[linux-2.6-block.git] / Documentation / dev-tools / sparse.rst

.. Copyright 2004 Linus Torvalds
.. Copyright 2004 Pavel Machek <pavel@ucw.cz>
.. Copyright 2006 Bob Copeland <me@bobcopeland.com>

Sparse
======

Sparse is a semantic checker for C programs; it can be used to find a
number of potential problems with kernel code.  See
https://lwn.net/Articles/689907/ for an overview of sparse; this document
contains some kernel-specific sparse information.
More information on sparse, mainly about its internals, can be found in
its official pages at https://sparse.docs.kernel.org.


Using sparse for typechecking
-----------------------------

"__bitwise" is a type attribute, so you have to do something like this::

        typedef int __bitwise pm_request_t;

        enum pm_request {
                PM_SUSPEND = (__force pm_request_t) 1,
                PM_RESUME = (__force pm_request_t) 2
        };

which makes PM_SUSPEND and PM_RESUME "bitwise" integers (the "__force" is
there because sparse will complain about casting to/from a bitwise type,
but in this case we really _do_ want to force the conversion). And because
the enum values are all the same type, now "enum pm_request" will be that
type too.

And with gcc, all the "__bitwise"/"__force stuff" goes away, and it all
ends up looking just like integers to gcc.

Quite frankly, you don't need the enum there. The above all really just
boils down to one special "int __bitwise" type.

So the simpler way is to just do::

        typedef int __bitwise pm_request_t;

        #define PM_SUSPEND ((__force pm_request_t) 1)
        #define PM_RESUME ((__force pm_request_t) 2)

and you now have all the infrastructure needed for strict typechecking.

One small note: the constant integer "0" is special. You can use a
constant zero as a bitwise integer type without sparse ever complaining.
This is because "bitwise" (as the name implies) was designed for making
sure that bitwise types don't get mixed up (little-endian vs big-endian
vs cpu-endian vs whatever), and there the constant "0" really _is_
special.

Using sparse for lock checking
------------------------------

The following macros are undefined for gcc and defined during a sparse
run to use the "context" tracking feature of sparse, applied to
locking.  These annotations tell sparse when a lock is held, with
regard to the annotated function's entry and exit.

__must_hold - The specified lock is held on function entry and exit.

__acquires - The specified lock is held on function exit, but not entry.

__releases - The specified lock is held on function entry, but not exit.

If the function enters and exits without the lock held, acquiring and
releasing the lock inside the function in a balanced way, no
annotation is needed.  The three annotations above are for cases where
sparse would otherwise report a context imbalance.

Getting sparse
--------------

You can get tarballs of the latest released versions from:
https://www.kernel.org/pub/software/devel/sparse/dist/

Alternatively, you can get snapshots of the latest development version
of sparse using git to clone::

        git://git.kernel.org/pub/scm/devel/sparse/sparse.git

Once you have it, just do::

        make
        make install

as a regular user, and it will install sparse in your ~/bin directory.

Using sparse
------------

Do a kernel make with "make C=1" to run sparse on all the C files that get
recompiled, or use "make C=2" to run sparse on the files whether they need to
be recompiled or not.  The latter is a fast way to check the whole tree if you
have already built it.

The optional make variable CF can be used to pass arguments to sparse.  The
build system passes -Wbitwise to sparse automatically.

Note that sparse defines the __CHECKER__ preprocessor symbol.
Commit	Line	Data
	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.