gio: gmemorymonitorbase: parent class of GMemoryMonitor backends

This class provides the shared functions, such as sending a signal and
string and value conversion. The backend classes should inherit this
class to get the shared functions.

It adds a configure time check for `sysinfo()`, as some systems don’t
have it.

gio/gmemorymonitorbase.h

@@ -0,0 +1,53 @@
+/* GIO - GLib Input, Output and Streaming Library
+ *
+ * Copyright 2025 Red Hat, Inc.
+ *
+ * SPDX-License-Identifier: LGPL-2.1-or-later
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef __G_MEMORY_MONITOR_BASE_H__
+#define __G_MEMORY_MONITOR_BASE_H__
+
+#include <glib-object.h>
+
+G_BEGIN_DECLS
+
+#define G_TYPE_MEMORY_MONITOR_BASE         (g_memory_monitor_base_get_type ())
+
+G_DECLARE_DERIVABLE_TYPE (GMemoryMonitorBase, g_memory_monitor_base, G, MEMORY_MONITOR_BASE, GObject)
+
+typedef enum
+{
+  G_MEMORY_MONITOR_LOW_MEMORY_LEVEL_INVALID = -1,
+  G_MEMORY_MONITOR_LOW_MEMORY_LEVEL_LOW = 0,
+  G_MEMORY_MONITOR_LOW_MEMORY_LEVEL_MEDIUM,
+  G_MEMORY_MONITOR_LOW_MEMORY_LEVEL_CRITICAL,
+  G_MEMORY_MONITOR_LOW_MEMORY_LEVEL_COUNT
+} GMemoryMonitorLowMemoryLevel;
+
+struct _GMemoryMonitorBaseClass
+{
+  GObjectClass parent_class;
+};
+
+void                        g_memory_monitor_base_send_event_to_user (GMemoryMonitorBase              *monitor,
+                                                                      GMemoryMonitorLowMemoryLevel     warning_level);
+GMemoryMonitorWarningLevel  g_memory_monitor_base_level_enum_to_byte (GMemoryMonitorLowMemoryLevel     level);
+gdouble                     g_memory_monitor_base_query_mem_ratio    (void);
+
+G_END_DECLS
+
+#endif /* __G_MEMORY_MONITOR_BASE_H__ */