[PATCH -perfbook 2/6] CodeSamples/datastruct: Move struct declaration from .c to .h

[Akira Yokosawa <akiyks@xxxxxxxxx>]

Forward declaration of a struct in .h is insufficient if there is code
that accesses the struct within the .h file depending on optimization
levels.

GCC of Ubuntu 22.04 with -O0 as well as -O3 ends up in these pair of
errors:

    make[1]: Entering directory 'perfbook/CodeSamples/datastruct/Issaquah'
    cc -g -O0 -Wall -o existence_test existence.c existence_test.c -lpthread -lurcu -lurcu-common -lurcu-signal
    In file included from /usr/include/x86_64-linux-gnu/urcu/pointer.h:39,
                     from /usr/include/x86_64-linux-gnu/urcu-pointer.h:1,
                     from /usr/include/x86_64-linux-gnu/urcu/rculist.h:30,
                     from ../../api.h:847,
                     from existence.c:30:
    existence.h: In function ‘existence_exists_relaxed’:
    existence.h:54:32: error: invalid use of undefined type ‘struct existence’
       54 |         struct existence *ep = rcu_dereference(*epp);
          |                                ^~~~~~~~~~~~~~~
    In file included from /usr/include/x86_64-linux-gnu/urcu/pointer.h:39,
                     from /usr/include/x86_64-linux-gnu/urcu-pointer.h:1,
                     from /usr/include/x86_64-linux-gnu/urcu/rculist.h:30,
                     from ../../api.h:847,
                     from existence_test.c:26:
    existence.h: In function ‘existence_exists_relaxed’:
    existence.h:54:32: error: invalid use of undefined type ‘struct existence’
       54 |         struct existence *ep = rcu_dereference(*epp);
          |                                ^~~~~~~~~~~~~~~
    make[1]: *** [Makefile:49: existence_test] Error 1
    make[1]: Leaving directory 'perfbook/CodeSamples/datastruct/Issaquah'

Move the declaration from .c to .h.

Signed-off-by: Akira Yokosawa <akiyks@xxxxxxxxx>
---
 CodeSamples/datastruct/Issaquah/existence.c | 6 ------
 CodeSamples/datastruct/Issaquah/existence.h | 7 ++++++-
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/CodeSamples/datastruct/Issaquah/existence.c b/CodeSamples/datastruct/Issaquah/existence.c
index 5e9ee70e..2d56a42d 100644
--- a/CodeSamples/datastruct/Issaquah/existence.c
+++ b/CodeSamples/datastruct/Issaquah/existence.c
@@ -37,12 +37,6 @@
 /* Existence-switch array. */
 const int existence_array[4] = { 1, 0, 0, 1 };
 
-/* Existence structure associated with each moving structure. */
-struct existence {
-	const int **existence_switch;
-	int offset;
-};
-
 /* Existence-group structure associated with multi-structure change. */
 struct existence_group {
 	struct existence outgoing;
diff --git a/CodeSamples/datastruct/Issaquah/existence.h b/CodeSamples/datastruct/Issaquah/existence.h
index 881c908f..c39a8d03 100644
--- a/CodeSamples/datastruct/Issaquah/existence.h
+++ b/CodeSamples/datastruct/Issaquah/existence.h
@@ -22,7 +22,12 @@
 #ifndef __EXISTENCE_H
 #define __EXISTENCE_H
 
-struct existence;
+/* Existence structure associated with each moving structure. */
+struct existence {
+	const int **existence_switch;
+	int offset;
+};
+
 struct existence_group;
 
 void existence_wire_call_rcu(void);
-- 
2.34.1