I don't understand the behavior of fonts in Xwindows - c

Question

Given the sample program:
/*
* Study for multiple windows.
*/
#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
Window w1, w2;
XEvent e;
const char* msg1 = "Hello, window 1";
const char* msg2 = "Hello, window 2";
int s1;
int s2;
GC gracxt1;
GC gracxt2;
XFontStruct* font;
XFontStruct* font2;
Display* d1;
Display* d2;
d1 = XOpenDisplay(NULL);
if (d1 == NULL) {
fprintf(stderr, "Cannot open display\n");
exit(1);
}
printf("d1: %p\n", d1);
d2 = XOpenDisplay(NULL);
if (d2 == NULL) {
fprintf(stderr, "Cannot open display\n");
exit(1);
}
printf("d2: %p\n", d2);
s1 = DefaultScreen(d1);
s2 = DefaultScreen(d1);
printf("s1: %d\n", s1);
printf("s2: %d\n", s2);
gracxt1 = XDefaultGC(d1, s1);
#if 0
gracxt2 = XDefaultGC(d2, s2);
#else
gracxt2 = XDefaultGC(d1, s2);
#endif
font = XLoadQueryFont(d1,
"-bitstream-courier 10 pitch-bold-r-normal--0-0-200-200-m-0-iso8859-1");
if (!font) {
fprintf(stderr, "*** No font ***\n");
exit(1);
}
XSetFont(d1, gracxt1, font->fid);
font2 = XLoadQueryFont(d1,
"-bitstream-courier 10 pitch-bold-r-normal--0-0-400-400-m-0-iso8859-1");
if (!font2) {
fprintf(stderr, "*** No font ***\n");
exit(1);
}
XSetFont(d1, gracxt2, font2->fid);
w1 = XCreateSimpleWindow(d1, RootWindow(d1, s1), 10, 10, 640, 480, 5,
BlackPixel(d1, s1), WhitePixel(d1, s1));
XSelectInput(d1, w1, ExposureMask|KeyPressMask|PointerMotionMask|StructureNotifyMask);
XMapWindow(d1, w1);
w2 = XCreateSimpleWindow(d1, RootWindow(d1, s1), 10, 10, 640, 480, 5,
BlackPixel(d1, s2), WhitePixel(d1, s2));
XSelectInput(d1, w2, ExposureMask|KeyPressMask|PointerMotionMask|StructureNotifyMask);
XMapWindow(d1, w2);
while (1) {
XNextEvent(d1, &e);
if (e.type == Expose) {
if (e.xany.window == w1) XDrawString(d1, e.xany.window, gracxt1, 10, 50, msg1, strlen(msg1));
else XDrawString(d1, e.xany.window, gracxt2, 10, 50, msg2, strlen(msg2));
}
if (e.type == KeyPress) break; /* exit on any key */
}
XCloseDisplay(d1);
XCloseDisplay(d2);
return 0;
}
It sets up two windows, and selects two different fonts, one for each window, and prints. My understanding of XWindows is that the font is selected into the graphics context, the GC. However, I get two different behaviors depending on which Display* is used:
#if 1
IE, each window with a different size font, or:
#if 0
I would think, and the documentation strongly implies, that the font is selected to the context gracxt1 or gracxt2 by the XSetFont() call. Thus I started this example with two GCs, but one Display* and one screen select. However, I don't get the behavior of font select per window until I give each window its own Display*.
The prints show:
d1: 0x55a5694692a0
d2: 0x55a569477170
s1: 0
s2: 0
For both, distinct Display* but same screen indexes. So where is the font select for the window stored? The Display*?
The documentation says "the font is selected to the GC, and the display just specifies the connection to the X server".
OS: Ubuntu 20.04
I printed the GContext numbers for each of gracxt1 and gracxt2:
#if 1 (working case)
d1: 0x55b4ae3d82a0
d2: 0x55b4ae3e6170
s1: 0
s2: 0
GCContext 1: 69206016
GCContext 2: 71303168
#if 0 (failing case)
d1: 0x55b4149672a0
d2: 0x55b414975170
s1: 0
s2: 0
GCContext 1: 69206016
GCContext 2: 69206016
The program refactored to function correctly:
/*
* Study for multiple windows.
*/
#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
Window w1, w2;
GC gracxt1, gracxt2;
XEvent e;
const char* msg1 = "Hello, window 1";
const char* msg2 = "Hello, window 2";
int s;
XFontStruct* font;
Display* d;
d = XOpenDisplay(NULL);
if (d == NULL) {
fprintf(stderr, "Cannot open display\n");
exit(1);
}
s = DefaultScreen(d);
s = DefaultScreen(d);
w1 = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 640, 480, 5,
BlackPixel(d, s), WhitePixel(d, s));
XSelectInput(d, w1, ExposureMask|KeyPressMask|PointerMotionMask|StructureNotifyMask);
XMapWindow(d, w1);
gracxt1 = XCreateGC(d, w1, 0, NULL);
w2 = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 640, 480, 5,
BlackPixel(d, s), WhitePixel(d, s));
XSelectInput(d, w2, ExposureMask|KeyPressMask|PointerMotionMask|StructureNotifyMask);
XMapWindow(d, w2);
gracxt2 = XCreateGC(d, w2, 0, NULL);
font = XLoadQueryFont(d,
"-bitstream-courier 10 pitch-bold-r-normal--0-0-200-200-m-0-iso8859-1");
if (!font) {
fprintf(stderr, "*** No font ***\n");
exit(1);
}
XSetFont(d, gracxt1, font->fid);
font = XLoadQueryFont(d,
"-bitstream-courier 10 pitch-bold-r-normal--0-0-400-400-m-0-iso8859-1");
if (!font) {
fprintf(stderr, "*** No font ***\n");
exit(1);
}
XSetFont(d, gracxt2, font->fid);
while (1) {
XNextEvent(d, &e);
if (e.type == Expose) {
if (e.xany.window == w1) XDrawString(d, e.xany.window, gracxt1, 10, 50, msg1, strlen(msg1));
else XDrawString(d, e.xany.window, gracxt2, 10, 50, msg2, strlen(msg2));
}
if (e.type == KeyPress) break; /* exit on any key */
}
XCloseDisplay(d);
return 0;
}

Answer 1

The font is part of the graphics context. Every graphics context has its own independent font setting.
I think your post has reversed the behaviors of the #if 0 and #if 1 cases. If so, then the behaviors you describe make perfect sense.
XOpenDisplay creates one graphics context per screen after connecting to the server. So d1 has a graphic context for screen 0, and d2 has a separate graphics context for screen 0. Therefore:
In both cases, gracxt1 uses the graphics context of d1 screen 0.
In the #if 0 case, gracxt2 uses the graphics context of d2 screen 0, which is separate from gracxt1. You set each GC's font to a different value. This is the behavior we see in your first screen shot: the two GCs have separate fonts.
In the #if 1 case, gracxt2 uses the graphics context of d1 screen 0, which is the same as gracxt1. When you set gracxt2's font, you are also setting gracxt1's font, overriding the earlier setting (because they are the same GC). This is the behavior we see in your second screen shot.