1 # Build System Best Practices 2 3 ## Read only source tree 4 5 Never write to the source directory during the build, always write to 6 `$OUT_DIR`. We expect to enforce this in the future. 7 8 If you want to verify / provide an update to a checked in generated source 9 file, generate that file into `$OUT_DIR` during the build, fail the build 10 asking the user to run a command (either a straight command, checked in script, 11 generated script, etc) to explicitly copy that file from the output into the 12 source tree. 13 14 ## Network access 15 16 Never access the network during the build. We expect to enforce this in the 17 future, though there will be some level of exceptions for tools like `distcc` 18 and `goma`. 19 20 ## Paths 21 22 Don't use absolute paths in Ninja files (with make's `$(abspath)` or similar), 23 as that could trigger extra rebuilds when a source directory is moved. 24 25 Assume that the source directory is `$PWD`. If a script is going to change 26 directories and needs to convert an input from a relative to absolute path, 27 prefer to do that in the script. 28 29 Don't encode absolute paths in build intermediates or outputs. This would make 30 it difficult to reproduce builds on other machines. 31 32 Don't assume that `$OUT_DIR` is `out`. The source and output trees are very 33 large these days, so some people put these on different disks. There are many 34 other uses as well. 35 36 Don't assume that `$OUT_DIR` is under `$PWD`, users can set it to a relative path 37 or an absolute path. 38 39 ## $(shell) use in Android.mk files 40 41 Don't use `$(shell)` to write files, create symlinks, etc. We expect to 42 enforce this in the future. Encode these as build rules in the build graph 43 instead. This can be problematic in a number of ways: 44 45 * `$(shell)` calls run at the beginning of every build, at minimum this slows 46 down build startup, but it can also trigger more build steps to run than are 47 necessary, since these files will change more often than necessary. 48 * It's no longer possible for a stripped-down product configuration to opt-out 49 of these created files. It's better to have actual rules and dependencies set 50 up so that space isn't wasted, but the files are there when necessary. 51 52 ## Headers 53 54 `LOCAL_COPY_HEADERS` is deprecated. Soong modules cannot use these headers, and 55 when the VNDK is enabled, System modules in Make cannot declare or use them 56 either. 57 58 The set of global include paths provided by the build system is also being 59 removed. They've been switched from using `-isystem` to `-I` already, and are 60 removed entirely in some environments (vendor code when the VNDK is enabled). 61 62 Instead, use `LOCAL_EXPORT_C_INCLUDE_DIRS`/`export_include_dirs`. These allow 63 access to the headers automatically if you link to the associated code. 64 65 If your library uses `LOCAL_EXPORT_C_INCLUDE_DIRS`/`export_include_dirs`, and 66 the exported headers reference a library that you link to, use 67 `LOCAL_EXPORT_SHARED_LIBRARY_HEADERS`/`LOCAL_EXPORT_STATIC_LIBRARY_HEADERS`/`LOCAL_EXPORT_HEADER_LIBRARY_HEADERS` 68 (`export_shared_lib_headers`/`export_static_lib_headers`/`export_header_lib_headers`) 69 to re-export the necessary headers to your users. 70 71 Don't use non-local paths in your `LOCAL_EXPORT_C_INCLUDE_DIRS`, use one of the 72 `LOCAL_EXPORT_*_HEADERS` instead. Non-local exported include dirs are not 73 supported in Soong. You may need to either move your module definition up a 74 directory (for example, if you have ./src/ and ./include/, you probably want to 75 define the module in ./Android.bp, not ./src/Android.bp), define a header 76 library and re-export it, or move the headers into a more appropriate location. 77 78 Prefer to use header libraries (`BUILD_HEADER_LIBRARY`/ `cc_library_headers`) 79 only if the headers are actually standalone, and do not have associated code. 80 Sometimes there are headers that have header-only sections, but also define 81 interfaces to a library. Prefer to split those header-only sections out to a 82 separate header-only library containing only the header-only sections, and 83 re-export that header library from the existing library. This will prevent 84 accidentally linking more code than you need (slower at build and/or runtime), 85 or accidentally not linking to a library that's actually necessary. 86 87 Prefer `LOCAL_EXPORT_C_INCLUDE_DIRS` over `LOCAL_C_INCLUDES` as well. 88 Eventually we'd like to remove `LOCAL_C_INCLUDES`, though significant cleanup 89 will be required first. This will be necessary to detect cases where modules 90 are using headers that shouldn't be available to them -- usually due to the 91 lack of ABI/API guarantees, but for various other reasons as well: layering 92 violations, planned deprecations, potential optimizations like C++ modules, 93 etc. 94 95 ## Use defaults over variables 96 97 Soong supports variable definitions in Android.bp files, but in many cases, 98 it's better to use defaults modules like `cc_defaults`, `java_defaults`, etc. 99 100 * It moves more information next to the values -- that the array of strings 101 will be used as a list of sources is useful, both for humans and automated 102 tools. This is even more useful if it's used inside an architecture or 103 target specific property. 104 * It can collect multiple pieces of information together into logical 105 inheritable groups that can be selected with a single property. 106 107 ## Custom build tools 108 109 If writing multiple files from a tool, declare them all in the build graph. 110 * Make: Use `.KATI_IMPLICIT_OUTPUTS` 111 * Android.bp: Just add them to the `out` list in genrule 112 * Custom Soong Plugin: Add to `Outputs` or `ImplicitOutputs` 113 114 Declare all files read by the tool, either with a dependency if you can, or by 115 writing a dependency file. Ninja supports a fairly limited set of dependency 116 file formats. You can verify that the dependencies are read correctly with: 117 118 ``` 119 NINJA_ARGS="-t deps <output_file>" m 120 ``` 121 122 Prefer to list input files on the command line, otherwise we may not know to 123 re-run your command when a new input file is added. Ninja does not treat a 124 change in dependencies as something that would invalidate an action -- the 125 command line would need to change, or one of the inputs would need to be newer 126 than the output file. If you don't include the inputs in your command line, you 127 may need to add the the directories to your dependency list or dependency file, 128 so that any additions or removals from those directories would trigger your 129 tool to be re-run. That can be more expensive than necessary though, since many 130 editors will write temporary files into the same directory, so changing a 131 README could trigger the directory's timestamp to be updated. 132 133 Only control output files based on the command line, not by an input file. We 134 need to know which files will be created before any inputs are read, since we 135 generate the entire build graph before reading source files, or running your 136 tool. This comes up with Java based tools fairly often -- they'll generate 137 different output files based on the classes declared in their input files. 138 We've worked around these tools with the "srcjar" concept, which is just a jar 139 file containing the generated sources. Our Java compilation tasks understand 140 *.srcjar files, and will extract them before passing them on to the compiler. 141 142 ## Libraries in PRODUCT_PACKAGES 143 144 Most libraries aren't necessary to include in `PRODUCT_PACKAGES`, unless 145 they're used dynamically via `dlopen`. If they're only used via 146 `LOCAL_SHARED_LIBRARIES` / `shared_libs`, then those dependencies will trigger 147 them to be installed when necessary. Adding unnecessary libraries into 148 `PRODUCT_PACKAGES` will force them to always be installed, wasting space. 149
