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These macros are used to embed information for tools in program source. A use of one of these macros is called an "annotation". A tool may define a set of such annotations which can then
be used to provide the tool with information that would otherwise be unavailable from the source code.
Annotations should, in general, provide documentation useful to the human reader. If information is of no use to a human trying to understand the code but is necessary for proper operation of a tool,
use another mechanism for conveying that information to the tool (one which does not involve adding to the source code), so as not to detract from the readability of the source. The following is an example
of an annotation which provides information of use to a tool and to the human reader (in this case, which data are protected by a particular lock, an annotation defined by the static lock analysis tool lock_lint).
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NOTE(MUTEX_PROTECTS_DATA(foo_lock, foo_list Foo))
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Such annotations do not represent executable code; they are neither statements nor declarations. They should not be followed by a semicolon. If a compiler or tool that analyzes C source does not
understand this annotation scheme, then the tool will ignore the annotations. (For such tools, NOTE(x) expands to nothing.)
Annotations may only be placed at particular places in the source. These places are where the following C constructs would be allowed:
- a top-level declaration (that is, a declaration not within a function or other construct)
- a declaration or statement within a block (including the block which defines a function)
- a member of a struct or union.
Annotations are not allowed in any other place. For example, the following are illegal:
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x = y + NOTE(...) z ;
typedef NOTE(...) unsigned int uint ;
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While NOTE and _NOTE may be used in the places described above, a particular type of annotation may only be allowed in a subset of those places. For example,
a particular annotation may not be allowed inside a struct or union definition.
NOTE vs _NOTE
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Ordinarily, NOTE should be used rather than _NOTE, since use of _NOTE technically makes a program non-portable. However, it may be inconvenient
to use NOTE for this purpose in existing code if NOTE is already heavily used for another purpose. In this case one should use a different macro and write a header
file similar to /usr/include/note.h which maps that macro to _NOTE in the same manner. For example, the following makes FOO such a macro:
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#ifndef _FOO_H
#define _FOO_H
#define FOO _NOTE
#include <sys/note.h>
#endif
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Public header files which span projects should use _NOTE rather than NOTE, since NOTE may already be used by a program which needs to include
such a header file.
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NoteInfo Argument
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The actual NoteInfo used in an annotation should be specified by a tool that deals with program source (see the documentation for the tool to determine which annotations,
if any, it understands).
NoteInfo must have one of the following forms:
where NoteName is simply an identifier which indicates the type of annotation, and Args is something defined by the tool that specifies the particular NoteName. The general restrictions on Args are that it be compatible with an ANSI C tokenizer and that unquoted parentheses be balanced (so that the end of the annotation
can be determined without intimate knowledge of any particular annotation).
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