how to include header file for each file compiled via bazel - c++

Question

I have a project that I'm building using bazel. My application references another bazel repository as a dependency, in its BUILD file, let's call this #dep. I cannot make any changes to the code in #dep, but I need to override a C macro defined in one of the header files in #dep.
I thought about using the compiler option -D to define the symbol defined at the top of the header file I want to replace, which contains the C macro, and then using -include to include a different header file with my macro, for all files in #dep which are being compiled via bazel. But in cc_binary, there is no option for -include, and copt = [] will only work for the target being compiled and not for its dependencies.
I came across this post but unfortunately the solution was not posted - How to specify preprocessor includes in Bazel? (-include common_header.h)

Answer 1

I don’t get why they don’t provide a way to use -include on the compiler command line for each file compiled, via the cc_binary build rule. They have defines = [] after all.
In general bazel doesn't provide a lot of mechanisms like that because it can cause scaling issues in large repositories. E.g. say project1 and project2 depend on the same tree of 1000 cc_library targets. Building both of them together results in 1000 cc_library targets being built. But if cc_binary had a way to change the way its dependencies are compiled, and project2 did something like that, then building the two projects together now requires building 2000 cc_library targets, which doubles memory requirements, remote caching requirements, remote execution requirements, increases analysis time.
For the defines attribute, that affects things that depend on the target with defines, but doesn't affect the dependencies of that target (it goes "up" the build graph, instead of "down", if cc_binary is at the top).
That said, there are ways to change the way dependencies of a target are compiled, these are called "configuration transitions". (e.g. android_binary uses one to compile multiple cpu architectures in a single app, but going from [x86] to [x86, arm64] ~doubles as above). Configuration transitions are little involved though.
There are easier options like --copt on the command line, which affect the entire build, which might work. The below example might do what you need. It relies on the macro being surrounded by "#ifndef". It doesn't seem to work without the #ifndef, some cursory searching says macros in source code can't be replaced on the command line (not a c/c++ expert myself). If overriding the macro really requires it being in a source file, that's tricky because there's no straight forward way to add a file to the inputs of every cc action (or some subset). If it does work, you can add build --copt=... to a .bazelrc file at the root of your workspace so you don't have to add it to the command line. Otherwise, the only other thing I can think of is a custom toolchain, as others have mentioned.
repo1$ bazel run foo
INFO: Analyzed target //:foo (1 packages loaded, 160 targets configured).
INFO: Found 1 target...
Target //:foo up-to-date:
bazel-bin/foo
INFO: Elapsed time: 0.318s, Critical Path: 0.09s
INFO: 5 processes: 1 internal, 4 linux-sandbox.
INFO: Build completed successfully, 5 total actions
INFO: Build completed successfully, 5 total actions
bar is 16
repo1$ bazel run foo --copt=-DBAZ=15
INFO: Build option --copt has changed, discarding analysis cache.
INFO: Analyzed target //:foo (0 packages loaded, 160 targets configured).
INFO: Found 1 target...
Target //:foo up-to-date:
bazel-bin/foo
INFO: Elapsed time: 0.252s, Critical Path: 0.09s
INFO: 5 processes: 1 internal, 4 linux-sandbox.
INFO: Build completed successfully, 5 total actions
INFO: Build completed successfully, 5 total actions
bar is 38
repo1/BUILD:
cc_binary(
name = "foo",
srcs = ["foo.c", "foo.h"],
deps = ["#repo2//:bar"],
)
repo1/WORKSPACE:
local_repository(
name = "repo2",
path = "../repo2",
)
repo1/foo.c:
#include "foo.h"
#include "bar.h"
#include "stdio.h"
int main() {
printf("bar is %d\n", bar());
return 0;
}
repo1/foo.h:
repo2/BUILD:
cc_library(
name = "bar",
srcs = ["bar.c"],
hdrs = ["bar.h"],
deps = [":baz"],
visibility = ["//visibility:public"],
)
cc_library(
name = "baz",
srcs = ["baz.c"],
hdrs = ["baz.h"],
visibility = ["//visibility:public"],
)
repo2/WORKSPACE:
repo2/bar.c:
#include "bar.h"
#include "baz.h"
int bar() {
return baz() + BAR;
}
repo2/bar.h:
#define BAR 8
int bar();
repo2/baz.c:
#include "baz.h"
int baz() {
return BAZ * 2;
}
repo2/baz.h:
#ifndef BAZ
#define BAZ 4
#endif
int baz();