Bazel link library: How to change to link to built library? - bazel

Question

I am new to bazel.
I tried the tutorial below
https://docs.bazel.build/versions/master/tutorial/cpp.html#use-multiple-packages
And to create a shared library, I added 'linkstatic = False' in cc_binary,
So BUILD file is
load("#rules_cc//cc:defs.bzl", "cc_binary", "cc_library")
cc_library(
name = "hello-greet",
srcs = ["hello-greet.cc"],
hdrs = ["hello-greet.h"],
)
cc_binary(
name = "hello-world",
srcs = ["hello-world.cc"],
deps = [
":hello-greet",
"//lib:hello-time"
],
linkstatic = False,
)
and after build, I confirmed that 'libhello-greet.so' was created under the bazel-bin directory.
However, if I check the hello-world executable with ldd, it is as follows.
$ ldd hello-world
~snip~
libmain_Slibhello-greet.so => /home/foo/.cache/bazel/_bazel_wrs/8e7a6d8989ff6d697373ddbabde23fa3/execroot/__main__/bazel-out/k8-fastbuild/bin/main/./../_solib_k8/libmain_Slibhello-greet 0x00007f2706f84000)
liblib_Slibhello-time.so => /home/foo/.cache/bazel/_bazel_wrs/8e7a6d8989ff6d697373ddbabde23fa3/execroot/__main__/bazel-out/k8-fastbuild/bin/main/./../_solib_k8/liblib_Slibhello-time.so 0x00007f2706f81000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f2706462000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2706071000)
/lib64/ld-linux-x86-64.so.2 (0x00007f2706da1000)
Why hello-world linking the library(libmain_Slibhello-greet.s) in the cache directory instead of linking the locally generated libhello-greet.so?
How do I change it link to libhello-greet.so?

Answer 1

i think that the locally created directories are all Symlinks to the output directory ~/.chache/bazel as you can check with
fed#fed-VirtualBox:~/cpp-tutorial/stage2/bazel-bin/main$ readlink -f libhello-greet.so
/home/.cache/bazel/_bazel_fed/d6678e06051dff35967a6c61fe201847/execroot/__main__/bazel-out/k8-fastbuild/bin/main/libhello-greet.so
You can find out more on the bazel output layout here https://docs.bazel.build/versions/master/output_directories.html .
However, an alternative to have your final executable linked to a shared object under a certain path is to set your environment variable
fed#fed-VirtualBox: LD_LIBRARY_PATH="/home/cpp-tutorial/stage2/main/lib/"
fed#fed-VirtualBox: export LD_LIBRARY_PATH
and then directly import the library in your BUILD file
cc_import(
name = "hello-greet",
hdrs = ["hello-greet.h"],
shared_library = "lib/libhello-greet.so",
)
cc_binary(
name = "hello-world",
srcs = ["hello-world.cc"],
deps = [
":hello-greet",
],
linkstatic = False,
)
and then your final executable will be linked to the .so under your path
fed#fed-VirtualBox:~/examples/cpp-tutorial/stage2$ ldd bazel-bin/main/hello-world
linux-vdso.so.1 (0x00007ffe72df2000)
libhello-greet.so => /home/cpp-tutorial/stage2/main/lib/libhello-greet.so (0x00007f3bba04d000)
libstdc++.so.6 => /lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f3bb9e5a000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f3bb9d0b000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f3bb9cf0000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f3bb9afe000)
/lib64/ld-linux-x86-64.so.2 (0x00007f3bba057000)

Answer 2

bazel will do name mangling for shared libraries. you can use below workaround to static the shared library's name.
# build a shared library
cc_binary(
name = "libhello-greet.so",
srcs = [
"hello-greet.h",
"hello-greet.cc",
],
linkshared = True,
)
# input the library
cc_import(
name = "hello-greet",
shared_library = "libhello-greet.so",
hdrs = ["hello-greet.h"],
)
cc_binary(
name = "hello-world",
srcs = ["hello-world.cc"],
deps = [
":hello-greet",
"//lib:hello-time",
],
)
ldd results are as follows. You can see that the name has not changed.
linux-vdso.so.1 (0x00007fff53195000)
libhello-greet.so => /home/zero/01opencode/examples/cpp-tutorial/stage3/bazel-bin/main/../_solib_k8/_U_S_Smain_Chello-greet___Umain/libhello-greet.so (0x00007f2ca7050000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f2ca6aa8000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f2ca670a000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f2ca64f2000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f2ca6101000)
/lib64/ld-linux-x86-64.so.2 (0x00007f2ca6e31000)