# Nonnull return value violation Detects when a function incorrectly returns null. ## Overview Use this check to detect when a function with the `returns_nonnull` attribute, or a function with a return type that has the `_Nonnull` annotation, returns null. Available in Xcode 9 and later. > Note: The nonnull violation check for return types with the `_Nonnull` annotation is off by default. You can turn it on by enabling the `-fsanitize=nullability-return` compiler flag. ### Violation of the nonnull attribute for a function in C In the following code, there is a violation of the `returns_nonnull` attribute of the `nonnull_returning_function` function: ```occ __attribute__((returns_nonnull)) int *nonnull_returning_function(int *p) { return p; // Warning: NULL can be returned here } nonnull_returning_function(NULL); // Error: nonnull return value attribute violation ``` #### Solution Correct logic errors, add any necessary null guards to the function, or remove the `returns_nonnull` attribute and rework the function caller logic accordingly. ### Violation of the nonnull annotation for a return type in C The following code violates the `_Nonnull` annotation of the return type for the `nonnull_returning_function` function: ```occ int *_Nonnull nonnull_returning_function(int *p) { return p; // Warning: NULL can be returned here } nonnull_returning_function(NULL); // Error: nonnull return value attribute violation ``` #### Solution Correct logic errors, add any necessary null guards to the function, or remove the `_Nonnull` annotation and rework the function caller logic accordingly. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)