Background#

Tools based on the C++ Abstract Syntax Tree need full information how to parse a translation unit. Usually this information is implicitly available in the build system, but running tools as part of the build system is not necessarily the best solution:

• Build systems are inherently change driven, so running multiple tools over the same code base without changing the code does not fit into the architecture of many build systems.

• Figuring out whether things have changed is often an IO bound process; this makes it hard to build low latency end user tools based on the build system.

• Build systems are inherently sequential in the build graph, for example due to generated source code. While tools that run independently of the build still need the generated source code to exist, running tools multiple times over unchanging source does not require serialization of the runs according to the build dependency graph.

Supported Systems#

Clang has the ability to generate compilation database fragments via -MJ argument <clang -MJ\<arg>>. You can concatenate those fragments together between [ and ] to create a compilation database.

Currently CMake (since 2.8.5) supports generation of compilation databases for Unix Makefile builds (Ninja builds in the works) with the option CMAKE_EXPORT_COMPILE_COMMANDS.

For projects on Linux, there is an alternative to intercept compiler calls with a tool called Bear.

Bazel can export a compilation database via this extractor extension. Bazel is otherwise resistant to Bear and other compiler-intercept techniques.

Clang’s tooling interface supports reading compilation databases; see the LibTooling documentation. libclang and its Python bindings also support this (since clang 3.2); see CXCompilationDatabase.h.

Format#

A compilation database is a JSON file, which consists of an array of “command objects”, where each command object specifies one way a translation unit is compiled in the project.

Each command object contains the translation unit’s main file, the working directory of the compile run and the actual compile command.

Example:

[
  { "directory": "/home/user/llvm/build",
    "arguments": ["/usr/bin/clang++", "-Irelative", "-DSOMEDEF=With spaces, quotes and \\-es.", "-c", "-o", "file.o", "file.cc"],
    "file": "file.cc" },

  { "directory": "/home/user/llvm/build",
    "command": "/usr/bin/clang++ -Irelative -DSOMEDEF=\"With spaces, quotes and \\-es.\" -c -o file.o file.cc",
    "file": "file2.cc" },

  ...
]

The contracts for each field in the command object are:

• directory: The working directory of the compilation. All paths specified in the command or file fields must be either absolute or relative to this directory. A relative directory is relative to the directory containing the compilation database file. Symlinks are not resolved: a symlink to the file is relative to the symlink’s directory.

• file: The main translation unit source processed by this compilation step. This is used by tools as the key into the compilation database. There can be multiple command objects for the same file, for example if the same source file is compiled with different configurations.

• arguments: The compile command argv as list of strings. This should run the compilation step for the translation unit file. arguments[0] should be the executable name, such as clang++. Arguments should not be escaped, but ready to pass to execvp().

• command: The compile command as a single shell-escaped string. Arguments may be shell quoted and escaped following platform conventions, with ‘"’ and ‘\’ being the only special characters. Shell expansion is not supported.

  Either arguments or command is required. arguments is preferred, as shell (un)escaping is a possible source of errors.

• output: The name of the output created by this compilation step. This field is optional. It can be used to distinguish different processing modes of the same input file.

Build System Integration#

The convention is to name the file compile_commands.json and put it at the top of the build directory. Clang tools are pointed to the top of the build directory to detect the file and use the compilation database to parse C++ code in the source tree.

Alternatives#

For simple projects, Clang tools also recognize a compile_flags.txt file. This should contain one argument per line. The same flags will be used to compile any file.

Example:

-xc++
-I
libwidget/include/

Here -I libwidget/include is two arguments, and so becomes two lines. Paths are relative to the directory containing compile_flags.txt.