1==============================================
2JSON Compilation Database Format Specification
3==============================================
4
5This document describes a format for specifying how to replay single
6compilations independently of the build system.
7
8Background
9==========
10
11Tools based on the C++ Abstract Syntax Tree need full information how to
12parse a translation unit. Usually this information is implicitly
13available in the build system, but running tools as part of the build
14system is not necessarily the best solution:
15
16-  Build systems are inherently change driven, so running multiple tools
17   over the same code base without changing the code does not fit into
18   the architecture of many build systems.
19-  Figuring out whether things have changed is often an IO bound
20   process; this makes it hard to build low latency end user tools based
21   on the build system.
22-  Build systems are inherently sequential in the build graph, for
23   example due to generated source code. While tools that run
24   independently of the build still need the generated source code to
25   exist, running tools multiple times over unchanging source does not
26   require serialization of the runs according to the build dependency
27   graph.
28
29Supported Systems
30=================
31
32Clang has the ability to generate compilation database fragments via
33``-MJ argument <clang -MJ\<arg>>``. You can concatenate those
34fragments together between ``[`` and ``]`` to create a compilation database.
35
36Currently `CMake <https://cmake.org>`_ (since 2.8.5) supports generation
37of compilation databases for Unix Makefile builds (Ninja builds in the
38works) with the option ``CMAKE_EXPORT_COMPILE_COMMANDS``.
39
40For projects on Linux, there is an alternative to intercept compiler
41calls with a tool called `Bear <https://github.com/rizsotto/Bear>`_.
42
43`Bazel <https://bazel.build>`_ can export a compilation database via
44`this extractor extension
45<https://github.com/hedronvision/bazel-compile-commands-extractor>`_.
46Bazel is otherwise resistant to Bear and other compiler-intercept
47techniques.
48
49Clang's tooling interface supports reading compilation databases; see
50the :doc:`LibTooling documentation <LibTooling>`. libclang and its
51python bindings also support this (since clang 3.2); see
52`CXCompilationDatabase.h </doxygen/group__COMPILATIONDB.html>`_.
53
54Format
55======
56
57A compilation database is a JSON file, which consist of an array of
58"command objects", where each command object specifies one way a
59translation unit is compiled in the project.
60
61Each command object contains the translation unit's main file, the
62working directory of the compile run and the actual compile command.
63
64Example:
65
66::
67
68    [
69      { "directory": "/home/user/llvm/build",
70        "arguments": ["/usr/bin/clang++", "-Irelative", "-DSOMEDEF=With spaces, quotes and \\-es.", "-c", "-o", "file.o", "file.cc"],
71        "file": "file.cc" },
72
73      { "directory": "/home/user/llvm/build",
74        "command": "/usr/bin/clang++ -Irelative -DSOMEDEF=\"With spaces, quotes and \\-es.\" -c -o file.o file.cc",
75        "file": "file2.cc" },
76
77      ...
78    ]
79
80The contracts for each field in the command object are:
81
82-  **directory:** The working directory of the compilation. All paths
83   specified in the **command** or **file** fields must be either
84   absolute or relative to this directory.
85-  **file:** The main translation unit source processed by this
86   compilation step. This is used by tools as the key into the
87   compilation database. There can be multiple command objects for the
88   same file, for example if the same source file is compiled with
89   different configurations.
90-  **arguments:** The compile command argv as list of strings.
91   This should run the compilation step for the translation unit ``file``.
92   ``arguments[0]`` should be the executable name, such as ``clang++``.
93   Arguments should not be escaped, but ready to pass to ``execvp()``.
94-  **command:** The compile command as a single shell-escaped string.
95   Arguments may be shell quoted and escaped following platform conventions,
96   with '``"``' and '``\``' being the only special characters. Shell expansion
97   is not supported.
98
99   Either **arguments** or **command** is required. **arguments** is preferred,
100   as shell (un)escaping is a possible source of errors.
101-  **output:** The name of the output created by this compilation step.
102   This field is optional. It can be used to distinguish different processing
103   modes of the same input file.
104
105Build System Integration
106========================
107
108The convention is to name the file compile\_commands.json and put it at
109the top of the build directory. Clang tools are pointed to the top of
110the build directory to detect the file and use the compilation database
111to parse C++ code in the source tree.
112
113Alternatives
114============
115For simple projects, Clang tools also recognize a ``compile_flags.txt`` file.
116This should contain one argument per line. The same flags will be used to
117compile any file.
118
119Example:
120
121::
122
123    -xc++
124    -I
125    libwidget/include/
126
127Here ``-I libwidget/include`` is two arguments, and so becomes two lines.
128Paths are relative to the directory containing ``compile_flags.txt``.
129
130