no web documentation for the api. However, the source code is pretty easy to read through. You pretty much only need iwlib.h and iwlib.c for this question.
While you can use iw_set_ext and iw_get_ext, the libiw implements a basic scanning function iw_scan, from which you can extract most of the information that you need.
Here is a simple program to get the ESSID for all available wireless networks. Compile with -liw and run with sudo.
#include <stdio.h>
#include <time.h>
#include <iwlib.h>
int main(void) {
wireless_scan_head head;
wireless_scan *result;
iwrange range;
int sock;
/* Open socket to kernel */
sock = iw_sockets_open();
/* Get some metadata to use for scanning */
if (iw_get_range_info(sock, "wlan0", &range) < 0) {
printf("Error during iw_get_range_info. Aborting.\n");
exit(2);
}
/* Perform the scan */
if (iw_scan(sock, "wlan0", range.we_version_compiled, &head) < 0) {
printf("Error during iw_scan. Aborting.\n");
exit(2);
}
/* Traverse the results */
result = head.result;
while (NULL != result) {
printf("%s\n", result->b.essid);
result = result->next;
}
exit(0);
}
DISCLAIMER: This is just a demonstration program. It's possible for some results to not have an essid. In addition, this assumes your wireless interface is "wlan0". You get the idea.
Read the iwlib source code!
The Wireless Tools package -- of which iwlist is a part -- also contains a Wireless Tools Helper Library. You need to include iwlib.h and link with libiw.a (i.e. add -liw). Then look up the documentation for the iw_set_ext function. The SIOCSIWSCAN parameter will be of most use. For an example of how to use this interface, take a look at the KWifiManager source in the KDE library (see: Interface_wireless_wirelessextensions::get_available_networks method). Alternatively, you can also download the Wireless Tools source code and take a look at how the iwlib iw_set_ext function is also used for scanning in iwlist.c.
As for privileges, I imagine the process will need to run as root to perform the scan. I'd love to know if this could be done otherwise as well.
Since you are using Ubuntu 8.04 the libiw-dev package should be of use.
You can use nmcli which does not require root permissions or name of WIFI interface.
nmcli -t -f ssid dev wifi
Is there a way to close the device/socket in VxWorks programmatically?
Meaning say I have the devices /tyco/0, /tyco/1 and /tyco/2 and I want to close/shutdown /tyco/1 and /tyco/2.
I would like to do something like remove("/tyco/1"). Something that would prevent even an open("/tyco/1") call later on in the code or from an outside source from opening the socket.
All devices available to VxWorks are part of the device list. The device list is accessible using the iosLib.
I've used the following code a lot to remove devices to generate errors in order to test my programs:
DEV_HDR *pDevice;
pDevice = iosDevFind("/xyz", NULL);
if (pDevice != NULL)
{
iosDevDelete(pDevice);
}
This works for all devices listed by the devs command which in your case will also work for "/tyco". I doubt that you can inhibit open calls to "/tyco/1" and "/tyco/2" but allow calls to "/tyco/0" using that method since it works on "devices".
If "/tyco/0" is your serial interface to the VxWorks shell then the method from above will work. Because removing a device from the device list will cause all following open calls to that device to fail but will not close already opened devices...
I am trying to understand basics of RPC using RPCGen. I followed a basic tutorial and wrote the follwing myrpc.x file
program MESSAGEPROG {
version EVALMESSAGEVERS {
int EVALMESSAGE(string) = 1;
} = 1;
} = 0x20000002;
I compile it by running
rpcgen -a -C myrpc.x
In the resulting server.c file, I added a printf statement as below
printf("Message is: %s,\n", *argp);
Then i run make -f Makefile.myrpc and start the server by running myrpc_server. Now when i run the client 'myrpc_client', I get the following message printed in the server
Message is: H���5�
Now my question is from where does this argument come from "H���5�" as this is not the argument which i am when running the client? Also can someone explain me how do i start running complex programs with rpcgen?
The garbage value is from code on line 15 in client.c, where is uninitialized variable used as an argument for your rpc call. My version of rpc show an error:
call failed: RPC: Can't encode arguments"
15 char * evalmessage_1_arg;
"How do I start running complex programs with rpc?" It' just on you. We cannot say when you need to use rpc. You probably have some reason for what you chose this implementation.
Some use case for rpc is thin client on slow computer, which needs some expensive computation. Client sends data to powerful server, that do the hard work and returns result.
is there an example of a full-duplex ALSA connection in C? I've read that it is supported, but all the introductory examples I saw did either record or play a sound sample, but I'd like to have one handler that can do both for my VoIP-app.
Big thanks for help,
Jens
Some guy named Alan has published this good (but old) tutorial, Full Duplex ALSA, which is written in C.
You provide a link to both handles and pump them in turn.
Here's alan's code elided and commented.
// the device plughw handle dynamic sample rate and type conversion.
// there are a range of alternate devices defined in your alsa.conf
// try:
// locate alsa.conf
// and check out what devices you have in there
//
// The following device is PLUG:HW:Device:0:Subdevice:0
// Often simply plug, plughw, plughw:0, will have the same effect
//
char *snd_device_in = "plughw:0,0";
char *snd_device_out = "plughw:0,0";
// handle constructs to populate with our links
snd_pcm_t *playback_handle;
snd_pcm_t *capture_handle;
//this is the usual construct... If not fail BLAH
if ((err = snd_pcm_open(&playback_handle, snd_device_out, SND_PCM_STREAM_PLAYBACK, 0)) < 0) {
fprintf(stderr, "cannot open output audio device %s: %s\n", snd_device_in, snd_strerror(err));
exit(1);
}
// And now the CAPTURE
if ((err = snd_pcm_open(&capture_handle, snd_device_in, SND_PCM_STREAM_CAPTURE, 0)) < 0) {
fprintf(stderr, "cannot open input audio device %s: %s\n", snd_device_out, snd_strerror(err));
exit(1);
}
then config and pump them.
A ring mod could do the job: http://soundprogramming.net/programming_and_apis/creating_a_ring_buffer or you could use alans way outlined above.
It was my first requirements to a Linux/Unix VoIP projects where I need to know about all of the available audio devices capability and name. Then I need to use these devices to capture and playback the audio.
For everyone's help I have made a (.so) library and a sample Application demonstrating the use of this library in c++.
The output of my library is like-
[root#~]# ./IdeaAudioEngineTest
HDA Intel plughw:0,0
HDA Intel plughw:0,2
USB Audio Device plughw:1,0
The library provides functionality to capture and playback real-time audio data.
Full source with documentation is available in IdeaAudio library with Duplex Alsa Audio
Library source is now open at github.com
See also latency.c, included in alsa-lib source; on the ALSA wiki:
http://www.alsa-project.org/main/index.php/Test_latency.c
I am using VC++ 2008 express edition for C. When I try to run this:
/* Demonstrates printer output. */
#include <stdio.h>
main()
{
float f = 2.0134;
fprintf(stdprn, "This message is printed.\n\n");
fprintf(stdprn, "And now some numbers:\n\n");
fprintf(stdprn, "The square of %f is %f.", f, f*f);
/* Send a form feed */
fprintf(stdprn, "\f");
}
I get four of these errors: error C2065: 'stdprn' : undeclared identifier.
On this forum, they wrote that it works to define the printer as follows:
FILE *printer;
printer = fopen("PRN", "w");
EDIT
It builds with a warning that fopen is unsafe. When it runs the error appears:
Debug Assertion fails.
File: f:\dd\vctools\crt_bld\self_x86\crt\src\fprintf.c
Line: 55
Expression: (str != NULL)
The stdprn stream was an extension provided by Borland compilers - as far as I know, MS have never supported it. Regarding the use of fopen to open the printer device, I don't think this will work with any recent versions of Windows, but a couple of things to try:
use PRN: as the name instead of PRN (note the colon)
try opening the specific device using (for example) LPT1: (once again, note the colon). This will of course not work if you don't have a printer attached.
don't depend on a printer dialog coming up - you are not really using the WIndows printing system when you take this approach (and so it probably won't solve your problem, but is worth a try).
I do not have a printer attached, but I do have the Microsoft XPS document writer installed, s it shoulod at least bring up the standard Windows Print dialog from which one can choose the printer.
No. It wouldn't bring up a dialogue. This is because you are flushing data out to a file. And not going through the circuitous Win32 API.
The print doesn't work because the data is not proper PDL -- something that the printer could understand. For the print to work fine, you need to push in a PDL file, with language specific constructs. This varies from printer to printer, a PS printer will need you to push in a PostScript snippet, a PCL -- a PCL command-set and in case of MXDW you will have to write up XML based page description markup and create a zip file (with all resources embedded in it) i.e. an XPS file to get proper printout.
The PDL constructs are important because otherwise the printer doesn't know where to put the data, which color to print it on, what orientation to use, how many copies to print and so on and so forth.
Edit: I am curious why you are doing this. I understand portability is probably something you are trying to address. But apart from that, I'd like to know, there may be better alternatives available. Win32 Print Subsytem APIs are something that you ought to lookup if you are trying to print programmatically on Windows with any degree of fidelity.
Edit#2:
EDIT It builds with a warning that fopen is unsafe.
This is because MS suggests you use the safer versions nowadays fopen_s . See Security Enhancements in the CRT.
When it runs the error appears:
Debug Assertion fails. File: f:\dd\vctools\crt_bld\self_x86\crt\src\fprintf.c Line: 55
Expression: (str != NULL)
This is because fopen (whose return value you do not check) returns a NULL pointer. The file open failed. Also, if it did succeed a matching fclose call is called for.
There's no such thing as stdprn in ANSI C, it was a nonstandard extension provided by some compilers many years ago.
Today to print you have to use the specific APIs provided on your platform; to print on Windows you have to use the printing APIs to manage the printing of the document and obtain a DC to the printer and the GDI APIs to perform the actual drawing on the DC.
On UNIX-like OSes, instead, usually CUPS is used.
You can substitute the printer using this command with net use, see here on the MSDN kb
NET USE LPT1 \\server_name\printer_name
There is an excellent chapter on printing in DOS using the BIOS, ok, its a bit antiquated but interesting to read purely for nostalgic sake.
Onto your problem, you may need to use CreateFile to open the LPT1 port, see here for an example, I have it duplicated it here, for your benefit.
HANDLE hFile;
hFile = CreateFile("LPT1", GENERIC_WRITE, 0,NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL);
if (hFile == INVALID_HANDLE_VALUE)
{
// handle error
}
OVERLAPPED ov = {};
ov.hEvent = CreateEvent(0, false, false, 0);
char szData[] = "1234567890";
DWORD p;
if (!WriteFile(hFile,szData, 10, &p, &ov))
{
if (GetLastError() != ERROR_IO_PENDING)
{
// handle error
}
}
// Wait for write op to complete (maximum 3 second)
DWORD dwWait = WaitForSingleObject(ov.hEvent, 3000);
if (dwWait == WAIT_TIMEOUT)
{
// it took more than 3 seconds
} else if (dwWait == WAIT_OBJECT_0)
{
// the write op completed,
// call GetOverlappedResult(...)
}
CloseHandle(ov.hEvent);
CloseHandle(hFile);
But if you insist on opening the LPT1 port directly, error checking is omitted...
FILE *prn = fopen("lpt1", "w");
fprintf(prn, "Hello World\n\f");
fclose(prn);
Hope this helps,
Best regards,
Tom.