I have little issue with XOpenDisplay function. In school I can run program and it works good when using XOpenDisplay("ip:0"), but on my local machine in home when I run program (changed ip on current) got "Segmentation fault (core dumped)", but with empy string XOpenDisplay("") it works fine. I need to be able to use ip. Used host +, but nothing changes.
My system is Kubuntu 14.04.1: 3.16.0-30-generic #40~14.04.1-Ubuntu SMP Thu Jan 15 17:43:14 UTC 2015
Here is code of program:
#include <X11/Xlib.h>
#include <X11/X.h>
#include <stdio.h>
Display *mydisplay;
Window mywindow;
XSetWindowAttributes mywindowattributes;
XGCValues mygcvalues;
GC mygc;
Visual *myvisual;
int mydepth;
int myscreen;
Colormap mycolormap;
XColor mycolor,mycolor1,dummy;
int i;
main()
{
mydisplay = XOpenDisplay("192.168.0.12:0");
myscreen = DefaultScreen(mydisplay);
myvisual = DefaultVisual(mydisplay,myscreen);
mydepth = DefaultDepth(mydisplay,myscreen);
mywindowattributes.background_pixel = XWhitePixel(mydisplay,myscreen);
mywindowattributes.override_redirect = True;
mywindow = XCreateWindow(mydisplay,XRootWindow(mydisplay,myscreen),
0,0,500,500,10,mydepth,InputOutput,
myvisual,CWBackPixel|CWOverrideRedirect,
&mywindowattributes);
mycolormap = DefaultColormap(mydisplay,myscreen);
XAllocNamedColor(mydisplay,mycolormap,"cyan",&mycolor,&dummy);
XAllocNamedColor(mydisplay,mycolormap,"red",&mycolor1,&dummy);
XMapWindow(mydisplay,mywindow);
mygc = DefaultGC(mydisplay,myscreen);
XSetForeground(mydisplay,mygc,mycolor.pixel);
XFillRectangle(mydisplay,mywindow,mygc,100,100,300,300);
XSetForeground(mydisplay,mygc,mycolor1.pixel);
XSetFunction(mydisplay,mygc,GXcopy);
XSetLineAttributes(mydisplay,mygc,10,LineSolid,CapProjecting,JoinMiter);
XDrawLine(mydisplay,mywindow,mygc,100,100,400,400);
XDrawLine(mydisplay,mywindow,mygc,100,400,400,100);
XFlush(mydisplay);
sleep(10);
XCloseDisplay(mydisplay);
exit(0);
}
I can only guess that need to set something, but have no idea where is that option.
You shall always check whether functions returned successfully, or not. It is not a Haskell, where all the checking done for you by monad, it is C. As for your particular case, the problem is that the function XOpenDisplay fails and returns null for you. In the next line you're trying to use DefaultScreen with the result. The DefaultScreen is defined as
#define DefaultScreen(dpy) ((dpy)->default_screen)
I.e. it just a macro, which using the first argument as a pointer. In your case it does ((0)->default_screen), i.e. it dereferencing the null pointer, and that leads to the segfault you see.
Also, about the XOpenDisplay("192.168.0.12:0"); — you didn't mentioned that you're trying to connect to another PC, so, if it's the same computer where the app running, try to call the function as XOpenDisplay("127.0.0.1:0");
UPD: okay, I tried to run the code at my PC, and the function doesn't work for me too. To find the reason I started the code under strace app, and saw
…
connect(3, {sa_family=AF_INET, sin_port=htons(6000), sin_addr=inet_addr("127.0.0.1")}, 16) = -1 ECONNREFUSED (Connection refused)
…
Aha! So, the app trying to connect to XServer, but Xserver refuses the connection. Actually, it have a security reason to disable it by default — so, that nobody would connect to your XServer from a network unless you specifically allowed it. For the function to work you need to launch your XServer with the option that allows such a connection. Right now DisplayManagers are the ones, who manages xsessions, so you need to set some option depending on your DM.
The solution for lightdm
Open the /etc/lightdm/lightdm.conf, and paste the line xserver-allow-tcp=true in the section [SeatDefaults](you will see it).
The solution for gdm
Edit the file /etc/gdm/gdm.schemas, you will find there something like
<schema>
<key>security/DisallowTCP</key>
<signature>b</signature>
<default>true</default>
</schema>
Change the true to false.
Related
I'm trying to make a daemon simulate a keypress. I got it already working for a non daemon process.
#include <xdo.h>
int main()
{
xdo_t * x = xdo_new(NULL);
xdo_enter_text_window(x, CURRENTWINDOW, "Hallo xdo!", 500000);
return 0;
}
If I try the same code for my daemon I get the following error
Error: Can't open display: (null)
Is there a way to still make it work with xdo or something else?
Your process must know the $DESKTOP environment variable specifying the desktop session to send these keys to, and yours doesn't seem to have that environment set.
Which also means you must realize it needs to have all the necessary privileges, which means access to ~/.Xauthority of the owner of the session, and the sockets in /tmp/.X11-unix
I have successfully connected and obtained data from Postgresql with my C application. My connector.pgc file looks like this:
#include <stdlib.h>
#include <stdio.h>
//#include "connect.h" // PROBLEM
int connect(); //NO PROBLEM
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
const char *target = "dbname#host:port";
const char *user = "user";
const char *passwd = "password";
EXEC SQL END DECLARE SECTION;
int main() {
EXEC SQL CONNECT TO :target AS con1 USER :user USING :passwd;
EXEC SQL SELECT pg_catalog.set_config('search_path', 'schema_name', false); EXEC SQL COMMIT;
EXEC SQL SET CONNECTION con1;
EXEC SQL SELECT current_database() INTO :dbname;
printf("current database is '%s'\n", dbname);
EXEC SQL DISCONNECT ALL;
// connect();
return 0;
}
// PROBLEM
//int connect(){
// printf("A\n");
// return 0;
//}
except, of course, username/password/schema_name/etc. are replaced with actual values.
I then do the following:
ecpg connector.pgc && gcc connector.c -o connectorXec -lecpg -L/usr/pgsql-12/lib && ./connectorXec
My operating system is a default CentOS 8.
ecpg --version returns ecpg (PostgreSQL) 13.2
psql --version returns psql (PostgreSQL) 12.8
The program compiles fine and I am able to connect without issues, the way I know this is printf extracts the correct database name, and I've also run other SQL SELECTS to confirm the data.
However, as soon as I uncomment the int connect() function, even without using it, it is called 3 times before the actual program, and I do not know why. To be more precise, I see the letter A being printed 3 times. Additionally, printf no longer displays the correct name, instead dbname is empty.
I tried putting int connect() function into a separate connect.c file, then linking it together with this program through connect.h, and the result is the same.
If I put the contents of main() into connect(), and then call it, the process just hangs indefinitely.
Truly, I am at a loss, and I do not have even the slightest idea of where to begin, since this situation seems impossible. I checked the connector.c file that ECPG produces and there are no additional calls. Most references I can find online contain only main() function, and in rare cases where they do not, there is no mention of any remotely similar issue.
Any ideas as to what's going on?
OK, so it turns out the connect() function is used somewhere, since renaming it solved the issue. I was renaming it in the created connector.c file, which is how I missed this obvious solution.
Laurenz Albe said that he tried this on his end and everything is fine, which means it is somehow unique to my setup.
To introduce my problem, I'm currently working on a project to remotely shut down stationary PCs via Siemens S7 PLC. Those PCs are used for an experimental manufactoring line for relays at my university. The principle is quiet simple, the PLC sends the IP of a computer via UDP to a special "always on" PC with monitoring functions on which a UDP server listens for packets (this PC starts up together with the manufactoring line; OS is Windows 10; the server is written in C). If it receives an UDP packet, it triggers a net use command followed by a shutdown command to that specific IP. This works just fine, if the server.exe is started manually. The goal is to get the server working when it's automatically started with e.g. the taskplaner. Exactly this is the main issue here, it's not working as background task. It receives the packets (I tested it) but then nothing happens, no computer shuts down. So I guessed it must be the net use or the shutdown command. At first I tried to set up the net use with a system()call:
char command[100] = { 0 };
snprintf(command, sizeof(command), "#NET USE \\\\%s\\IPC$ %s /USER:%s", ip, pw, usr);
system(command);
As this won't work I found following statement for system()on the Windows API reference page:
This API cannot be used in applications that execute in the Windows Runtime.
So I figured I had to find an alternative, which leads me to my next try with the ShellExecute() function. It seemed like there are no problems concerning the execution out of the runtime, cause I could't find any word about it at the reference page. I tried:
char programpath[100] = "C:\\Windows\\System32\\net.exe";
char cmdline[100] = { 0 };
snprintf(cmdline, sizeof(cmdline), "Use \\\\%s\\IPC$ %s /USER:%s", ip, pw, usr);
ShellExecute(NULL, "open", programpath, cmdline, NULL, SW_HIDE);
But nope, won't work either from the background. When I asked my prof about it, he said he is more into Linux, so he could only guess that there might be a problem with the permissions. He meant that my server has possibly no rights to open those two programs, when calling them as background process. But even after a long time of investigating through the internet, I can't find a proper solution which fits to my problem.
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 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.