Bear is a tool that generates a compilation database for clang tooling.
The JSON compilation database is used in the clang project to provide information on how a single compilation unit is processed. With this, it is easy to re-run the compilation with alternate programs.
One way to get a compilation database is to use cmake as the build
tool. Passing -DCMAKE_EXPORT_COMPILE_COMMANDS=ON to cmake generates
the compile_commands.json file into the current directory.
For non-cmake projects, Bear generates the JSON file during the build process.
The concept behind Bear is: to execute the original build command and
intercept the exec calls issued by the build tool. To achieve that,
Bear uses the LD_PRELOAD or DYLD_INSERT_LIBRARIES mechanisms provided
by the dynamic linker.
Bear has two components: the library and the binary. The library
redefines the exec methods to be used by all child processes. The
executable enables the use of the library for child processes and
writes the output file.
Bear should be quite portable on UNIX operating systems. It has been tested on FreeBSD, GNU/Linux and OS X.
- an ANSI C compiler, to compile the sources.
- cmake, to configure the build process.
- make, to run the build. The makefiles are generated by
cmake. - python is a runtime dependency. The
bearcommand is written in Python. (version >= 2.7) - lit is an optional dependency to run the functional tests
Ideally, you should build Bear in a separate build directory.
cmake $BEAR_SOURCE_DIR
make all
make install # to install
make check # to run tests
make package # to make packages
You can configure the build process with passing arguments to cmake.
After installation the usage is like this:
bear <your-build-command>
The output file called compile_commands.json found in current directory.
For more options you can check the man page or pass --help parameter.
Side note: Since bear is executing the build command only those commands
will be recorded which were actually executed. Which means if you were already
built your project and you re-run the build command with Bear you probably end
up to have an empty output. (Practically it means you need to run make clean
before you run bear make.)
Because Bear uses LD_PRELOAD or DYLD_INSERT_LIBRARIES environment variables,
it does not append to it, but overrides it. So builds which are using these
variables might not work. (I don't know any build tool which does that, but
please let me know if you do.)
Multilib is one of the solutions allowing users to run applications built for various application binary interfaces (ABIs) of the same architecture. The most common use of multilib is to run 32-bit applications on 64-bit kernel.
For OSX this is not an issue. The build commands from previous section will work, Bear will intercept compiler calls for 32-bit and 64-bit applications.
For Linux, a small tune is needed at build time. Need to compile libear.so
libray for 32-bit and for 64-bit too. Then install these libraries to the OS
preferred multilib directories. And replace the libear.so path default
value with a single path, which matches both. (The match can be achieved by
the $LIB token expansion from the dynamic loader. See man ld.so for more.)
Debian derivatives are using lib/i386-linux-gnu and lib/x86_64-linux-gnu,
while many other distributions are simple lib and lib64. Here comes an
example build script to install a multilib capable Bear. It will install Bear
under /opt/bear on a non Debian system.
(cd ~/build32; cmake "$BEAR_SOURCE_DIR" -DCMAKE_C_COMPILER_ARG1="-m32"; VERBOSE=1 make all;)
(cd ~/build64; cmake "$BEAR_SOURCE_DIR" -DCMAKE_C_COMPILER_ARG1="-m64" -DDEFAULT_PRELOAD_FILE='/opt/bear/$LIB/libear.so'; VERBOSE=1 make all;)
sudo install -m 0644 ~/build32/libear/libear.so /opt/bear/lib/libear.so
sudo install -m 0644 ~/build64/libear/libear.so /opt/bear/lib64/libear.so
sudo install -m 0555 ~/build64/bear/bear" /opt/bear/bin/bear
To check you installation, install lit and run the test suite.
PATH=/opt/bear/bin:$PATH lit -v test
PATH=/opt/bear/bin:$PATH lit -v test -DMULTILIB=true
Security extension/modes on different operating systems might disable library
preloads. This case Bear behaves normally, but the result compilation database
will be empty. (Please make sure it's not the case when reporting bugs.)
Notable examples for enabled security modes are: OS X 10.11 (check with
csrutil status | grep 'System Integrity Protection'), and Fedora, CentOS, RHEL
(check with sestatus | grep 'SELinux status').
Workaround could be to disable the security feature while running Bear. (This
might involve reboot of your computer, so might be heavy workaround.) Another
option if the build tool is not installed under certain directories.
Or use tools which are using compiler wrappers. (It injects a fake compiler
which does record the compiler invocation and calls the real compiler too.)
An example for such tool might be scan-build. The build system
shall respect CC and CXX environment variables.
The two main constraints to intercept compiler execution from bazel builds are: bazel runs a daemon which runs the compilations, and it creates an isolated environment to run the compiler. These problems are not just hard to circumvent, but the workaround would not be stable to support it by this tool.
The good news is: there are extensions for bazel to generate the compilation database.
Currently Bear based on dynamic linker load mechanism, executions made by statically linked binaries are not captured. It means, if the build tool is statically linked binary, compiler calls won't be recorded by Bear.
If you find a bug in this documentation or elsewhere in the program or would like to propose an improvement, please use the project's github issue tracker. Please describing the bug and where you found it. If you have a suggestion how to fix it, include that as well. Patches are also welcome.