# Alpha Checkers

Experimental checkers in addition to the Default Checkers. These are checkers with known issues or limitations that keep them from being on by default. They are likely to have false positives. Bug reports are welcome but will likely not be investigated for some time. Patches welcome!

- Core Alpha Checkers
- C++ Alpha Checkers
- Dead Code Alpha Checkers
- OS X Alpha Checkers
- Security Alpha Checkers
- Unix Alpha Checkers

### Core Alpha Checkers

Name, Description | Example

alpha.core.BoolAssignment (ObjC)
Warn about assigning non-{0,1} values to boolean variables.

    void test() {
      BOOL b = -1; // warn
    }

alpha.core.CastSize (C)
Check when casting a malloc'ed type T, whether the size is a multiple of the size of T (Works only with unix.Malloc or alpha.unix.MallocWithAnnotations checks enabled).

    void test() {
      int *x = (int *)malloc(11); // warn
    }

alpha.core.CastToStruct (C, C++)
Check for cast from non-struct pointer to struct pointer.

    // C
    struct s {};

    void test(int *p) {
      struct s *ps = (struct s *) p; // warn
    }

    // C++
    class c {};

    void test(int *p) {
      c *pc = (c *) p; // warn
    }

alpha.core.FixedAddr (C)
Check for assignment of a fixed address to a pointer.

    void test() {
      int *p;
      p = (int *) 0x10000; // warn
    }

alpha.core.IdenticalExpr (C, C++)
Warn about suspicious uses of identical expressions.

    // C
    void test() {
      int a = 5;
      int b = a | 4 | a; // warn: identical expr on both sides
    }

    // C++
    bool f(void);

    void test(bool b) {
      int i = 10;
      if (f()) { // warn: true and false branches are identical
        do {
          i--;
        } while (f());
      } else {
        do {
          i--;
        } while (f());
      }
    }

alpha.core.PointerArithm (C)
Check for pointer arithmetic on locations other than array elements.

    void test() {
      int x;
      int *p;
      p = &x + 1; // warn
    }

alpha.core.PointerSub (C)
Check for pointer subtractions on two pointers pointing to different memory chunks.

    void test() {
      int x, y;
      int d = &y - &x; // warn
    }

alpha.core.SizeofPtr (C)
Warn about unintended use of `sizeof()` on pointer expressions.

    struct s {};

    int test(struct s *p) {
      return sizeof(p);
        // warn: sizeof(ptr) can produce an unexpected result
    }

### C++ Alpha Checkers

Name, Description | Example

alpha.cplusplus.NewDeleteLeaks (C++)
Check for memory leaks. Traces memory managed by `new`/`delete`.

    void test() {
      int *p = new int;
    } // warn

alpha.cplusplus.VirtualCall (C++)
Check virtual member function calls during construction or destruction.

    class A {
    public:
      A() {
        f(); // warn
      }
      virtual void f();
    };

    class A {
    public:
      ~A() {
        this->f(); // warn
      }
      virtual void f();
    };

### Dead Code Alpha Checkers

Name, Description | Example

alpha.deadcode.UnreachableCode (C, C++, ObjC)
Check unreachable code.

    // C
    int test() {
      int x = 1;
      while(x);
      return x; // warn
    }

    // C++
    void test() {
      int a = 2;

      while (a > 1)
        a--;

      if (a > 1)
        a++; // warn
    }

    // Objective-C
    void test(id x) {
      return;
      [x retain]; // warn
    }

### OS X Alpha Checkers

Name, Description | Example

alpha.osx.cocoa.Dealloc (ObjC)
Warn about Objective-C classes that lack a correct implementation of `-dealloc`.

    @interface MyObject : NSObject {
      id _myproperty;
    }
    @end

    @implementation MyObject // warn: lacks 'dealloc'
    @end

    @interface MyObject : NSObject {}
    @property(assign) id myproperty;
    @end

    @implementation MyObject // warn: does not send 'dealloc' to super
    - (void)dealloc {
      self.myproperty = 0;
    }
    @end

    @interface MyObject : NSObject {
      id _myproperty;
    }
    @property(retain) id myproperty;
    @end

    @implementation MyObject
    @synthesize myproperty = _myproperty;
      // warn: var was retained but wasn't released
    - (void)dealloc {
      [super dealloc];
    }
    @end

    @interface MyObject : NSObject {
      id _myproperty;
    }
    @property(assign) id myproperty;
    @end

    @implementation MyObject
    @synthesize myproperty = _myproperty;
      // warn: var wasn't retained but was released
    - (void)dealloc {
      [_myproperty release];
      [super dealloc];
    }
    @end

alpha.osx.cocoa.DirectIvarAssignment (ObjC)
Check that Objective C properties follow the following rule: the property should be set with the setter, not though a direct assignment.

    @interface MyClass : NSObject {}
    @property (readonly) id A;
    - (void) foo;
    @end

    @implementation MyClass
    - (void) foo {
      _A = 0; // warn
    }
    @end

alpha.osx.cocoa.DirectIvarAssignmentForAnnotatedFunctions (ObjC)
Check for direct assignments to instance variables in the methods annotated with `objc_no_direct_instance_variable_assignment`.

    @interface MyClass : NSObject {}
    @property (readonly) id A;
    - (void) fAnnotated __attribute__((
        annotate("objc_no_direct_instance_variable_assignment")));
    - (void) fNotAnnotated;
    @end

    @implementation MyClass
    - (void) fAnnotated {
      _A = 0; // warn
    }
    - (void) fNotAnnotated {
      _A = 0; // no warn
    }
    @end

alpha.osx.cocoa.InstanceVariableInvalidation (ObjC)
Check that the invalidatable instance variables are invalidated in the methods annotated with `objc_instance_variable_invalidator`.

    @protocol Invalidation <NSObject>
    - (void) invalidate
      __attribute__((annotate("objc_instance_variable_invalidator")));
    @end

    @interface InvalidationImpObj : NSObject <Invalidation>
    @end

    @interface SubclassInvalidationImpObj : InvalidationImpObj {
      InvalidationImpObj *var;
    }
    - (void)invalidate;
    @end

    @implementation SubclassInvalidationImpObj
    - (void) invalidate {}
    @end
    // warn: var needs to be invalidated or set to nil

alpha.osx.cocoa.MissingInvalidationMethod (ObjC)
Check that the invalidation methods are present in classes that contain invalidatable instance variables.

    @protocol Invalidation <NSObject>
    - (void)invalidate
      __attribute__((annotate("objc_instance_variable_invalidator")));
    @end

    @interface NeedInvalidation : NSObject <Invalidation>
    @end

    @interface MissingInvalidationMethodDecl : NSObject {
      NeedInvalidation *Var; // warn
    }
    @end

    @implementation MissingInvalidationMethodDecl
    @end

### Security Alpha Checkers

Name, Description | Example

alpha.security.ArrayBound (C)
Warn about buffer overflows (older checker).

    void test() {
      char *s = "";
      char c = s[1]; // warn
    }

    struct seven_words {
      int c[7];
    };

    void test() {
      struct seven_words a, *p;
      p = &a;
      p[0] = a;
      p[1] = a;
      p[2] = a; // warn
    }

    // note: requires unix.Malloc or
    // alpha.unix.MallocWithAnnotations checks enabled.
    void test() {
      int *p = malloc(12);
      p[3] = 4; // warn
    }

    void test() {
      char a[2];
      int *b = (int*)a;
      b[1] = 3; // warn
    }

alpha.security.ArrayBoundV2 (C)
Warn about buffer overflows (newer checker).

    void test() {
      char *s = "";
      char c = s[1]; // warn
    }

    void test() {
      int buf[100];
      int *p = buf;
      p = p + 99;
      p[1] = 1; // warn
    }

    // note: compiler has internal check for this.
    // Use -Wno-array-bounds to suppress compiler warning.
    void test() {
      int buf[100][100];
      buf[0][-1] = 1; // warn
    }

    // note: requires alpha.security.taint check turned on.
    void test() {
      char s[] = "abc";
      int x = getchar();
      char c = s[x]; // warn: index is tainted
    }

alpha.security.MallocOverflow (C)
Check for overflows in the arguments to `malloc()`.

    void test(int n) {
      void *p = malloc(n * sizeof(int)); // warn
    }

alpha.security.ReturnPtrRange (C)
Check for an out-of-bound pointer being returned to callers.

    static int A[10];

    int *test() {
      int *p = A + 10;
      return p; // warn
    }

    int test(void) {
      int x;
      return x; // warn: undefined or garbage returned
    }

alpha.security.taint.TaintPropagation (C)
Generate taint information used by other checkers.

    void test() {
      char x = getchar(); // 'x' marked as tainted
      system(&x); // warn: untrusted data is passed to a system call
    }

    // note: compiler internally checks if the second param to
    // sprintf is a string literal or not.
    // Use -Wno-format-security to suppress compiler warning.
    void test() {
      char s[10], buf[10];
      fscanf(stdin, "%s", s); // 's' marked as tainted

      sprintf(buf, s); // warn: untrusted data as a format string
    }

    void test() {
      size_t ts;
      scanf("%zd", &ts); // 'ts' marked as tainted
      int *p = (int *)malloc(ts * sizeof(int));
        // warn: untrusted data as bufer size
    }

### Unix Alpha Checkers

Name, Description | Example

alpha.unix.Chroot (C)
Check improper use of `chroot`.

    void f();

    void test() {
      chroot("/usr/local");
      f(); // warn: no call of chdir("/") immediately after chroot
    }

alpha.unix.MallocWithAnnotations (C)
Check for memory leaks, double free, and use-after-free problems. Assumes that all user-defined functions which might free a pointer are annotated.

    void __attribute((ownership_returns(malloc))) *my_malloc(size_t);

    void test() {
      int *p = my_malloc(1);
    } // warn: potential leak

    void __attribute((ownership_returns(malloc))) *my_malloc(size_t);
    void __attribute((ownership_takes(malloc, 1))) my_free(void *);

    void test() {
      int *p = my_malloc(1);
      my_free(p);
      my_free(p); // warn: attempt to free released
    }

    void __attribute((ownership_returns(malloc))) *my_malloc(size_t);
    void __attribute((ownership_holds(malloc, 1))) my_hold(void *);

    void test() {
      int *p = my_malloc(1);
      my_hold(p);
      free(p); // warn: attempt to free non-owned memory
    }

    void __attribute((ownership_takes(malloc, 1))) my_free(void *);

    void test() {
      int *p = malloc(1);
      my_free(p);
      *p = 1; // warn: use after free
    }

alpha.unix.PthreadLock (C)
Simple lock -> unlock checker; applies to:
pthread_mutex_lock
pthread_rwlock_rdlock
pthread_rwlock_wrlock
lck_mtx_lock
lck_rw_lock_exclusive
lck_rw_lock_shared
pthread_mutex_trylock
pthread_rwlock_tryrdlock
pthread_rwlock_tryrwlock
lck_mtx_try_lock
lck_rw_try_lock_exclusive
lck_rw_try_lock_shared
pthread_mutex_unlock
pthread_rwlock_unlock
lck_mtx_unlock
lck_rw_done

    pthread_mutex_t mtx;

    void test() {
      pthread_mutex_lock(&mtx);
      pthread_mutex_lock(&mtx);
        // warn: this lock has already been acquired
    }

    lck_mtx_t lck1, lck2;

    void test() {
      lck_mtx_lock(&lck1);
      lck_mtx_lock(&lck2);
      lck_mtx_unlock(&lck1);
        // warn: this was not the most recently acquired lock
    }

    lck_mtx_t lck1, lck2;

    void test() {
      if (lck_mtx_try_lock(&lck1) == 0)
        return;

      lck_mtx_lock(&lck2);
      lck_mtx_unlock(&lck1);
        // warn: this was not the most recently acquired lock
    }

alpha.unix.SimpleStream (C)
Check for misuses of stream APIs:
fopen
fclose
(demo checker, the subject of the demo (Slides ,Video) by Anna Zaks and Jordan Rose presented at the 2012 LLVM Developers' Meeting).

    void test() {
      FILE *F = fopen("myfile.txt", "w");
    } // warn: opened file is never closed

    void test() {
      FILE *F = fopen("myfile.txt", "w");

      if (F)
        fclose(F);

      fclose(F); // warn: closing a previously closed file stream
    }

alpha.unix.Stream (C)
Check stream handling functions:
fopen
tmpfile
fclose
fread
fwrite
fseek
ftell
rewind
fgetpos
fsetpos
clearerr
feof
ferror
fileno

    void test() {
      FILE *p = fopen("foo", "r");
    } // warn: opened file is never closed

    void test() {
      FILE *p = fopen("foo", "r");
      fseek(p, 1, SEEK_SET); // warn: stream pointer might be NULL
      fclose(p);
    }

    void test() {
      FILE *p = fopen("foo", "r");

      if (p)
        fseek(p, 1, 3);
         // warn: third arg should be SEEK_SET, SEEK_END, or SEEK_CUR

      fclose(p);
    }

    void test() {
      FILE *p = fopen("foo", "r");
      fclose(p);
      fclose(p); // warn: already closed
    }

    void test() {
      FILE *p = tmpfile();
      ftell(p); // warn: stream pointer might be NULL
      fclose(p);
    }

alpha.unix.cstring.BufferOverlap (C)
Checks for overlap in two buffer arguments; applies to:
memcpy
mempcpy

    void test() {
      int a[4] = {0};
      memcpy(a + 2, a + 1, 8); // warn
    }

alpha.unix.cstring.NotNullTerminated (C)
Check for arguments which are not null-terminated strings; applies to:
strlen
strnlen
strcpy
strncpy
strcat
strncat

    void test() {
      int y = strlen((char *)&test); // warn
    }

alpha.unix.cstring.OutOfBounds (C)
Check for out-of-bounds access in string functions; applies to:
strncopy
strncat

    void test(char *y) {
      char x[4];
      if (strlen(y) == 4)
        strncpy(x, y, 5); // warn
    }