library: vmstat redesign now using 'stack' vs. 'chain'

In addition to that text shown below the line which is
common to several commit messages, this patch contains
several minor changes with lessor impact upon the API:

. Standard copyright boilerplate was added in .c file.

. The #include header files are ordered alphabetically
now, with all those <sys/??> types separately grouped.

. The header file follows the conventions of indenting
(by 4 spaces) those parameters too lengthy for 1 line.

------------------------------------------------------
. The former 'chains' have now become 'stacks' without
the 'next' pointer in each result struct. The pointers
initially seemed to offer some flexibility with memory
allocations and benefits for the library access logic.
However, user access was always via displacement and a
a statically allocated chain was cumbersome to define.

. An enumerator ending in '_noop' will no longer serve
as a fencepost delimiter. Rather, it has become a much
more important and flexible user oriented tool. Adding
one or more such 'items' in any items list passed into
the library becomes the means of extending the 'stack'
to also include user (not just library) data. Any such
data is guaranteed to never be altered by the library.

. Anticipating PID support, where many different types
must be represented in a result structure, we'll adopt
a common naming standard. And, while not every results
structure currently needs to reflect disparate types a
union will be employed so the same dot qualifier ('.')
can be used consistently when accessing all such data.

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/libprocps.sym

@@ -78,7 +78,7 @@ global:
 	procps_vmstat_ref;
 	procps_vmstat_unref;
 	procps_vmstat_get;
-	procps_vmstat_get_chain;
+	procps_vmstat_getstack;
 local:
 	*;
 };

proc/vmstat.c

@@ -1,11 +1,29 @@
+/*
+ * libprocps - Library to read proc filesystem
+ *
+ * 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, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+
 #include <sys/types.h>
 #include <sys/stat.h>
-#include <fcntl.h>
 
 #include <proc/vmstat.h>
 #include "procps-private.h"
@@ -162,36 +180,40 @@ PROCPS_EXPORT unsigned long procps_vmstat_get (
             return info->data.pswpin;
         case PROCPS_VMSTAT_PSWPOUT:
             return info->data.pswpout;
-    }
+        default:
             return 0;
+    }
 }
 
-PROCPS_EXPORT int procps_vmstat_get_chain (
+PROCPS_EXPORT int procps_vmstat_getstack (
         struct procps_vmstat *info,
-        struct vmstat_result *item)
+        struct vmstat_result *these)
 {
-    if (item == NULL)
+    if (these == NULL)
         return -EINVAL;
 
-    do {
-        switch (item->item) {
+    for (;;) {
+        switch (these->item) {
             case PROCPS_VMSTAT_PGPGIN:
-                item->result = info->data.pgpgin;
+                these->result.ul_int = info->data.pgpgin;
                 break;
             case PROCPS_VMSTAT_PGPGOUT:
-                item->result = info->data.pgpgout;
+                these->result.ul_int = info->data.pgpgout;
                 break;
             case PROCPS_VMSTAT_PSWPIN:
-                item->result = info->data.pswpin;
+                these->result.ul_int = info->data.pswpin;
                 break;
             case PROCPS_VMSTAT_PSWPOUT:
-                item->result = info->data.pswpout;
+                these->result.ul_int = info->data.pswpout;
                 break;
+            case PROCPS_VMSTAT_noop:
+                // don't disturb potential user data in the result struct
+                break;
+            case PROCPS_VMSTAT_stack_end:
+                return 0;
             default:
                 return -EINVAL;
         }
-        item = item->next;
-    } while (item);
-
-    return 0;
+        ++these;
+    }
 }

proc/vmstat.h

@@ -28,16 +28,19 @@
 __BEGIN_DECLS
 
 enum vmstat_item {
-    PROCPS_VMSTAT_PGPGIN,
-    PROCPS_VMSTAT_PGPGOUT,
-    PROCPS_VMSTAT_PSWPIN,
-    PROCPS_VMSTAT_PSWPOUT
+    PROCPS_VMSTAT_PGPGIN,      // ul_int
+    PROCPS_VMSTAT_PGPGOUT,     // ul_int
+    PROCPS_VMSTAT_PSWPIN,      // ul_int
+    PROCPS_VMSTAT_PSWPOUT,     // ul_int
+    PROCPS_VMSTAT_noop,        // n/a
+    PROCPS_VMSTAT_stack_end    // n/a
 };
 
 struct vmstat_result {
     enum vmstat_item item;
-    unsigned long result;
-    struct vmstat_result *next;
+    union {
+        unsigned long ul_int;
+    } result;
 };
 
 struct procps_vmstat;
@@ -48,8 +51,13 @@ int procps_vmstat_read (struct procps_vmstat *info);
 int procps_vmstat_ref (struct procps_vmstat *info);
 int procps_vmstat_unref (struct procps_vmstat **info);
 
-unsigned long procps_vmstat_get (struct procps_vmstat *info, enum vmstat_item item);
-int procps_vmstat_get_chain (struct procps_vmstat *info, struct vmstat_result *item);
+unsigned long procps_vmstat_get (
+    struct procps_vmstat *info,
+    enum vmstat_item item);
+
+int procps_vmstat_getstack (
+    struct procps_vmstat *info,
+    struct vmstat_result *these);
 
 __END_DECLS
 #endif