container_of: Add container_of_or_null()

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@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 0449935056f51a84af2a84f5b2dd7fcbf48c7678..47a34d853b4c73f0280e360fea413480e4df8229 100644 (file)
--- 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_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
 /**
  * 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 5da440a1e5d09ca05c3c36f03d04a149d694a9ab..18207f60521f418d75df8cee15293c062acc1a21 100644 (file)
--- 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;
 
        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(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);
 
        ok1(container_of_var(intp, &foo, a) == &foo);
        ok1(container_of_var(charp, &foo, b) == &foo);