I write an xlib C application. I need different behavior of it, depending if it running with xinit ./myapp from CLI when no X server started, from running in normal KDE desktop.
argv[0] returns same value in both cases (i.e. not sees xinit prefix).
How i can determine when xinit was used for starting my app?
Thanks for comments. I do not know how to describe my task more precisely, not really i need about exactly xinit (or X server or so). Maybe yes i need to know is Window manager is running. In fact i just need to determine if kwin's color inversion is active (and this is answer why i need to detect, even when i know that A X program should not have to care under which window manager it is displayed. in URL above). But it is quite complex to determine inversion exactly. But because inversion is turned on permanently in KDE for me, i can maybe test for Window manager is running...
Currently, using suggested solution, i was able to solve it at least for my particular case. But not sure if it portable enough.
fp = popen("/usr/bin/xprop -root _NET_SUPPORTING_WM_CHECK", "r");
if (fp == NULL) {
printf("Failed to run `/usr/bin/xprop -root _NET_SUPPORTING_WM_CHECK`\n" );
exit(1);
}
fgets(cmdout, sizeof(cmdout), fp);
pclose(fp);
i = strncmp(cmdout, "_NET_SUPPORTING_WM_CHECK(WINDOW): window id #", 40);
if (i == 0) {
inverted_colors = 1;
printf ("Window manager detected.\n");
} else {
inverted_colors = 0;
printf ("No window manager detected.\n");
}
Related
I'm working on an assignment that involves creating files and manipulating characters within them using what the lecturer describes as "File and System I/O calls" in C.
Specifically, I am using open, creat, read, write, lseek, close, and unlink.
Without revealing too much and violating academic integrity, essentially the assignment just entails creating files, copying characters between them, changing the characters, etc.
All of this was going perfectly until a certain point of the program after which read just... Wouldn't work. It doesn't matter what file I am accessing, or what other calls I put before it- for instance, I have tried closing and re-opening a file before trying to read, writing before reading (worked perfectly, put the characters right at the file position the lseek intended), etc.
I have checked the returns of all previous commands, none are giving errors other than the read, which is giving me errorno 9. Having looked this up, it appears to refer to having an incorrect file descriptor, but this doesn't make sense to me as I can use the same fid for any other command. Using ls -l, I confirmed that I have read and write permission (groups and public do not, if that helps).
I am at a total loss of where to go troubleshooting from here, any help would be immensely appreciated. Here is the code snippet in question:
readStatus = lseek(WWWfid,500,0);
if (readStatus<0) {
printf("error with lseek");
return 0;
}
/*printf("Read status: %i\n", readStatus);/*DEBUG*/
writeStatus = write(WWWfid, "wtf", 3);
if (writeStatus<0) {
printf("error with write");
return 0;
}
readStatus = read(WWWfid, buffer, 26);
int errorNum = errno;
/*buffer[27] = '\0';*/
if (readStatus<0) {
printf("error with read before loop, error %i\n", errorNum);
return 0;
}
I can likely include more without invoking the wroth of my uni but I'd prefer not to- also seems likely to be irrelevant given that all proceeding code appears to be functioning correctly.
Thanks for reading, please let me know if you have any insight at all
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
I'm writing a plugin for my statusbar to print MPD state, currently using the libmpdclient library. It has to be robust to properly handle lost connections in case MPD is restarted, but simple checking with mpd_connection_get_error on existing mpd_connection object does not work – it can detect the error only when the initial mpd_connection_new fails.
This is a simplified code I'm working with:
#include <stdio.h>
#include <unistd.h>
#include <mpd/client.h>
int main(void) {
struct mpd_connection* m_connection = NULL;
struct mpd_status* m_status = NULL;
char* m_state_str;
m_connection = mpd_connection_new(NULL, 0, 30000);
while (1) {
// this check works only on start up (i.e. when mpd_connection_new failed),
// not when the connection is lost later
if (mpd_connection_get_error(m_connection) != MPD_ERROR_SUCCESS) {
fprintf(stderr, "Could not connect to MPD: %s\n", mpd_connection_get_error_message(m_connection));
mpd_connection_free(m_connection);
m_connection = NULL;
}
m_status = mpd_run_status(m_connection);
if (mpd_status_get_state(m_status) == MPD_STATE_PLAY) {
m_state_str = "playing";
} else if (mpd_status_get_state(m_status) == MPD_STATE_STOP) {
m_state_str = "stopped";
} else if (mpd_status_get_state(m_status) == MPD_STATE_PAUSE) {
m_state_str = "paused";
} else {
m_state_str = "unknown";
}
printf("MPD state: %s\n", m_state_str);
sleep(1);
}
}
When MPD is stopped during the execution of the above program, it segfaults with:
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007fb2fd9557e0 in mpd_status_get_state () from /usr/lib/libmpdclient.so.2
The only way I can think of to make the program safe is to establish a new connection in every iteration, which I was hoping to avoid. But then what if the connection is lost between individual calls to libmpdclient functions? How often, and more importantly how exactly, should I check if the connection is still alive?
The only way I could find that really works (beyond reestablishing a connection with each run) is using the idle command. If mpd_recv_idle (or mpd_run_idle) returns 0, it is an error condition, and you can take that as a cue to free your connection and run from there.
It's not a perfect solution, but it does let you keep a live connection between runs, and it helps you avoid segfaults (though I don't think you can completely avoid them, because if you send a command and mpd is killed before you recv it, I'm pretty sure the library still segfaults). I'm not sure if there is a better solution. It would be fantastic if there was a reliable way to detect if your connection was still alive via the API, but I can't find anything of the sort. It doesn't seem like libmpdclient is well-built for very long-lived connections that have to deal with mpd instances that go up and down over time.
Another lower-level option is to use sockets to interact with MPD through its protocol directly, though in doing that you'd likely reimplement much of libmpdclient itself anyway.
EDIT: Unfortunately, the idle command does block until something happens, and can sit blocking for as long as a single audio track will last, so if you need your program to do other things in the interim, you have to find a way to implement it asynchronously or in another thread.
Assuming "conn" is a connection created with "mpd_connection_new":
if (mpd_connection_get_error(conn) == MPD_ERROR_CLOSED) {
// mpd_connection_get_error_message(conn)
// will return "Connection closed by the server"
}
You can run this check after almost any libmpdclient call, including "mpd_recv_idle" or (as per your example) "mpd_run_status".
I'm using libmpdclient 2.18, and this certainly works for me.
I'm trying to make a game in C using SDL_ttf to display the score every time the diplay is refreshed. The code looks like :
SDL_Surface *score = NULL;
TTF_Font *font;
SDL_Color color = { 255, 255, 255 };
font = TTF_OpenFont( "/home/sophie/Bureau/snake/data/ubuntu.ttf", 28 );
if (font == NULL) {
printf("%s\n", TTF_GetError());
}
score = TTF_RenderText_Solid( font, "score to display", color );
SDL_BlitSurface( score, NULL, screen, NULL );
SDL_Flip(screen);
When I launch the game, everything works properly, but after a while the game crashes and I get the following error :
Couldn't open /home/sophie/Bureau/snake/data/ubuntu.ttf
libgcc_s.so.1 must be installed for pthread_cancel to work
Abandon (core dumped)
I tried different fonts but I still have this problem.
Then I used a counter in the main loop of the game and found that the game always crashes after the 1008th time, regardless of the speed I wanted it to work at (in snake everything goes faster when you score points).
I don't know where does the problem comes from, nor what exactly does the error message mean.
Please tell me if you have any ideas, or if my question is poorly formulated. I looked on several forums and found nothing corresponding to my case, I could use any help now !
Thanks in advance
It looks like you're repeatedly opening the font every time you go through this function:
font = TTF_OpenFont( "/home/sophie/Bureau/snake/data/ubuntu.ttf", 28 );
While it may not be in the main game loop as Jongware suspected, you mentioned that after 1008 executions through this code path, the code crashes.
What is happening is that some resource is being leaked. Either the resource needs to be released by calling TTF_CloseFont() or (more efficient) hold onto the handle after the first time you open it and re-use it each time. Use a static declaration for the font and initialize to NULL:
static TTF_Font *font = NULL;
Then, if it hasn't been opened yet, open it:
if (!font) {
font = TTF_OpenFont( "/home/sophie/Bureau/snake/data/ubuntu.ttf", 28 );
}
This will initialize font the first time while subsequent iterations over the code will not unnecessarily re-do the process and leak the resource.
You mentioned that the code crashes after 1008 times through this function. That's pretty close to 1024. As memory serves, Linux has a limit of 1024 file handles per process (this is probably tunable in the kernel but I have run into this limitation in debugging resource leaks before). There are probably 16 other file handles open by your process and then 1 process being leaked by each invocation of TTF_OpenFont. Once you go above 1024, boom.
You can check the number of open file handles of a particular process (<pid>) by inspecting the number of file descriptors in /proc/<pid>/fd/.
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.