[ccan] [PATCH 1/3] container_of: Add container_of_or_null()
David Gibson
david at gibson.dropbear.id.au
Wed Sep 10 00:22:52 EST 2014
It's quite common to have a pointer which could be either a pointer to a
structure member, or NULL. This needs special casing with container_of(),
or it will convert NULL into something strange.
This patch adds container_of_or_null(), which will return NULL if passed
(an appropriately typed) NULL, or the containining structure as
container_of() otherwise.
Signed-off-by: David Gibson <david at gibson.dropbear.id.au>
---
ccan/container_of/container_of.h | 36 ++++++++++++++++++++++++++++++++++++
ccan/container_of/test/run.c | 6 +++++-
2 files changed, 41 insertions(+), 1 deletion(-)
diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 0449935..47a34d8 100644
--- a/ccan/container_of/container_of.h
+++ b/ccan/container_of/container_of.h
@@ -36,6 +36,42 @@
- container_off(containing_type, member)) \
+ check_types_match(*(member_ptr), ((containing_type *)0)->member))
+
+/**
+ * container_of_or_null - get pointer to enclosing structure, or NULL
+ * @member_ptr: pointer to the structure member
+ * @containing_type: the type this member is within
+ * @member: the name of this member within the structure.
+ *
+ * Given a pointer to a member of a structure, this macro does pointer
+ * subtraction to return the pointer to the enclosing type, unless it
+ * is given NULL, in which case it also returns NULL.
+ *
+ * Example:
+ * struct foo {
+ * int fielda, fieldb;
+ * // ...
+ * };
+ * struct info {
+ * int some_other_field;
+ * struct foo my_foo;
+ * };
+ *
+ * static struct info *foo_to_info_allowing_null(struct foo *foo)
+ * {
+ * return container_of_or_null(foo, struct info, my_foo);
+ * }
+ */
+static inline char *container_of_or_null_(void *member_ptr, size_t offset)
+{
+ return member_ptr ? (char *)member_ptr - offset : NULL;
+}
+#define container_of_or_null(member_ptr, containing_type, member) \
+ ((containing_type *) \
+ container_of_or_null_(member_ptr, \
+ container_off(containing_type, member)) \
+ + check_types_match(*(member_ptr), ((containing_type *)0)->member))
+
/**
* container_off - get offset to enclosing structure
* @containing_type: the type this member is within
diff --git a/ccan/container_of/test/run.c b/ccan/container_of/test/run.c
index 5da440a..18207f6 100644
--- a/ccan/container_of/test/run.c
+++ b/ccan/container_of/test/run.c
@@ -12,9 +12,13 @@ int main(int argc, char *argv[])
int *intp = &foo.a;
char *charp = &foo.b;
- plan_tests(8);
+ plan_tests(12);
ok1(container_of(intp, struct foo, a) == &foo);
ok1(container_of(charp, struct foo, b) == &foo);
+ ok1(container_of_or_null(intp, struct foo, a) == &foo);
+ ok1(container_of_or_null(charp, struct foo, b) == &foo);
+ ok1(container_of_or_null((int *)NULL, struct foo, a) == NULL);
+ ok1(container_of_or_null((char *)NULL, struct foo, b) == NULL);
ok1(container_of_var(intp, &foo, a) == &foo);
ok1(container_of_var(charp, &foo, b) == &foo);
--
1.9.3
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