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Cake Static Analysis — Ownership & Nullable Contracts

Last Updated: August 2026

A hands-on guide to Cake's ownership and nullable pointer annotations - with working examples, enforced rules, and an incremental migration strategy for existing codebases.

Introduction

Cake provides a set of contract annotations recognized by its static analyzer. With ownership annotations, you can achieve the same guarantees as C++ RAII and often stronger ones. Cake also introduces nullable pointer annotations, making it explicit when a pointer may be null and preventing mistakes like accidentally dereferencing a null pointer.

This manual walks you through each concept with working code examples, explains the rules enforced by the analyzer, and shows you how to adopt these features incrementally in an existing codebase.

Chapter 1: Nullable Pointers

The _Opt pointer annotation

The _Opt pointer annotation explicitly marks a pointer as nullable. The absence of _Opt means the pointer is non-nullable. The annotation is placed after the *.

For example, the following declaration says that strdup() accepts a non-nullable pointer and returns a nullable one:

char * _Opt strdup(const char * src);

A pointer without _Opt is always assumed to be non-nullable.

Enabling Nullable Rules: #pragma nullable

Because existing C code was not written with nullability in mind, Cake provides a pragma to control when the new rules apply:

// new rules apply: absence of _Opt = non-nullable
#pragma nullable enable   

// Unannotated source: all unannotated pointers are nullable
#pragma nullable disable

This lets you migrate code incrementally enabling the rules file-by-file or region-by-region. Only static analysis behavior changes; the runtime behavior of your program is unaffected.

Example 1: Assigning Nullable to a Non-Nullable Pointer

Once nullable rules are enabled, assigning nullptr to an unqualified pointer generates a warning:

#pragma nullable enable

int main() {
  int * p = nullptr;  /* warning: p is non-nullable */
}

Example 2: Converting Non-Nullable to Nullable

Assigning a non-nullable pointer to a nullable variable is always safe and explicitly allowed:

#pragma nullable enable

char * get_name();   // returns non-nullable

int main() {
  char * _Opt s = get_name();  // ok: widening to nullable
}

Example 3: Nullable Pointer to a Non-Nullable Parameter

Narrowing from nullable to non-nullable requires a null check. Without one, the analyzer warns you:

#pragma nullable enable

char * _Opt strdup(const char * src);
void f(char *s);   // s is non-nullable

int main() {
  char * _Opt s1 = strdup("a");
  f(s1);  /* warning: s1 may be null */
}

To fix this, add a null check before the call:

#pragma nullable enable

char * _Opt strdup(const char * src);
void f(char *s);   // s is non-nullable

int main() {
  char * _Opt s1 = strdup("a");
  if (s1)
     f(s1);  // ok: flow analysis confirms s1 is not null here
}

Cake uses flow analysis to track possible nullability through branches. Once you check a pointer, the analyzer knows it is non-null inside the guarded block.

//lint

Because Cake's analysis is not inter-procedural, it cannot infer postconditions from called functions.

When the analyzer cannot determine a pointer's state on its own, you can use //lint warning-number:

#pragma safety enable

struct X { int * _Opt data; };

bool is_empty(struct X * p) {
    return p->data == nullptr;
}

void f(struct X * p) {
   if (!is_empty(p)) {
      *p->data = 1; //lint 33 (guaranteed by is_empty)
   }
}

Non-Nullable Initialization

Non-nullable pointers can be initialized with {}, meaning they are set to zero; however, they are still in an invalid state despite having a value. This is very similar to being uninitialized. For instance:

#pragma nullable enable

struct X { char * text; };

struct X f() {
    struct X x;
    return x;  // warning: returning uninitialized 'x.text'
}

Compare with:

#pragma nullable enable

struct X { char * text; };

struct X f() {
    struct X x = {};
    return x;  // warning: returning possible null pointer 'x.text'
}

In both cases, the object is in an invalid state. In the first case, x.text is uninitialized (it has no defined value). In the second case, x.text is initialized to zero (null), which is a defined value but still invalid for a non-nullable pointer.

#pragma nullable enable

char * _Opt strdup(const char * src);  

struct X { char * text; };  

void f() {  
   char * _Opt s = strdup("a");
   if (s == nullptr)
     return;
   struct X x;
   x.text = s; //ok
}

Using malloc() and calloc() with Structs

Cake understands the built-in semantics of malloc() and calloc() through the _Uninitialized and _Clear pointee annotations declared on their return types:

void * _Owner _Opt _Uninitialized malloc(size_t size);
void * _Owner _Opt _Clear calloc(size_t nmemb, size_t size);

#pragma nullable enable

char * _Opt strdup(const char * src);  
void * _Owner _Opt _Uninitialized malloc(unsigned int sz);

struct X {  char * text; };  

void f() {     
   struct X * _Owner _Opt pX = malloc(sizeof *pX);
   if (pX)
   {
      char * _Opt s = strdup("a");
      if (s != nullptr){
        pX->text = s; //ok
      }     
   }
}

#pragma safety enable  

char * _Opt strdup(const char * src);  
void * _Owner _Opt _Clear calloc(unsigned int n, unsigned int sz);
void free(void * _Owner _Opt p);

struct X {  
    char * text; //non-nullable
};  

void f0(struct X* p) { }

void f() {     
   struct X * _Owner _Opt pX = calloc(1, sizeof * pX);
   if (pX)
   {
      f0(pX); //warning 33: non-nullable pointer 'pX.text' may be null          
      free(pX);
   }
}

Chapter 2: Object Lifetime and Ownership

Object lifetime as the portion of program execution during which storage is reserved for that object. Cake's ownership system gives you many compile-time enforcement of these rules.

Enabling Ownership Checks

// enables lifetime checks
#pragma ownership enable  

// equivalent to: nullable enable + ownership enable
#pragma safety enable

Note: _Owner annotations are parsed even when ownership is disabled, but have no effect. Use #pragma safety enable as a shorthand for both features.

Owner References and the _Owner annotation

An owner reference is an object that manages the lifetime of the thing it references. The most common form is an owner pointer — a pointer to a heap-allocated object that is responsible for freeing it.

Declare an owner pointer by adding _Owner after the *:

#pragma safety enable

typedef int FILE;

FILE * _Owner _Opt fopen( const char *filename, const char *mode );
void fclose(FILE * _Owner p); /* p is not nullable */

int main() {
    FILE * _Owner _Opt f = fopen("file.txt", "r");
    if (f)
        fclose(f);  /* ownership is transferred to fclose */
}

If you comment out fclose(), the analyzer warns that f leaves scope without its ownership being transferred:

} // warning: 'f' leaves scope with an owned resource

The Two Core Ownership Rules

Rule: An owner reference is always the unique owner of the referenced object. Copying an owner reference transfers ownership to the destination.

Rule: Before the end of its lifetime, an owner reference must move ownership of the objects it owns.

Ownership transfer happens through assignment or by passing the owner as a function argument. Calling fclose(f) is exactly like assigning f to fclose's parameter — the callee takes ownership:

FILE * _Owner _Opt f2 = f;  // ownership transferred: f is now 'moved'
if (f2) fclose(f2);         // f2's ownership transferred to fclose

Additional Owner Rules

Rule: Owner objects cannot be discarded. Calling fopen() and ignoring the return value is an error.

fopen("file.txt", "r");  // warning: discarding owner return value

Rule: A non-owner object cannot be copied to an owner object.

Rule: The null pointer constant can be used to initialize owner objects (it represents an empty owner).

Non-Pointer Owner References

Ownership is not limited to pointers. Berkeley sockets, for example, use an integer file descriptor. You can mark any type as an owner:

_Owner int server_socket = socket(AF_INET, SOCK_STREAM, 0);
/* ... */
close(server_socket);

Note: The _Owner annotation belongs to the storage holding the reference, not to the pointed-to type. Its placement follows the same rules as const.

View References

A view reference accesses an object without managing its lifetime. Regular (non-_Owner) pointers are view references by default. The _View annotation is used on struct types, not on pointer declarations.

Rule: The lifetime of the referenced object must exceed the lifetime of the view reference.

The analyzer detects when a view pointer outlives what it points to:

struct X { int i; };

int main()
{
    struct X * _Opt p = nullptr;

    {
        struct X x = {};
        p = &x;
    }

    p->i = 1;  // warning: 'x' lifetime ended
    static_debug(p);
}

The _View annotation

The _View annotation enables assignment without ownership transfer. A _View object does not own the resource.

#pragma safety enable

struct X { char * _Owner _Opt text; };

void f(_View struct X x) { /* read-only access, no ownership */ }
void free(void* _Owner _Opt p);

int main() {
    struct X x = {};
    f(x); /* not moved */
    
    _View struct X x2 = x; /* not moved */

    free(x.text);   /* x is the owner */
}

Deleting Owner Pointers

Owner pointers take responsibility for both the pointed-to object and its storage. A common pattern is to implement a delete function that releases both:

#pragma safety enable
#include <stdlib.h>

struct X { char * _Owner text; };

void x_delete(struct X * _Owner _Opt p) {
  if (p) {
    free(p->text);
    free(p); /* storage */
  }
}

Note: void * _Owner owns only the storage.

Note: Converting T * _Owner to void * _Owner requires the pointed-to object to be empty first. This is how the analyzer distinguishes between moving the object and moving only the raw storage.

Comparison with C++ RAII

RAII is a pattern, not a static check: a constructor/destructor pair runs automatically at scope exit, but nothing verifies that a raw resource outside such a wrapper was released. T* p = new T; with no matching delete compiles without complaint in C++. The guarantee only exists for types that actually implement it.

Cake's _Owner takes the opposite approach: it enforces the same discipline directly on ordinary pointers and values, without requiring a wrapper type. Leaving scope with an unmoved _Owner is a compile-time warning, not a convention you have to trust. The fopen/fclose example earlier in this chapter is exactly the RAII pattern, but checked by the analyzer instead of relying on a class to run the cleanup.

The two approaches provide different guarantees. RAII executes the destructor automatically at scope exit. Cake's Out/Dtor are contracts that the analyzer verifies were fulfilled, but no code is generated to call them automatically. As a result, there are code paths that RAII would still clean up automatically, whereas Cake relies on the analyzer being able to verify that the required cleanup was written.

Cake also proves the ownership rules statically, with no runtime overhead. std::unique_ptr, which is the closest analog to _Owner (single, move-only ownership), still performs a runtime check in its destructor. After a move, its internal pointer is set to null so that delete ptr; becomes a no-op. Every destructor therefore executes a null check, whether or not the object was ever moved from. Cake proves the equivalent fact "this owner still holds something to release" ahead of time by tracking ownership and moved/null state through flow analysis, so no unconditional runtime check needs to be emitted. When the analyzer cannot prove that an owner is null, an explicit if (ptr) provides the missing proof, allowing the cleanup to be verified while keeping the common case free of unnecessary runtime checks.

Chapter 3: Parameter annotations

The _Dtor parameter annotation

_Dtor tells the analyzer that the function will move out all owner contents of the pointed-to struct, leaving it uninitialized. The compiler also verifies that the implementation fulfills this contract.

#pragma safety enable
#include <stdlib.h>

struct X { char * _Owner _Opt text; };

void x_destroy(_Dtor struct X * x) {
    free(x->text);  // moves x->text; contract fulfilled
}

int main() {
    struct X x = {};
    /* ... */
    x_destroy(&x);  // contents of x are now moved
}

Note: _Dtor is implicitly _Opt on every member it reaches — the pointee is only guaranteed to be partially constructed, so each member is treated as possibly-null regardless of its own declared nullability. This lets _Dtor accept an object that failed halfway through initialization. It does not excuse the callee from releasing a member that does turn out to be non-null: the caller is excused from that release (its ownership ends at the call), so the callee must actually do it, typically behind an if (x->member) ... guard.

You can build a delete function on top of x_destroy:

void x_delete(_Opt struct X * _Owner _Opt p) {
    if (p) {
        x_destroy(p);  // moves *p's contents
        free(p);       // releases storage
    }
}

The _Clear parameter annotation

_Clear tells the analyzer that the function will zero out every member of the pointed-to struct before returning. The compiler verifies that the implementation fulfills this contract — each member must be provably zero at every exit point:

#pragma safety enable

struct X { int a; int b; };

void zero_out(_Clear struct X* p) {
    p->a = 0;
    p->b = 0;  // both members must be zero at exit
}

Omitting a member (or leaving it possibly non-zero) is a warning:

void zero_out(_Clear struct X* p) {
    p->a = 0;
    // p->b never touched
} // warning: _Clear parameter 'p' pointee (.b) is not zero at exit

_Clear follows the same placement rules as _Out/_Dtor: it annotates the pointee, not the pointer, and the pointee cannot also be const (the callee has to write it).

The _Out parameter annotation

_Out is the inverse of _Dtor. It tells the analyzer that the function expects an uninitialized object as input and initializes it on return. This is the pattern for init-style functions.

#pragma safety enable
#include <string.h>

struct X { char * _Owner _Opt text; };

int init(_Out struct X *p, const char * text) {
    p->text = strdup(text);  // safe: p->text is uninitialized
}

int main() {
    struct X x;
    init(&x, "hello");  // x is fully initialized after this
    free(x.text);
}

Contrast this with a setter, which operates on an already-initialized object and must free the old value first:

#pragma safety enable
#include <string.h>

struct X { char * _Owner _Opt text; };

int set(struct X *p, const char * text) {
    // old value must be released
    //free(p->text);
    p->text = strdup(text);
}

Comparison with C#'s out parameters

_Out looks like C#'s out: both let the callee receive storage the caller has not initialized, on the understanding that the callee will initialize it. C#'s out is backed by definite assignment — the compiler rejects any code path in the callee that returns without assigning the out parameter — and _Out is checked the same way, against the implementation, not just assumed:

#pragma safety enable

char* _Owner _Opt strdup(const char* src);

struct X { char* _Owner _Opt text; };

void x_init_forgot(_Out struct X* p) {
    /* p->text is never written */
} // warning: _Out parameter 'p' pointee (.text) is possibly not initialized at exit

Unlike _Dtor — which only obligates _Owner members, since a plain scalar has no lifetime to end — _Out covers every member of the pointee, the same breadth as out. Leaving a non-_Owner field untouched is just as much a violation as leaving an _Owner one uninitialized, because the caller is trusting that the whole object is now well-formed, not only its owned resources:

#pragma safety enable

char* _Owner _Opt strdup(const char* src);

struct X { char* _Owner _Opt text; int i; };

void x_init(_Out struct X* p) {
    p->text = strdup("hello");
    p->i = 0;
} // ok: every member initialized

void x_init_partial(_Out struct X* p) {
    p->text = strdup("hello");
    /* p->i never touched */
} // warning: _Out parameter 'p' pointee (.i) is possibly not initialized at exit

Type Annotations in Arrays parameters

In C, array types in arguments are pointers. This characteristics is preserved.

To use the owner annotation in an array we do. (Just like const)

#pragma safety enable

void free(void * _Owner _Opt p);

void f(int a[_Owner])
{
  free(a);
}

But I think this is quite uncommon.

Chapter 4: Flow Analysis and Object States

Flow analysis is the foundation of Cake's nullable and ownership checks. It tracks the possible states of every variable at every point in your program.

Enabling Flow Analysis

#pragma flow enable    // standalone
#pragma safety enable  // enables flow automatically
// or compile with: -fanalyzer

The State Model

The analyzer tracks the following states for each variable:

uninitialized — the variable has been declared but not yet assigned a value.

moved — ownership was transferred to another variable or passed into a function call.

null — the pointer is null, or the owner holds no resource.

not-null — the pointer is known to reference a live object.

zero — the value of a non-pointer object is zero. This is distinct from null because, for non-pointer owners like sockets, zero does not necessarily mean "no resource".

not-zero — the value of a non-pointer object is non-zero.

lifetime-ended — the object the pointer was referencing has gone out of scope.

Note: uninitialized, moved, and lifetime-ended are imaginary states: they exist in flow analysis but not at runtime, and have no corresponding value in memory. The other states (null, not-null, zero, not-zero) describe something actually stored in the variable. But nothing at runtime is "uninitialized," "moved," or "lifetime-ended" — the bits sitting in memory are just bits (a moved-from or lifetime-ended pointer can still hold a perfectly valid-looking address). These states exist only so the analyzer can track what it knows: that no meaningful value has been given yet, that the variable is no longer accountable for the value it held, or that the object it once pointed to is no longer around.

Inspecting State with static_debug

Use this built-in declaration to inspect a variable's tracked state during development:

#pragma safety enable

int main() {
    int a;
    static_debug(a);   // prints state to compiler output
}

Proving Facts with compile_assert

compile_assert(expr) asks the analyzer to prove expr using only what it already knows at that point — across every value alternative the flow tracker has recorded, not just one possible path. If it can't prove expr on every alternative, it warns:

#pragma safety enable

void f(int c) {
    int a, b;
    if (c) { a = 1; b = 2; } else { a = 3; b = 4; }

    compile_assert(a + b == 3 || a + b == 7);  // ok: true on every path
    compile_assert(a + b == 3);                // warning: false on the c==0 path
}

Unlike assert() and _Assert(), compile_assert is purely a compile-time check:

#pragma safety enable

void f(int* _Opt p) {
    compile_assert(p != 0);  // warning: not provable here (p could be null)
    *p = 1;                  // warning: possible null pointer dereference (unaffected by the assert above)
}

Because of this, compile_assert is mainly a development and regression-testing tool: use it to pin down exactly what the analyzer can currently prove at a given point, and to catch the analyzer regressing on a previously-provable fact. Prefer assert()/_Assert() in real code where you actually want to establish a fact for the analysis (and the runtime) going forward.

Uninitialized State

Reading an uninitialized variable triggers a warning. This also applies after a move:

int * _Owner p = f();
free(p);   // p is now uninitialized (moved into free)
free(p);   // warning: p is uninitialized

Moved State

When you assign an owner to another owner in local scope, the source enters the moved state. The analyzer knows the pointed-to object is still valid but the source no longer owns it:

int * _Owner p = f();
int * _Owner _Opt p2 = 0;
p2 = p;       // p is now 'moved'
free(p);      // warning: p was moved
free(p2);     // ok

Null and Not-Null States

After a null check, the analyzer refines the state inside each branch:

void * _Owner _Opt p = malloc(1);
if (p) {
    compile_assert(p != 0);  // ok: provable inside the check
}
free(p);

Chapter 5: Known Limitations

Backward goto Is Not Analyzed as a Loop

Unlike while/for, which run a two-pass analysis to approximate repeated execution, a label is visited only once, in top-to-bottom order. A backward goto (jumping to a label above it) does not re-trigger analysis of the label's body, so the analyzer under-approximates what the jump can do:

#pragma safety enable

void f(int n) {
    int a = 0;
L:
    a = a + 1;
    if (n > 0) {
        n = n - 1;
        goto L;
    }
    // Expected (sound): a is an unbounded value, since the loop can run any
    // number of times.
    // Actual (today): a is seen as exactly 1, as if the backward jump had
    // no effect on the analysis.
    compile_assert(a == 1);
}

Workaround: Prefer while/for over backward goto where possible; those constructs get the full two-pass loop analysis.

Pointer Equality Between Distinct Pointers

Two unrelated pointers may still compare equal at runtime, but the analyzer currently treats a concrete (synthetic) pointer value as foldable, so p == q for two distinct pointees folds to always-false and the true branch is (incorrectly) treated as dead:

#pragma safety enable

void ptr_eq(int* _Opt p, int* _Opt q) {
    if (p == q) {
        // This branch is reachable at runtime and must still be analyzed,
        // but flow3 currently folds the comparison to always-false here.
        if (p) {
            compile_assert(p != 0);
        }
    }
}

Note: Comparisons against the literal 0/nullptr are unaffected — only two non-constant pointer operands trigger this.

These are implementation constraints, not flaws in the ownership model itself.

Chapter 6: Code Transition Strategy

Adopting Cake's static analysis in an existing codebase does not require a big-bang migration. The recommended approach is incremental:

  1. Create safe.h — define all Cake extensions (_Owner, _Opt, _View, _Dtor, _Out, compile_assert, static_debug) as empty macros. This lets your code compile cleanly with a standard C compiler before you begin annotating.

  2. Enable nullable rules one file at a time — add #pragma nullable enable to one translation unit, fix its warnings, then move to the next.

  3. Enable ownership rules — once nullable warnings are clean in a file, add #pragma ownership enable (or #pragma safety enable).

  4. Annotate signatures progressively — add _Owner, _Opt, _Out, and _Dtor annotations as you work through each file. The pragma-controlled rollout ensures you always have a compiling codebase.

Quick Reference

Type Annotations

_Opt — the pointer may be null (nullable). Without this type annotation, a pointer is treated as non-nullable when nullable rules are enabled.

_Owner — the reference manages the lifetime of the object it references. Ownership is transferred on assignment or when passed to a function.

_View on struct — strips _Owner from all members for the duration of that variable's scope. Used to pass an owner struct without transferring ownership.

_Out — the parameter must be uninitialized on entry; the function is responsible for initializing it before returning.

_Dtor — the parameter must be fully initialized on entry; the function is responsible for moving out all owner contents before returning.

_Clear — the function is responsible for setting every member of the pointee to zero before returning. Also used on a function's return type (e.g. calloc()) to describe an already zero-initialized pointee.

_Uninitialized — marks the pointee as uninitialized. Used on a function's return type (e.g. malloc()) to describe a freshly allocated, uninitialized pointee.

Pragmas

#pragma nullable enable / #pragma nullable disable — toggles non-nullable enforcement. When enabled, any pointer without _Opt is treated as non-nullable.

#pragma ownership enable — enables owner reference lifetime checks.

#pragma safety enable — enables both nullable and ownership checks at once. Equivalent to combining both pragmas above.

#pragma flow enable — enables flow analysis without enabling nullable or ownership rules.

Built-in Intrinsics

compile_assert(expr) — asks the analyzer to prove expr from what it currently knows; warns if it can't. Purely compile-time — no runtime effect, and it does not narrow later state.

static_debug(var) — prints var's current state to the compiler output. Use this to explore what the analyzer knows about a variable.

References