Xlib perversity

[MK27]

Anyone used Xlib much? I just want to ring the speaker/bell/beeeeep with it.

#include <stdio.h>
#include <X11/Xlib.h>

int main(void) {
	Display *display = NULL;
	
	perror("Now");
	if ((display = XOpenDisplay(":0.0")) == NULL) {
		perror("Can't connect to server");
	}
	printf("%p\n",display);

	perror("Now");
	XBell(display,100);
	perror("Now");
	return 0;
}

Complile -lX11.

What's weird to my mind here is this output:

Now: Success
0xf3a010
Now: Resource temporarily unavailable
Now: Resource temporarily unavailable

Apparently errno is getting set before the XBell call, but XOpenDisplay returned a valid pointer...

[brewbuck]

It might seem odd, but this is normal behavior for errno. You are never expected to clear errno upon success, only to set it upon failure.

The general rule is to rely on the API's method for reporting error status, and ONLY if an error occurred can you examine errno. For the scant few functions where errno is the only way of indicating failure, you should actually set errno = 0 yourself before calling those functions.

Probably, inside XOpenDisplay(), the client tries several different strategies to find the server, and one or more of these resulted in an error.

Ask yourself, have you made it a habit to continuously set errno to 0 whenever you try something and it succeeds? Nobody else does that either, including library code.

[MK27]

Probably, inside XOpenDisplay(), the client tries several different strategies to find the server, and one or more of these resulted in an error.

Okay, that makes sense. I did actually try setting errno before I posted, the outcome is much the same obviously...but I was not aware of this explanation. Thanks brewbuck. Seems strange to me that functions that could set errno would not themselves clear it first, as you say.

Anyway my bell still doesn't ring!

[Kennedy]

I would guess that XOpenDisplay() doesn't set errno. I would think it would be more on the lines of errno is set by a function that XOpenDisplay() uses. I'm basing this on the following

If successful, XOpenDisplay() returns a pointer to a Display structure, which is defined in X11/Xlib.h. If XOpenDisplay() does not succeed, it returns NULL. After a successful call to XOpenDisplay() all of the screens in the display can be used by the client. The screen number specified in the display_name argument is returned by the DefaultScreen() macro (or the XDefaultScreen() function). You can access elements of the Display and Screen structures only by using the information macros or functions. For information about using macros and functions to obtain information from the Display structure, see Display Macros.

Note that it says on failure it returns NULL and never mentions errno. Probably there is a couple calls to open(), ioctl(), bind() etc.