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authorElia Pinto <gitter.spiros@gmail.com>2015-04-30 14:44:14 +0200
committerJunio C Hamano <gitster@pobox.com>2015-05-05 15:26:48 -0700
commit89c855ed3cbe64bea93ea081d0e96077e9ee8517 (patch)
tree72ce4c2b43a79f93f955ceafd7cf8f4ebdab6fb7 /git-compat-util.h
parentfdf96a20acf96a6ac538df8113b2aafd6ed71d50 (diff)
downloadgit-89c855ed3cbe64bea93ea081d0e96077e9ee8517.tar.gz
git-compat-util.h: implement a different ARRAY_SIZE macro for for safely deriving the size of arrayep/do-not-feed-a-pointer-to-array-size
To get number of elements in an array git use the ARRAY_SIZE macro defined as: #define ARRAY_SIZE(x) (sizeof(x)/sizeof((x)[0])) The problem with it is a possibility of mistakenly passing to it a pointer instead an array. The ARRAY_SIZE macro as conventionally defined does not provide good type-safety and the open-coded approach is more fragile, more verbose and provides no improvement in type-safety. Use instead a different but compatible ARRAY_SIZE() macro, which will also break compile if you try to use it on a pointer. This implemention revert to the original code if the compiler doesn't know the typeof and __builtin_types_compatible_p GCC extensions. This can ensure our code is robust to changes, without needing a gratuitous macro or constant. A similar ARRAY_SIZE implementation also exists in the linux kernel. Credits to Rusty Russell and his ccan library. Signed-off-by: Elia Pinto <gitter.spiros@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'git-compat-util.h')
-rw-r--r--git-compat-util.h54
1 files changed, 53 insertions, 1 deletions
diff --git a/git-compat-util.h b/git-compat-util.h
index 400e921086..7fad5aad9f 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -3,6 +3,23 @@
#define _FILE_OFFSET_BITS 64
+
+/* Derived from Linux "Features Test Macro" header
+ * Convenience macros to test the versions of gcc (or
+ * a compatible compiler).
+ * Use them like this:
+ * #if GIT_GNUC_PREREQ (2,8)
+ * ... code requiring gcc 2.8 or later ...
+ * #endif
+*/
+#if defined(__GNUC__) && defined(__GNUC_MINOR__)
+# define GIT_GNUC_PREREQ(maj, min) \
+ ((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
+#else
+ #define GIT_GNUC_PREREQ(maj, min) 0
+#endif
+
+
#ifndef FLEX_ARRAY
/*
* See if our compiler is known to support flexible array members.
@@ -25,7 +42,42 @@
#endif
#endif
-#define ARRAY_SIZE(x) (sizeof(x)/sizeof(x[0]))
+
+/*
+ * BUILD_ASSERT_OR_ZERO - assert a build-time dependency, as an expression.
+ * @cond: the compile-time condition which must be true.
+ *
+ * Your compile will fail if the condition isn't true, or can't be evaluated
+ * by the compiler. This can be used in an expression: its value is "0".
+ *
+ * Example:
+ * #define foo_to_char(foo) \
+ * ((char *)(foo) \
+ * + BUILD_ASSERT_OR_ZERO(offsetof(struct foo, string) == 0))
+ */
+#define BUILD_ASSERT_OR_ZERO(cond) \
+ (sizeof(char [1 - 2*!(cond)]) - 1)
+
+#if defined(__GNUC__) && (__GNUC__ >= 3)
+# if GIT_GNUC_PREREQ(3, 1)
+ /* &arr[0] degrades to a pointer: a different type from an array */
+# define BARF_UNLESS_AN_ARRAY(arr) \
+ BUILD_ASSERT_OR_ZERO(!__builtin_types_compatible_p(__typeof__(arr), \
+ __typeof__(&(arr)[0])))
+# else
+# define BARF_UNLESS_AN_ARRAY(arr) 0
+# endif
+#endif
+/*
+ * ARRAY_SIZE - get the number of elements in a visible array
+ * <at> x: the array whose size you want.
+ *
+ * This does not work on pointers, or arrays declared as [], or
+ * function parameters. With correct compiler support, such usage
+ * will cause a build error (see the build_assert_or_zero macro).
+ */
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]) + BARF_UNLESS_AN_ARRAY(x))
+
#define bitsizeof(x) (CHAR_BIT * sizeof(x))
#define maximum_signed_value_of_type(a) \