Index: src/OFLocalization.h
==================================================================
--- src/OFLocalization.h
+++ src/OFLocalization.h
@@ -79,11 +79,11 @@
  *	    case, you need to manually allocate an instance and call
  *	    @ref init once.
  *
  * @return The shared OFLocalization instance
  */
-+ (instancetype)sharedLocalization;
++ (nullable instancetype)sharedLocalization;
 
 /**
  * @brief Returns the language of the locale.
  *
  * If the language is unknown, `nil` is returned.
@@ -116,11 +116,11 @@
 /*!
  * @brief Returns the decimal point of the system's locale.
  *
  * @return The decimal point of the system's locale
  */
-+ (OFString *)decimalPoint;
++ (nullable OFString *)decimalPoint;
 
 #ifdef OF_HAVE_FILES
 /*!
  * @brief Adds a directory to scan for language files.
  *

Index: src/OFRunLoop.h
==================================================================
--- src/OFRunLoop.h
+++ src/OFRunLoop.h
@@ -59,18 +59,18 @@
 /*!
  * @brief Returns the run loop for the main thread.
  *
  * @return The run loop for the main thread
  */
-+ (OFRunLoop *)mainRunLoop;
++ (nullable OFRunLoop *)mainRunLoop;
 
 /*!
  * @brief Returns the run loop for the current thread.
  *
  * @return The run loop for the current thread
  */
-+ (OFRunLoop *)currentRunLoop;
++ (nullable OFRunLoop *)currentRunLoop;
 
 /*!
  * @brief Adds an OFTimer to the run loop.
  *
  * @param timer The timer to add

Index: src/OFSystemInfo.h
==================================================================
--- src/OFSystemInfo.h
+++ src/OFSystemInfo.h
@@ -49,11 +49,11 @@
  * On Windows, it uses the `APPDATA` environment variable.@n
  * On Haiku, it uses the `B_USER_SETTINGS_DIRECTORY` directory.
  *
  * @return The path where user data for the application can be stored
  */
-+ (OFString *)userDataPath;
++ (nullable OFString *)userDataPath;
 
 /*!
  * @brief Returns the path where user configuration for the application can be
  *	  stored.
  *
@@ -63,11 +63,11 @@
  * On Windows, it uses the `APPDATA` environment variable.@n
  * On Haiku, it uses the `B_USER_SETTINGS_DIRECTORY` directory.
  *
  * @return The path where user configuration for the application can be stored
  */
-+ (OFString *)userConfigPath;
++ (nullable OFString *)userConfigPath;
 
 /*!
  * @brief Returns the vendor of the CPU.
  *
  * If the vendor could not be determined, `nil` is returned instead.

Index: src/OFThread.h
==================================================================
--- src/OFThread.h
+++ src/OFThread.h
@@ -110,26 +110,26 @@
 /*!
  * @brief Returns the current thread.
  *
  * @return The current thread
  */
-+ (OFThread *)currentThread;
++ (nullable OFThread *)currentThread;
 
 /*!
  * @brief Returns the main thread.
  *
  * @return The main thread
  */
-+ (OFThread *)mainThread;
++ (nullable OFThread *)mainThread;
 
 /*!
  * @brief Returns a dictionary to store thread-specific data, meaning it
  *	  returns a different dictionary for every thread.
  *
  * @return A dictionary to store thread-specific data.
  */
-+ (OFMutableDictionary *)threadDictionary;
++ (nullable OFMutableDictionary *)threadDictionary;
 #endif
 
 /*!
  * @brief Suspends execution of the current thread for the specified time
  *	  interval.