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