Index: src/OFList.h
==================================================================
--- src/OFList.h
+++ src/OFList.h
@@ -29,15 +29,15 @@
  *
  * A struct that contains a pointer to the next list object, the previous list
  * object and the object.
  */
 struct of_list_object_t {
-	/// A pointer to the next list object in the list
-	of_list_object_t *next;
-	/// A pointer to the previous list object in the list
-	of_list_object_t *previous;
-	/// The object for the list object
+	/*! A pointer to the next list object in the list */
+	of_list_object_t *_Nullable next;
+	/*! A pointer to the previous list object in the list */
+	of_list_object_t *_Nullable previous;
+	/*! The object for the list object */
 	id __unsafe_unretained object;
 };
 
 /*!
  * @class OFList OFList.h ObjFW/OFList.h

Index: src/OFMapTable.h
==================================================================
--- src/OFMapTable.h
+++ src/OFMapTable.h
@@ -249,15 +249,15 @@
 /*!
  * @brief Returns the next object.
  *
  * @return The next object
  */
-- (void *)nextObject;
+- (nullable void *)nextObject;
 
 /*!
  * @brief Resets the enumerator, so the next call to @ref nextKey returns the
  *	  first key again.
  */
 - (void)reset;
 @end
 
 OF_ASSUME_NONNULL_END

Index: src/OFObject.h
==================================================================
--- src/OFObject.h
+++ src/OFObject.h
@@ -646,26 +646,28 @@
  * @brief Allocates memory and stores it in the object's memory pool.
  *
  * It will be freed automatically when the object is deallocated.
  *
  * @param size The size of the memory to allocate
- * @return A pointer to the allocated memory
+ * @return A pointer to the allocated memory. May return NULL if the specified
+ *	   size is 0.
  */
-- (void *)allocMemoryWithSize: (size_t)size;
+- (nullable void *)allocMemoryWithSize: (size_t)size;
 
 /*!
  * @brief Allocates memory for the specified number of items and stores it in
  *	  the object's memory pool.
  *
  * It will be freed automatically when the object is deallocated.
  *
  * @param size The size of each item to allocate
  * @param count The number of items to allocate
- * @return A pointer to the allocated memory
+ * @return A pointer to the allocated memory. May return NULL if the specified
+ *	   size or count is 0.
  */
-- (void *)allocMemoryWithSize: (size_t)size
-			count: (size_t)count;
+- (nullable void *)allocMemoryWithSize: (size_t)size
+				 count: (size_t)count;
 
 /*!
  * @brief Resizes memory in the object's memory pool to the specified size.
  *
  * If the pointer is NULL, this is equivalent to allocating memory.

Index: src/OFString+XMLUnescaping.h
==================================================================
--- src/OFString+XMLUnescaping.h
+++ src/OFString+XMLUnescaping.h
@@ -58,12 +58,12 @@
  *
  * @param string The string which contains the unknown entity
  * @param entity The name of the entity that is unknown
  * @return A substitution for the entity or `nil`
  */
--	  (OFString *)string: (OFString *)string
-  containsUnknownEntityNamed: (OFString *)entity;
+- (nullable OFString *)string: (OFString *)string
+   containsUnknownEntityNamed: (OFString *)entity;
 @end
 
 @interface OFString (XMLUnescaping)
 /*!
  * @brief Unescapes XML in the string.

Index: src/OFTarArchive.h
==================================================================
--- src/OFTarArchive.h
+++ src/OFTarArchive.h
@@ -84,9 +84,9 @@
  *	    @ref OFReadFailedException!
  *
  * @return The next entry from the tar archive or `nil` if all entries have
  *	   been read
  */
-- (OFTarArchiveEntry *)nextEntry;
+- (nullable OFTarArchiveEntry *)nextEntry;
 @end
 
 OF_ASSUME_NONNULL_END

Index: src/OFXMLElement.h
==================================================================
--- src/OFXMLElement.h
+++ src/OFXMLElement.h
@@ -276,21 +276,21 @@
  * @brief Returns the attribute with the specified name.
  *
  * @param attributeName The name of the attribute
  * @return The attribute with the specified name
  */
-- (OFXMLAttribute *)attributeForName: (OFString *)attributeName;
+- (nullable OFXMLAttribute *)attributeForName: (OFString *)attributeName;
 
 /*!
  * @brief Returns the attribute with the specified name and namespace.
  *
  * @param attributeName The name of the attribute
  * @param attributeNS The namespace of the attribute
  * @return The attribute with the specified name and namespace
  */
-- (OFXMLAttribute *)attributeForName: (OFString *)attributeName
-			   namespace: (nullable OFString *)attributeNS;
+- (nullable OFXMLAttribute *)attributeForName: (OFString *)attributeName
+				    namespace: (nullable OFString *)attributeNS;
 
 /*!
  * @brief Removes the attribute with the specified name.
  *
  * @param attributeName The name of the attribute
@@ -397,11 +397,11 @@
  * @brief Returns the first child element with the specified name.
  *
  * @param elementName The name of the element
  * @return The first child element with the specified name
  */
-- (OFXMLElement *)elementForName: (OFString *)elementName;
+- (nullable OFXMLElement *)elementForName: (OFString *)elementName;
 
 /*!
  * @brief Returns the child elements with the specified name.
  *
  * @param elementName The name of the elements
@@ -415,12 +415,12 @@
  *
  * @param elementName The name of the element
  * @param elementNS The namespace of the element
  * @return The first child element with the specified name and namespace
  */
-- (OFXMLElement *)elementForName: (OFString *)elementName
-		       namespace: (nullable OFString *)elementNS;
+- (nullable OFXMLElement *)elementForName: (OFString *)elementName
+				namespace: (nullable OFString *)elementNS;
 
 /*!
  * @brief Returns the child elements with the specified name and namespace.
  *
  * @param elementName The name of the elements

Index: src/OFXMLParser.h
==================================================================
--- src/OFXMLParser.h
+++ src/OFXMLParser.h
@@ -114,12 +114,12 @@
  *
  * @param parser The parser which found an unknown entity
  * @param entity The name of the entity the XML parser didn't know
  * @return A substitution for the entity or `nil`
  */
--      (OFString *)parser: (OFXMLParser *)parser
-  foundUnknownEntityNamed: (OFString *)entity;
+- (nullable OFString *)parser: (OFXMLParser *)parser
+      foundUnknownEntityNamed: (OFString *)entity;
 @end
 
 /*!
  * @class OFXMLParser OFXMLParser.h ObjFW/OFXMLParser.h
  *