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