1.. Copyright 2004 Linus Torvalds
2.. Copyright 2004 Pavel Machek <pavel@ucw.cz>
3.. Copyright 2006 Bob Copeland <me@bobcopeland.com>
4
5Sparse
6======
7
8Sparse is a semantic checker for C programs; it can be used to find a
9number of potential problems with kernel code.  See
10https://lwn.net/Articles/689907/ for an overview of sparse; this document
11contains some kernel-specific sparse information.
12More information on sparse, mainly about its internals, can be found in
13its official pages at https://sparse.docs.kernel.org.
14
15
16Using sparse for typechecking
17-----------------------------
18
19"__bitwise" is a type attribute, so you have to do something like this::
20
21        typedef int __bitwise pm_request_t;
22
23        enum pm_request {
24                PM_SUSPEND = (__force pm_request_t) 1,
25                PM_RESUME = (__force pm_request_t) 2
26        };
27
28which makes PM_SUSPEND and PM_RESUME "bitwise" integers (the "__force" is
29there because sparse will complain about casting to/from a bitwise type,
30but in this case we really _do_ want to force the conversion). And because
31the enum values are all the same type, now "enum pm_request" will be that
32type too.
33
34And with gcc, all the "__bitwise"/"__force stuff" goes away, and it all
35ends up looking just like integers to gcc.
36
37Quite frankly, you don't need the enum there. The above all really just
38boils down to one special "int __bitwise" type.
39
40So the simpler way is to just do::
41
42        typedef int __bitwise pm_request_t;
43
44        #define PM_SUSPEND ((__force pm_request_t) 1)
45        #define PM_RESUME ((__force pm_request_t) 2)
46
47and you now have all the infrastructure needed for strict typechecking.
48
49One small note: the constant integer "0" is special. You can use a
50constant zero as a bitwise integer type without sparse ever complaining.
51This is because "bitwise" (as the name implies) was designed for making
52sure that bitwise types don't get mixed up (little-endian vs big-endian
53vs cpu-endian vs whatever), and there the constant "0" really _is_
54special.
55
56Using sparse for lock checking
57------------------------------
58
59The following macros are undefined for gcc and defined during a sparse
60run to use the "context" tracking feature of sparse, applied to
61locking.  These annotations tell sparse when a lock is held, with
62regard to the annotated function's entry and exit.
63
64__must_hold - The specified lock is held on function entry and exit.
65
66__acquires - The specified lock is held on function exit, but not entry.
67
68__releases - The specified lock is held on function entry, but not exit.
69
70If the function enters and exits without the lock held, acquiring and
71releasing the lock inside the function in a balanced way, no
72annotation is needed.  The three annotations above are for cases where
73sparse would otherwise report a context imbalance.
74
75Getting sparse
76--------------
77
78You can get tarballs of the latest released versions from:
79https://www.kernel.org/pub/software/devel/sparse/dist/
80
81Alternatively, you can get snapshots of the latest development version
82of sparse using git to clone::
83
84        git://git.kernel.org/pub/scm/devel/sparse/sparse.git
85
86Once you have it, just do::
87
88        make
89        make install
90
91as a regular user, and it will install sparse in your ~/bin directory.
92
93Using sparse
94------------
95
96Do a kernel make with "make C=1" to run sparse on all the C files that get
97recompiled, or use "make C=2" to run sparse on the files whether they need to
98be recompiled or not.  The latter is a fast way to check the whole tree if you
99have already built it.
100
101The optional make variable CF can be used to pass arguments to sparse.  The
102build system passes -Wbitwise to sparse automatically.
103