# Nonnull argument violation Detects when an argument incorrectly receives a null value. ## Overview Use this check to detect when a function that has an argument with the `nonnull` attribute or the `_Nonnull` annotation receives a null value. Available in Xcode 9 and later. > Note: The nonnull violation check for arguments with the `_Nonnull` annotation is off by default. You can turn it on by enabling the `-fsanitize=nullability-arg` compiler flag. ### Violation of the nonnull parameter attribute in C In the following example, the call to the `has_nonnull_argument` function breaks the `nonnull` attribute of the parameter `p`: ```occ void has_nonnull_argument(__attribute__((nonnull)) int *p) { // ... } has_nonnull_argument(NULL); // Error: nonnull parameter attribute violation ``` #### Solution Correct logic errors, or remove the `nonnull` attribute and rework the called function’s logic accordingly. ### Violation of the nonnull annotation for an argument in C In the following example, the call to the `has_nonnull_argument` function breaks the `_Nonnull` annotation of the parameter `p`: ```occ void has_nonnull_argument(int * _Nonnull p) { // ... } has_nonnull_argument(NULL); // Error: _Nonnull annotation violation ``` #### Solution Correct logic errors, or remove the `_Nonnull` attribute and rework the called function’s 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)