Make liberty-xui load PNG program icons

X11 applications now have a dependency on libpng.

This makes use of a new related liberty-xdg module,
which can be used separately.

2 files changed, 808 insertions, 8 deletions

diff --git a/liberty-xdg.c b/liberty-xdg.c
new file mode 100644
index 0000000..2c95668
--- /dev/null
+++ b/liberty-xdg.c
@@ -0,0 +1,766 @@
+/*
+ * liberty-xdg.c: the ultimate C unlibrary: freedesktop.org specifications
+ *
+ * Copyright (c) 2023 - 2024, Přemysl Eric Janouch <p@janouch.name>
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ */
+
+// This files assumes you've already included liberty.c.
+
+#ifdef LIBERTY_XDG_WANT_X11
+#include <X11/Xatom.h>
+#include <X11/Xlib.h>
+
+#include <png.h>
+#endif // LIBERTY_XDG_WANT_X11
+
+// --- XSettings ---------------------------------------------------------------
+
+#ifdef LIBERTY_XDG_WANT_X11
+
+struct xdg_xsettings_setting
+{
+	enum xdg_xsettings_type
+	{
+		XDG_XSETTINGS_INTEGER,
+		XDG_XSETTINGS_STRING,
+		XDG_XSETTINGS_COLOR,
+	}
+	type;                               ///< What's stored in the union
+	uint32_t serial;                    ///< Serial of the last change
+	union
+	{
+		int32_t integer;
+		struct str string;
+		struct { uint16_t red, green, blue, alpha; } color;
+	};
+};
+
+static void
+xdg_xsettings_setting_destroy (struct xdg_xsettings_setting *self)
+{
+	if (self->type == XDG_XSETTINGS_STRING)
+		str_free (&self->string);
+	free (self);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+struct xdg_xsettings
+{
+	struct str_map settings;            ///< Name -> xdg_xsettings_setting
+};
+
+static void
+xdg_xsettings_free (struct xdg_xsettings *self)
+{
+	str_map_free (&self->settings);
+}
+
+static struct xdg_xsettings
+xdg_xsettings_make (void)
+{
+	return (struct xdg_xsettings)
+	{
+		.settings =
+			str_map_make ((str_map_free_fn) xdg_xsettings_setting_destroy),
+	};
+}
+
+static void
+xdg_xsettings_update (struct xdg_xsettings *self, Display *dpy)
+{
+	// TODO: We're supposed to lock the server.
+	// TODO: We're supposed to trap X errors.
+	char *selection = xstrdup_printf ("_XSETTINGS_S%d", DefaultScreen (dpy));
+	Window owner
+		= XGetSelectionOwner (dpy, XInternAtom (dpy, selection, True));
+	free (selection);
+	if (!owner)
+		return;
+
+	Atom actual_type = None;
+	int actual_format = 0;
+	unsigned long nitems = 0, bytes_after = 0;
+	unsigned char *buffer = NULL;
+	Atom xsettings = XInternAtom (dpy, "_XSETTINGS_SETTINGS", True);
+	int status = XGetWindowProperty (dpy,
+		owner,
+		xsettings,
+		0L,
+		LONG_MAX,
+		False,
+		xsettings,
+		&actual_type,
+		&actual_format,
+		&nitems,
+		&bytes_after,
+		&buffer);
+	if (status != Success || !buffer)
+		return;
+
+	if (actual_type != xsettings
+	 || actual_format != 8
+	 || nitems < 12)
+		goto fail;
+
+	const struct peeker *peeker = NULL;
+	if (buffer[0] == LSBFirst)
+		peeker = &peeker_le;
+	else if (buffer[0] == MSBFirst)
+		peeker = &peeker_be;
+	else
+		goto fail;
+
+	// We're ignoring the serial for now.
+	uint32_t n_settings = peeker->u32 (buffer + 8);
+	size_t offset = 12;
+	struct str name = str_make ();
+	struct xdg_xsettings_setting *setting = xcalloc (1, sizeof *setting);
+	while (n_settings--)
+	{
+		if (nitems < offset + 4)
+			goto fail_item;
+
+		setting->type = buffer[offset];
+		uint16_t name_len = peeker->u16 (buffer + offset + 2);
+		offset += 4;
+		if (nitems < offset + name_len)
+			goto fail_item;
+
+		str_append_data (&name, buffer + offset, name_len);
+		offset += ((name_len + 3) & ~3);
+		if (nitems < offset + 4)
+			goto fail_item;
+
+		setting->serial = peeker->u32 (buffer + offset);
+		offset += 4;
+		switch (setting->type)
+		{
+		case XDG_XSETTINGS_INTEGER:
+			if (nitems < offset + 4)
+				goto fail_item;
+
+			setting->integer = (int32_t) peeker->u32 (buffer + offset);
+			offset += 4;
+			break;
+		case XDG_XSETTINGS_STRING:
+		{
+			if (nitems < offset + 4)
+				goto fail_item;
+
+			uint32_t value_len = peeker->u32 (buffer + offset);
+			offset += 4;
+			if (nitems < offset + value_len)
+				goto fail_item;
+
+			setting->string = str_make ();
+			str_append_data (&setting->string, buffer + offset, value_len);
+			offset += ((value_len + 3) & ~3);
+			break;
+		}
+		case XDG_XSETTINGS_COLOR:
+			if (nitems < offset + 8)
+				goto fail_item;
+
+			setting->color.red   = peeker->u16 (buffer + offset);
+			setting->color.green = peeker->u16 (buffer + offset + 2);
+			setting->color.blue  = peeker->u16 (buffer + offset + 4);
+			setting->color.alpha = peeker->u16 (buffer + offset + 6);
+			offset += 8;
+			break;
+		default:
+			goto fail_item;
+		}
+
+		// TODO(p): Change detection, by comparing existence and serials.
+		str_map_set (&self->settings, name.str, setting);
+		setting = xcalloc (1, sizeof *setting);
+		str_reset (&name);
+	}
+fail_item:
+	xdg_xsettings_setting_destroy (setting);
+	str_free (&name);
+fail:
+	XFree (buffer);
+}
+
+#endif // LIBERTY_XDG_WANT_X11
+
+// --- Desktop file parser -----------------------------------------------------
+
+// Useful for parsing desktop-file-spec, icon-theme-spec, trash-spec,
+// mime-apps-spec.  This code is not designed for making changes to the files.
+
+struct desktop_file
+{
+	struct str_map groups;              ///< Group name → Key → Value
+};
+
+static void
+desktop_file_free_group (void *value)
+{
+	str_map_free (value);
+	free (value);
+}
+
+static void
+desktop_file_free (struct desktop_file *self)
+{
+	str_map_free (&self->groups);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+static void
+desktop_file_parse_line (struct desktop_file *self,
+	char **group_name, const char *line, const char *end)
+{
+	struct str_map *group = NULL;
+	if (*group_name)
+		group = str_map_find (&self->groups, *group_name);
+
+	if (*line == '[')
+	{
+		bool ok = *--end == ']';
+		for (const char *p = ++line; ok && p != end; p++)
+			ok = (unsigned char) *p >= 32 && (unsigned char) *p <= 127
+				&& *p != '[' && *p != ']';
+		if (!ok)
+		{
+			cstr_set (group_name, NULL);
+			print_debug ("invalid desktop file group header");
+			return;
+		}
+
+		cstr_set (group_name, xstrndup (line, end - line));
+		if (str_map_find (&self->groups, *group_name))
+		{
+			print_debug ("duplicate desktop file group: %s", *group_name);
+			return;
+		}
+
+		group = xcalloc (1, sizeof *group);
+		*group = str_map_make (free);
+		str_map_set (&self->groups, *group_name, group);
+		return;
+	}
+	if (!group)
+	{
+		print_debug ("unexpected desktop file entry outside of a group");
+		return;
+	}
+
+	const char *key_end = line;
+	while (key_end != end && (isalnum_ascii (*key_end) || *key_end == '-'))
+		key_end++;
+
+	// We could validate these further, but we just search in them anyway.
+	if (key_end != end && *key_end == '[')
+	{
+		while (++key_end != end && *key_end != ']')
+			;
+		if (key_end != end && *key_end == ']')
+			key_end++;
+	}
+
+	const char *value = key_end;
+	while (value != end && *value == ' ')
+		value++;
+	if (value == end || *value++ != '=')
+	{
+		print_debug ("invalid desktop file entry");
+		return;
+	}
+	while (value != end && *value == ' ')
+		value++;
+
+	char *key = xstrndup (line, key_end - line);
+	if (str_map_find (group, key))
+		print_debug ("duplicate desktop file entry for: %s", key);
+	else
+		str_map_set (group, key, xstrndup (value, end - value));
+	free (key);
+}
+
+static struct desktop_file
+desktop_file_make (const char *data, size_t len)
+{
+	struct desktop_file self = (struct desktop_file)
+		{ .groups = str_map_make (desktop_file_free_group) };
+
+	char *group_name = NULL;
+	const char *p = data, *data_end = p + len;
+	while (p != data_end)
+	{
+		const char *line = p, *line_end = line;
+		while (line_end != data_end && *line_end != '\n')
+			line_end++;
+		if ((p = line_end) != data_end && *p == '\n')
+			p++;
+
+		if (line != line_end && *line != '#')
+			desktop_file_parse_line (&self, &group_name, line, line_end);
+	}
+	free (group_name);
+	return self;
+}
+
+static const char *
+desktop_file_get (struct desktop_file *self, const char *group, const char *key)
+{
+	// TODO(p): Ideally, also implement localised keys.
+	struct str_map *group_map = str_map_find (&self->groups, group);
+	if (!group_map)
+		return NULL;
+
+	return str_map_find (group_map, key);
+}
+
+static struct strv
+desktop_file_unescape (const char *value, bool is_list)
+{
+	struct strv result = strv_make ();
+	struct str s = str_make ();
+
+	// XXX: The unescaping behaviour is underspecified.
+	//   It might make sense to warn about unrecognised escape sequences.
+	bool escape = false;
+	for (const char *p = value; *p; p++)
+	{
+		if (escape)
+		{
+			switch (*p)
+			{
+			break; case 's': str_append_c (&s, ' ');
+			break; case 'n': str_append_c (&s, '\n');
+			break; case 't': str_append_c (&s, '\t');
+			break; case 'r': str_append_c (&s, '\r');
+			break; default:  str_append_c (&s, *p);
+			}
+		}
+		else if (*p == '\\' && p[1])
+			escape = true;
+		else if (*p == ';' && is_list)
+		{
+			strv_append_owned (&result, str_steal (&s));
+			s = str_make ();
+		}
+		else
+			str_append_c (&s, *p);
+	}
+
+	if (!is_list || s.len != 0)
+		strv_append_owned (&result, str_steal (&s));
+	else
+		str_free (&s);
+	return result;
+}
+
+static char *
+desktop_file_get_string (struct desktop_file *self,
+	const char *group, const char *key)
+{
+	const char *value = desktop_file_get (self, group, key);
+	if (!value)
+		return NULL;
+
+	struct strv values = desktop_file_unescape (value, false /* is_list */);
+	char *unescaped = strv_steal (&values, 0);
+	strv_free (&values);
+	return unescaped;
+}
+
+static struct strv
+desktop_file_get_stringv (struct desktop_file *self,
+	const char *group, const char *key)
+{
+	const char *value = desktop_file_get (self, group, key);
+	if (!value)
+		return strv_make ();
+
+	return desktop_file_unescape (value, true /* is_list */);
+}
+
+static bool
+desktop_file_get_bool (struct desktop_file *self,
+	const char *group, const char *key)
+{
+	const char *value = desktop_file_get (self, group, key);
+	if (!value)
+		return false;
+
+	// Let's be compatible with pre-1.0 files when it costs us so little.
+	if (!strcmp (value, "true")
+	 || !strcmp (value, "1"))
+		return true;
+	if (!strcmp (value, "false")
+	 || !strcmp (value, "0"))
+		return false;
+
+	print_debug ("invalid desktop file boolean for '%s': %s", key, value);
+	return false;
+}
+
+// Nothing uses the "numeric" type.
+// "icon-theme-spec" uses "integer" and doesn't say what it is.
+static long
+desktop_file_get_integer (struct desktop_file *self,
+	const char *group, const char *key)
+{
+	const char *value = desktop_file_get (self, group, key);
+	if (!value)
+		return 0;
+
+	char *end = NULL;
+	long parsed = (errno = 0, strtol (value, &end, 10));
+	if (errno != 0 || *end)
+		print_debug ("invalid desktop file integer for '%s': %s", key, value);
+	return parsed;
+}
+
+// --- Icon themes -------------------------------------------------------------
+
+// This implements part of the Icon Theme Specification.
+
+struct icon_theme_icon
+{
+	uint32_t width;                     ///< Width of argb in pixels
+	uint32_t height;                    ///< Height of argb in pixels
+	uint32_t argb[];                    ///< ARGB32 data, unassociated alpha
+};
+
+static void
+icon_theme_open_on_error (png_structp pngp, const char *error)
+{
+	print_debug ("%s: %s", (const char *) png_get_error_ptr (pngp), error);
+	png_longjmp (pngp, 1);
+}
+
+static void
+icon_theme_open_on_warning (png_structp pngp, const char *warning)
+{
+	(void) pngp;
+	(void) warning;
+	// Fuck your "gamma value does not match libpng estimate".
+}
+
+// For simplicity, only support PNG icons, using the most popular library.
+static struct icon_theme_icon *
+icon_theme_open (const char *path)
+{
+	volatile png_bytep buffer = NULL;
+	volatile png_bytepp row_pointers = NULL;
+	volatile struct icon_theme_icon *result = NULL;
+	FILE *fp = fopen (path, "rb");
+	if (!fp)
+	{
+		if (errno != ENOENT)
+			print_debug ("%s: %s", path, strerror (errno));
+		return NULL;
+	}
+
+	// The simplified and high-level APIs aren't powerful enough.
+	png_structp pngp = png_create_read_struct (PNG_LIBPNG_VER_STRING,
+		(png_voidp) path, icon_theme_open_on_error, icon_theme_open_on_warning);
+	png_infop infop = png_create_info_struct (pngp);
+	if (!infop)
+	{
+		print_debug ("%s: %s", path, strerror (errno));
+		goto fail;
+	}
+	if (setjmp (png_jmpbuf (pngp)))
+		goto fail;
+
+	png_init_io (pngp, fp);
+	png_read_info (pngp, infop);
+
+	// Asking for at least 8-bit channels.  This call is a superset of:
+	//  - png_set_palette_to_rgb(),
+	//  - png_set_tRNS_to_alpha(),
+	//  - png_set_expand_gray_1_2_4_to_8().
+	png_set_expand (pngp);
+
+	// Reduce the possibilities further to RGB or RGBA...
+	png_set_gray_to_rgb (pngp);
+
+	// ...and /exactly/ 8-bit channels.
+	// Alternatively, use png_set_expand_16() above to obtain 16-bit channels.
+	png_set_scale_16 (pngp);
+
+	// PNG uses RGBA order, let's change that to ARGB (both in memory order).
+	// This doesn't change a row's `color_type` in png_do_read_filler(),
+	// and the following transformation thus ignores it.
+	png_set_add_alpha (pngp, 0xFFFF, PNG_FILLER_BEFORE);
+	png_set_swap_alpha (pngp);
+
+	(void) png_set_interlace_handling (pngp);
+
+	png_read_update_info (pngp, infop);
+	if (png_get_bit_depth (pngp, infop) != 8
+	 || png_get_channels (pngp, infop) != 4
+	 || png_get_color_type (pngp, infop) != PNG_COLOR_TYPE_RGB_ALPHA)
+		png_error (pngp, "result not A8R8G8B8");
+
+	size_t row_bytes = png_get_rowbytes (pngp, infop);
+	size_t height = png_get_image_height (pngp, infop);
+	buffer = xcalloc (row_bytes, height);
+	row_pointers = xcalloc (height, sizeof buffer);
+	for (size_t y = 0; y < height; y++)
+		row_pointers[y] = buffer + y * row_bytes;
+
+	png_read_image (pngp, row_pointers);
+
+	result = xcalloc (1, sizeof *result + row_bytes * height);
+	result->width = png_get_image_width (pngp, infop);
+	result->height = height;
+
+	uint32_t *dst = (uint32_t *) result->argb, *src = (uint32_t *) buffer;
+	for (size_t pixels = result->width * result->height; pixels--; )
+		*dst++ = ntohl (*src++);
+
+fail:
+	free (buffer);
+	free (row_pointers);
+	png_destroy_read_struct (&pngp, &infop, NULL);
+	fclose (fp);
+	return (struct icon_theme_icon *) result;
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+struct icon_theme_find_context
+{
+	struct strv base;                   ///< Base directories
+	struct str_map visited;             ///< Cycle prevention
+
+	ARRAY (struct icon_theme_icon *, icons)
+};
+
+static void
+icon_theme_find__fallback (struct icon_theme_find_context *ctx,
+	const char *name)
+{
+	for (size_t i = 0; i < ctx->base.len; i++)
+	{
+		char *path = xstrdup_printf ("%s/%s.png", ctx->base.vector[i], name);
+		struct icon_theme_icon *icon = icon_theme_open (path);
+		free (path);
+		if (icon)
+		{
+			ARRAY_RESERVE (ctx->icons, 1);
+			ctx->icons[ctx->icons_len++] = icon;
+			return;
+		}
+	}
+}
+
+static struct desktop_file
+icon_theme_find__index (struct icon_theme_find_context *ctx, const char *theme)
+{
+	struct str data = str_make ();
+	for (size_t i = 0; i < ctx->base.len; i++)
+	{
+		struct error *e = NULL;
+		char *path = xstrdup_printf ("%s/%s/index.theme",
+			ctx->base.vector[i], theme);
+		read_file (path, &data, &e);
+		free (path);
+		if (!e)
+			break;
+
+		if (errno != ENOENT)
+			print_debug ("%s", e->message);
+		error_free (e);
+	}
+
+	struct desktop_file index = desktop_file_make (data.str, data.len);
+	str_free (&data);
+	return index;
+}
+
+static void
+icon_theme_find__named (struct icon_theme_find_context *ctx,
+	const char *theme, const char *name)
+{
+	// Either a cycle, or a common ancestor of inherited themes, which is valid.
+	if (str_map_find (&ctx->visited, theme))
+		return;
+
+	str_map_set (&ctx->visited, theme, (void *) (intptr_t) 1);
+	struct desktop_file index = icon_theme_find__index (ctx, theme);
+
+	char *directories =
+		desktop_file_get_string (&index, "Icon Theme", "Directories");
+	if (!directories)
+		goto out;
+
+	// NOTE: The sizes are not deduplicated, and priorities are uncertain.
+	struct strv dirs = strv_make ();
+	cstr_split (directories, ",", true, &dirs);
+	free (directories);
+	for (size_t d = 0; d < dirs.len; d++)
+	{
+		// The hicolor icon theme stuffs everything in Directories.
+		if (desktop_file_get (&index, dirs.vector[d], "Scale")
+		 && desktop_file_get_integer (&index, dirs.vector[d], "Scale") != 1)
+			continue;
+
+		for (size_t i = 0; i < ctx->base.len; i++)
+		{
+			char *path = xstrdup_printf ("%s/%s/%s/%s.png",
+				ctx->base.vector[i], theme, dirs.vector[d], name);
+			struct icon_theme_icon *icon = icon_theme_open (path);
+			free (path);
+			if (icon)
+			{
+				ARRAY_RESERVE (ctx->icons, 1);
+				ctx->icons[ctx->icons_len++] = icon;
+				break;
+			}
+		}
+	}
+	strv_free (&dirs);
+	if (ctx->icons_len)
+		goto out;
+
+	char *inherits =
+		desktop_file_get_string (&index, "Icon Theme", "Inherits");
+	if (inherits)
+	{
+		struct strv parents = strv_make ();
+		cstr_split (inherits, ",", true, &parents);
+		free (inherits);
+		for (size_t i = 0; i < parents.len; i++)
+		{
+			icon_theme_find__named (ctx, parents.vector[i], name);
+			if (ctx->icons_len)
+				break;
+		}
+		strv_free (&parents);
+	}
+
+out:
+	desktop_file_free (&index);
+}
+
+/// Return all base directories appropriate for icon search.
+static struct strv
+icon_theme_get_base_directories (void)
+{
+	struct strv dirs = strv_make ();
+	struct str icons = str_make ();
+	(void) str_append_env_path (&icons, "HOME", false);
+	str_append (&icons, "/.icons");
+	strv_append_owned (&dirs, str_steal (&icons));
+
+	// Note that we use XDG_CONFIG_HOME as well, which might be intended.
+	struct strv xdg = strv_make ();
+	get_xdg_data_dirs (&xdg);
+	for (size_t i = 0; i < xdg.len; i++)
+		strv_append_owned (&dirs, xstrdup_printf ("%s/icons", xdg.vector[i]));
+	strv_free (&xdg);
+
+	strv_append (&dirs, "/usr/share/pixmaps");
+	return dirs;
+}
+
+static int
+icon_theme_find__compare (const void *a, const void *b)
+{
+	const struct icon_theme_icon **ia = (const struct icon_theme_icon **) a;
+	const struct icon_theme_icon **ib = (const struct icon_theme_icon **) b;
+	double pa = (double) (*ia)->width * (*ia)->height;
+	double pb = (double) (*ib)->width * (*ib)->height;
+	return (pa > pb) - (pa < pb);
+}
+
+/// Return all sizes of the named icon.  When the theme name is not NULL,
+/// use it as the preferred theme.  Always consult fallbacks locations.
+/// Ignore icon scales other than 1.
+static struct icon_theme_icon **
+icon_theme_find (const char *theme, const char *name, size_t *len)
+{
+	struct icon_theme_find_context ctx = {};
+	ctx.base = icon_theme_get_base_directories ();
+	ctx.visited = str_map_make (NULL);
+	ARRAY_INIT (ctx.icons);
+
+	if (theme)
+		icon_theme_find__named (&ctx, theme, name);
+	if (!ctx.icons_len)
+		icon_theme_find__named (&ctx, "hicolor", name);
+	if (!ctx.icons_len)
+		icon_theme_find__fallback (&ctx, name);
+
+	strv_free (&ctx.base);
+	str_map_free (&ctx.visited);
+
+	ARRAY_RESERVE (ctx.icons, 1);
+	ctx.icons[ctx.icons_len] = NULL;
+	if (!ctx.icons_len)
+	{
+		free (ctx.icons);
+		return NULL;
+	}
+
+	qsort (ctx.icons,
+		ctx.icons_len, sizeof *ctx.icons, icon_theme_find__compare);
+	*len = ctx.icons_len;
+	return ctx.icons;
+}
+
+static void
+icon_theme_free (struct icon_theme_icon **icons)
+{
+	for (struct icon_theme_icon **p = icons; *p; p++)
+		free (*p);
+	free (icons);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
+#ifdef LIBERTY_XDG_WANT_X11
+
+static void
+icon_theme_set_window_icon (Display *dpy,
+	Window window, const char *theme, const char *name)
+{
+	size_t icons_len = 0;
+	struct icon_theme_icon **icons = icon_theme_find (theme, name, &icons_len);
+	if (!icons)
+		return;
+
+	size_t n = 0;
+	for (size_t i = 0; i < icons_len; i++)
+		n += 2 + icons[i]->width * icons[i]->height;
+
+	unsigned long *data = xcalloc (n, sizeof *data), *p = data;
+	for (size_t i = 0; i < icons_len; i++)
+	{
+		*p++ = icons[i]->width;
+		*p++ = icons[i]->height;
+
+		uint32_t *q = icons[i]->argb;
+		for (size_t k = icons[i]->width * icons[i]->height; k--; )
+			*p++ = *q++;
+	}
+
+	XChangeProperty (dpy, window, XInternAtom (dpy, "_NET_WM_ICON", False),
+		XA_CARDINAL, 32, PropModeReplace, (const unsigned char *) data, n);
+	free (data);
+	icon_theme_free (icons);
+}
+
+#endif // LIBERTY_XDG_WANT_X11
diff --git a/liberty-xui.c b/liberty-xui.c
index 45a6d90..761d7c6 100644
--- a/liberty-xui.c
+++ b/liberty-xui.c
@@ -1,7 +1,7 @@
 /*
  * liberty-xui.c: the ultimate C unlibrary: hybrid terminal/X11 UI
  *
- * Copyright (c) 2016 - 2023, Přemysl Eric Janouch <p@janouch.name>
+ * Copyright (c) 2016 - 2024, Přemysl Eric Janouch <p@janouch.name>
  *
  * Permission to use, copy, modify, and/or distribute this software for any
  * purpose with or without fee is hereby granted.
@@ -17,6 +17,7 @@
  */
 
 // This file includes some common stuff to build terminal/X11 applications with.
+// It assumes you've already included liberty.c, and may include liberty-xdg.c.
 
 #include <ncurses.h>
 
@@ -66,6 +67,9 @@ enum { XUI_KEYMOD_DOUBLE_CLICK = 1 << 15 };
 #include <X11/keysym.h>
 #include <X11/XKBlib.h>
 #include <X11/Xft/Xft.h>
+
+#define LIBERTY_XDG_WANT_X11
+#include "liberty-xdg.c"
 #endif  // LIBERTY_XUI_WANT_X11
 
 // The application needs to implement these.
@@ -690,7 +694,10 @@ struct xui
 	XftDraw *xft_draw;                  ///< Xft rendering context
 	struct x11_font *xft_fonts;         ///< Font collection
 	char *x11_selection;                ///< CLIPBOARD selection
+	struct xdg_xsettings x11_xsettings; ///< XSETTINGS
 
+	int32_t x11_double_click_time;      ///< Maximum delay for double clicks
+	int32_t x11_double_click_distance;  ///< Maximum distance for double clicks
 	const char *x11_fontname;           ///< Fontconfig font name
 	const char *x11_fontname_monospace; ///< Fontconfig monospace font name
 	XRenderColor *x_fg;                 ///< Foreground per attribute
@@ -1375,6 +1382,7 @@ x11_destroy (void)
 	LIST_FOR_EACH (struct x11_font, font, g_xui.xft_fonts)
 		x11_font_destroy (font);
 	cstr_set (&g_xui.x11_selection, NULL);
+	xdg_xsettings_free (&g_xui.x11_xsettings);
 
 	free (g_xui.x_fg);
 	free (g_xui.x_bg);
@@ -1623,9 +1631,10 @@ on_x11_input_event (XEvent *ev)
 	unsigned int button = ev->xbutton.button;
 	int modifiers = x11_state_to_modifiers (ev->xbutton.state);
 	if (ev->type == ButtonPress
-	 && ev->xbutton.time - last_press_event.xbutton.time < 500
-	 && abs (last_press_event.xbutton.x - x) < 5
-	 && abs (last_press_event.xbutton.y - y) < 5
+	 && ev->xbutton.time - last_press_event.xbutton.time
+		< (Time) g_xui.x11_double_click_time
+	 && abs (last_press_event.xbutton.x - x) < g_xui.x11_double_click_distance
+	 && abs (last_press_event.xbutton.y - y) < g_xui.x11_double_click_distance
 	 && last_press_event.xbutton.button == button)
 	{
 		modifiers |= XUI_KEYMOD_DOUBLE_CLICK;
@@ -1891,6 +1900,13 @@ x11_init (struct poller *poller, struct attrs *app_attrs, size_t app_attrs_len)
 
 	x11_init_attributes (app_attrs, app_attrs_len);
 
+	// https://www.freedesktop.org/wiki/Specifications/XSettingsRegistry/
+	// TODO: Try to use Xft/{Antialias,DPI,HintStyle,Hinting,RGBA}
+	//   from XSETTINGS.  Sadly, Gtk/FontName is in the Pango format,
+	//   which is rather difficult to parse.
+	g_xui.x11_xsettings = xdg_xsettings_make ();
+	xdg_xsettings_update (&g_xui.x11_xsettings, g_xui.dpy);
+
 	if (!FcInit ())
 		print_warning ("Fontconfig initialization failed");
 	if (!(g_xui.xft_fonts = x11_font_open (0)))
@@ -1942,6 +1958,25 @@ x11_init (struct poller *poller, struct attrs *app_attrs, size_t app_attrs_len)
 		XSetWMName (g_xui.dpy, g_xui.x11_window, &prop);
 	XFree (prop.value);
 
+	// It should not be outlandish to expect to find a program icon,
+	// although it should be possible to use a "DBus well-known name".
+	const char *icon_theme_name = NULL;
+	const struct xdg_xsettings_setting *setting =
+		str_map_find (&g_xui.x11_xsettings.settings, "Net/IconThemeName");
+	if (setting != NULL && setting->type == XDG_XSETTINGS_STRING)
+		icon_theme_name = setting->string.str;
+	icon_theme_set_window_icon (g_xui.dpy,
+		g_xui.x11_window, icon_theme_name, name);
+
+	if ((setting = str_map_find (&g_xui.x11_xsettings.settings,
+			"Net/DoubleClickTime"))
+	 && setting->type == XDG_XSETTINGS_INTEGER && setting->integer >= 0)
+		g_xui.x11_double_click_time = setting->integer;
+	if ((setting = str_map_find (&g_xui.x11_xsettings.settings,
+			"Net/DoubleClickDistance"))
+	 && setting->type == XDG_XSETTINGS_INTEGER && setting->integer >= 0)
+		g_xui.x11_double_click_distance = setting->integer;
+
 	// TODO: It is possible to do, e.g., on-the-spot.
 	XIMStyle im_style = XIMPreeditNothing | XIMStatusNothing;
 	XIMStyles *im_styles = NULL;
@@ -2147,11 +2182,10 @@ xui_preinit (void)
 	// Presumably, although not necessarily; unsure if queryable at all.
 	g_xui.focused = true;
 
-	// TODO: Try to use Gtk/FontName from the _XSETTINGS_S%d selection,
-	//   as well as Net/DoubleClick*.  See the XSETTINGS proposal for details.
-	//   https://www.freedesktop.org/wiki/Specifications/XSettingsRegistry/
-	//   Note that this needs an X11 connection already.
 #ifdef LIBERTY_XUI_WANT_X11
+	// Note that XSETTINGS overrides some values in the init.
+	g_xui.x11_double_click_time = 500;
+	g_xui.x11_double_click_distance = 5;
 	g_xui.x11_fontname = "sans\\-serif-11";
 	g_xui.x11_fontname_monospace = "monospace-11";
 #endif  // LIBERTY_XUI_WANT_X11