[libvirt] [PATCH] build: silence stupid gcc warning on STREQ_NULLABLE

[Eric Blake]

Our existing STRNEQ_NULLABLE() triggered a warning in gcc 4.7 when
used with a literal NULL argument:

qemumonitorjsontest.c: In function 'testQemuMonitorJSONGetMachines':
qemumonitorjsontest.c:289:5: error: null argument where non-null required (argument 1) [-Werror=nonnull]

even though the strcmp is provably dead when a null argument is
present.  Squelch the warning by refactoring things so that gcc
never sees strcmp() called with NULL arguments (we still compare
NULL as not equal to "", this rewrite merely aids gcc).

Next, gcc has a valid warning about a literal NULLSTR(NULL):

qemumonitorjsontest.c:289:5: error: invalid application of 'sizeof' to a void type [-Werror=pointer-arith]

Of course, you'd never write NULLSTR(NULL) directly, but it is
handy to use through macros.  And since we only really need the
code to be warning-free when developing with modern gcc, and
merely compiled correctly elsewhere, we can rely on gcc extensions
to avoid dereferencing NULL even inside a sizeof operation.

* src/internal.h (STREQ_NULLABLE, STRNEQ_NULLABLE): Avoid gcc 4.7
stupidity.
(NULLSTR): Allow passing compile-time constants via macros.
---
 src/internal.h | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/src/internal.h b/src/internal.h
index 300de3a..c27ffc5 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -79,10 +79,9 @@
 # define STRSKIP(a,b) (STRPREFIX(a,b) ? (a) + strlen(b) : NULL)

 # define STREQ_NULLABLE(a, b)                           \
-    ((!(a) && !(b)) || ((a) && (b) && STREQ((a), (b))))
+    ((a) ? (b) && STREQ((a) ? (a) : "", (b) ? (b) : "") : !(b))
 # define STRNEQ_NULLABLE(a, b)                          \
-    ((!(a) ^ !(b)) || ((a) && (b) && STRNEQ((a), (b))))
-
+    ((a) ? !(b) || STRNEQ((a) ? (a) : "", (b) ? (b) : "") : !!(b))

 # define NUL_TERMINATE(buf) do { (buf)[sizeof(buf)-1] = '\0'; } while (0)
 # define ARRAY_CARDINALITY(Array) (sizeof(Array) / sizeof(*(Array)))
@@ -206,9 +205,22 @@
 /*
  * Use this when passing possibly-NULL strings to printf-a-likes.
  */
-# define NULLSTR(s) \
-    ((void)verify_true(sizeof(*(s)) == sizeof(char)),   \
-     (s) ? (s) : "(null)")
+# if __GNUC_PREREQ(4, 6)
+char *link_error_due_to_bad_NULLSTR_type(void);
+#  define NULLSTR(s)                                                    \
+    ({                                                                  \
+        const char *_tmp;                                               \
+        if (__builtin_constant_p(s) ||                                  \
+            __builtin_types_compatible_p(typeof(s), char *) ||          \
+            __builtin_types_compatible_p(typeof(s), const char *))      \
+            _tmp = (s) ? (s) : "(null)";                                \
+        else                                                            \
+            _tmp = link_error_due_to_bad_NULLSTR_type();                \
+        _tmp;                                                           \
+    })
+# else
+#  define NULLSTR(s) ((s) ? (s) : "(null)")
+# endif

 /**
  * TODO:
-- 
1.7.11.4