Compiler Attributes: nonnull

Apr 16, 2025

You can use nonnull attribute to write safer, clearer, and faster code.

GCC and Clang support the __attribute__((nonnull)) (or [[gnu::nonnull]] in C++) to mark function pointer arguments as never NULL. This helps:

1. Catch NULL arguments at compile time.
2. Enable better compiler optimizations.
3. Document intent clearly.

• __attribute__((nonnull)): All pointer args must be non-NULL.
• __attribute__((nonnull(1, 3))): Only specific arguments (1-based indexing).

For example:

void f(int *p) __attribute__((nonnull(1)));
f(NULL); // May trigger warning with -Wall

In C++:

[[gnu::nonnull(2)]] void g(int a, int *b);

Note that

• Add -Wall to enable warnings.
• Apply nonnull in declarations, not definitions.
• Only catches statically known NULLs—runtime checks are still needed.