Always verify that pointers and function return values are not null before dereferencing or using them. This prevents crashes and undefined behavior, especially when working with C functions or manual memory management. Key practices: - Check C function return values that can return null: `display := C.XOpenDisplay(0)` should be followed by a null check since XOpenDisplay is documented to return null on failure - Assert pointer validity in tests: `assert green_arena.data != unsafe { nil }` and `assert green_arena.head != unsafe { nil }` - Add defensive null checks in critical paths: `if g.fn_decl == unsafe { nil } || g.fn_decl.return_type == ast.void_type` prevents crashes when fn_decl might be uninitialized - Set pointers to nil after freeing to make use-after-free bugs easier to detect: `current.next = unsafe { nil }` after `free(current)` Example from discussions: ```v // Bad: No null check for C function return display := C.XOpenDisplay(0) // Immediate use without verification // Good: Check for null return display := C.XOpenDisplay(0) if display == unsafe { nil } { return error('Failed to open display') } defer { C.XCloseDisplay(display) } ``` This practice is essential for robust code that interfaces with C libraries or manages pointers manually.