Annotating Structs and Classes

You can annotate struct and class members by using annotations that act like invariants—they are presumed to be true at any function call or function entry/exit that involves the enclosing structure as a parameter or a result value.

Struct and Class Annotations

  • _Field_range_(low, high)

    The field is in the range (inclusive) from low to high. Equivalent to _Satisfies_(_Curr_ >= low && _Curr_ <= high) applied to the annotated object by using the appropriate pre or post conditions.

  • _Field_size_(size), _Field_size_opt_(size), _Field_size_bytes_(size), _Field_size_bytes_opt_(size)

    A field that has a writable size in elements (or bytes) as specified by size.

  • _Field_size_part_(size, count), _Field_size_part_opt_(size, count), _Field_size_bytes_part_(size, count), _Field_size_bytes_part_opt_(size, count)

    A field that has a writable size in elements (or bytes) as specified by size, and the count of those elements (bytes) that are readable.

  • _Field_size_full_(size), _Field_size_full_opt_(size), _Field_size_bytes_full_(size), _Field_size_bytes_full_opt_(size)

    A field that has both readable and writable size in elements (or bytes) as specified by size.

  • _Field_z_

    A field that has a null-terminated string.

  • _Struct_size_bytes_(size)

    Applies to struct or class declaration. Indicates that a valid object of that type may be larger than the declared type, with the number of bytes being specified by size. For example:

    typedef _Struct_size_bytes_(nSize)
    struct MyStruct {
        size_t nSize;

    The buffer size in bytes of a parameter pM of type MyStruct * is then taken to be:

    min(pM->nSize, sizeof(MyStruct))


#include <sal.h>

// This _Struct_size_bytes_ is equivalent to what below _Field_size_ means.
_Struct_size_bytes_(__builtin_offsetof(MyBuffer, buffer) + bufferSize * sizeof(int))
struct MyBuffer
    static int MaxBufferSize;

    const char* name;

    int firstField;

    // ... other fields

    _Field_range_(1, MaxBufferSize)
    int bufferSize;

    _Field_size_(bufferSize)        // Prefered way - easier to read and maintain.
    int buffer[]; // Using C99 Flexible array member

Notes for this example:

  • _Field_z_ is equivalent to _Null_terminated_. _Field_z_ for the name field specifies that the name field is a null-terminated string.
  • _Field_range_ for bufferSize specifies that the value of bufferSize should be within 1 and MaxBufferSize (both inclusive).
  • The end results of the _Struct_size_bytes_ and _Field_size_ annotations are equivalent. For structures or classes that have a similar layout, _Field_size_ is easier to read and maintain, because it has fewer references and calculations than the equivalent _Struct_size_bytes_ annotation. _Field_size_ doesn’t require conversion to the byte size. If byte size is the only option, for example, for a void pointer field, _Field_size_bytes_ can be used. If both _Struct_size_bytes_ and _Field_size_ exist, both will be available to tools. It is up to the tool what to do if the two annotations disagree.

See also