c++ - How to write files to current directory instead of bazel-out

Question

I have the following directory structure:

my_dir
|
 --> src
|    |
|     --> foo.cc
|     --> BUILD
|
 --> WORKSPACE
|
 --> bazel-out/ (symlink)
| 
| ...

src/BUILD contains the following code:

cc_binary(
    name = "foo",
    srcs = ["foo.cc"]
)

The file foo.cc creates a file named bar.txt using the regular way with <fstream> utilities.

However, when I invoke Bazel with bazel run //src:foo the file bar.txt is created and placed in bazel-out/darwin-fastbuild/bin/src/foo.runfiles/foo/bar.txt instead of my_dir/src/bar.txt, where the original source is.

I tried adding an outs field to the foo rule, but Bazel complained that outs is not a recognized attribute for cc_binary.

I also thought of creating a filegroup rule, but there is no deps field where I can declare foo as a dependency for those files.

How can I make sure that the files generated by running the cc_binary rule are placed in my_dir/src/bar.txt instead of bazel-out/...?

Answer 1

Bazel doesn't allow you to modify the state of your workspace, by design.

The short answer is that you don't want the results of the past builds to modify the state of your workspace, hence potentially modifying the results of the future builds. It'll violate reproducibility if running Bazel multiple times on the same workspace results in different outputs.

Given your example: imagine calling bazel run //src:foo which inserts

#define true false
#define false true

at the top of the src/foo.cc. What happens if you call bazel run //src:foo again?

The long answer: https://docs.bazel.build/versions/master/rule-challenges.html#assumption-aim-for-correctness-throughput-ease-of-use-latency

Here's more information on the output directory: https://docs.bazel.build/versions/master/output_directories.html#documentation-of-the-current-bazel-output-directory-layout