This article describes some Clang header modules features which apply to #include. They enforce a more explicit dependency graph. Strict dependency information provides documentation purposes and makes refactoring convenient. Include What You Use describes the benefits of clean header inclusions well, so I will not repeat it here.

When C++20 modules are used, the features apply to #include in a global module fragment (module;) but have no effect for import declarations.

Layering check

-fmodules-decluse

For a #include directive, this option emits an error if the following conditions are satisfied (see clang/lib/Lex/ModuleMap.cpp diagnoseHeaderInclusion):

• The main file is within a module (called "source module", say, A).
• The main file or an included file from the source module includes a file from another module B.
• A does not have a use-declaration of B (no use B).

For the first condition, -fmodule-map-file= is needed to load the source module map and -fmodule-name=A is needed to indicate that the source file is logically part of module A.

For the second condition, the module map defining B must be loaded by specifying -fimplicit-module-maps (implied by -fmodules and -fcxx-modules) or a -fmodule-map-file=.

Here is an example:

1
2
3
4
5
6
7
8
9
10
11
12
13

cat > a.cc <<'eof'
#include "dir/b.h"
#include "dir/c.h"
int main() {}
eof
echo 'module A { header "a.h" use B }' > module.modulemap
mkdir -p dir
echo '#include "c.h"' > dir/b.h
echo 'void fc();' > dir/c.h
cat > dir/module.modulemap <<'eof'
module B { header "b.h" use C }
module C { header "c.h" }
eof

The following commands lead to an error about dir/c.h. #include "dir/b.h" is allowed because module A has a use-declaration on module B.

1
2
3
4
5
6
7
8
9
10

% clang -fsyntax-only -fmodules-decluse -fmodule-map-file=module.modulemap -fmodule-name=A -fimplicit-module-maps a.cc 
a.cc:1:10: error: module A does not depend on a module exporting 'dir/c.h'
#include "dir/c.h"
         ^
1 error generated.
% clang -fsyntax-only -fmodules-decluse -fmodule-map-file=module.modulemap -fmodule-name=A -fmodule-map-file=dir/module.modulemap a.cc
a.cc:1:10: error: module A does not depend on a module exporting 'dir/c.h'
#include "dir/c.h"
         ^
1 error generated.

textual header "c.h" triggers the error as well.

If we remove -fmodule-name=A, we won't see an error: Clang does not know a.cc logically belongs to module A.

-fmodules-strict-decluse

This is a strict variant of -fmodules-decluse. If an included file is not within a module, -fmodules-decluse allows the inclusion while -fmodules-strict-decluse reports an error.

Use the previous example, but drop -fimplicit-module-maps and -fmodule-map-file=dir/module.modulemap so that Clang thinks dir/c.h is not within a module. Let's see the distinction between -fmodules-decluse and -fmodules-strict-decluse.

1
2
3
4
5
6

% clang -fsyntax-only -fmodules-decluse -fmodule-map-file=module.modulemap -fmodule-name=A a.cc
% clang -fsyntax-only -fmodules-strict-decluse -fmodule-map-file=module.modulemap -fmodule-name=A a.cc
a.cc:1:10: error: module A does not depend on a module exporting 'dir/c.h'
#include "dir/c.h"
         ^
1 error generated.

Many systems do not ship Clang module map files for C/C++ standard libraries, so -fmodules-strict-decluse is not suitable.

1
2
3
4
5

% clang -fsyntax-only -fmodules-strict-decluse -fmodule-map-file=module.modulemap -fmodule-name=A -fimplicit-module-maps a.cc
a.cc:1:10: error: module A does not depend on a module exporting 'stdio.h'
#include <stdio.h>
         ^
...

In Bazel, tools/cpp/generate_system_module_map.sh generates a module map listing all system headers.

-Wprivate-header

In the Clang module map language, a header with the private specifier cannot be included from outside the module itself. -Wprivate-header is an enabled-by-default warning enforcing this rule. The warning is orthogonal to -fmodules-decluse/-fmodules-strict-decluse.

To see its effect, change dir/module.modulemap by making b.h private:

1
2

module B { private header "b.h" use C }
module C { header "c.h" }

Then clang -fsyntax-only -fmodule-map-file=module.modulemap -fmodule-name=A -fimplicit-module-maps a.cc will report an error:

1
2
3
4

a.cc:2:10: error: use of private header from outside its module: 'dir/c.h' [-Wprivate-header]
#include "dir/c.h"
         ^
1 error generated.

Relation with include-what-you-use and dependency-related linker options

To make full power of the layering check features, the source files must have clean header inclusions.

In the following example, a.cc gets dir/c.h declarations transitively via dir/b.h but does not include dir/b.h directly. -fmodules-strict-decluse cannot flag this case.

1
2
3
4
5
6
7
8
9
10
11
12

cat > a.cc <<'eof'
#include "dir/b.h"
int main() { fc(); }
eof
echo 'module A { header "a.h" use B }' > module.modulemap
mkdir -p dir
echo '#include "c.h"' > dir/b.h
echo 'void fc();' > dir/c.h
cat > dir/module.modulemap <<'eof'
module B { header "b.h" use C }
module C { header "c.h" }
eof

If #include "dir/b.h" is added due to clean header inclusions, -fmodules-decluse will report an error.

In the absence of clean header inclusions, dependency-related linker options (-z defs, --no-allow-shlib-undefined, and --warn-backrefs) can mitigate some brittle build problems.

With C++20 modules, if B does not export-import C, a.cc cannot get C's declarations from import B;.

Build system support

Bazel is the first build system implementing the layering check feature and as of today the only build system.

Bazel

Bazel has implemented the built-in feature layering_check (https://github.com/bazelbuild/bazel/pull/11440) using both -fmodules-strict-decluse and -Wprivate-header.

Bazel generates .cppmap module files from deps attributes. hdrs and textual_hdrs files are converted to textual header declarations while srcs headers are converted to private textual header declarations. deps attributes are converted to use declarations.

When building a target with Clang and layering_check is enabled for the target, Bazel passes a list of -fmodule-map-file= (according to the build target and its direct dependencies) and -fmodule-name= to Clang.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28

touch ./WORKSPACE
cat > ./a.cc <<'eof'
#include "a.h"
int main() { fc(); }
eof
echo '#include "c.h"' > ./a.h
echo '#include "c.h"' > ./b.h
cat > ./BUILD <<'eof'
cc_binary(
    name = "a",
    srcs = ["a.cc", "a.h"],
    deps = [":b"],  # :c is missing
)

cc_library(
    name = "b",
    hdrs = ["b.h"],
    deps = [":c"],
)

cc_library(
    name = "c",
    srcs = ["c.cc"],
    hdrs = ["c.h"],
)
eof
echo 'void fc() {}' > ./c.cc
echo 'void fc();' > ./c.h

The following build command gives an error with Clang 16: a.h's inclusion of c.h does not have a corresponding use declaration. (Clang before 16.0 did not check -fmodules-decluse in textual headers: https://reviews.llvm.org/D132779)

1
2
3
4
5
6
7
8
9
10
11

% CC=/tmp/Rel/bin/clang bazel build --features=layering_check :a
INFO: Analyzed target //:a (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
ERROR: /tmp/f/BUILD:1:10: Compiling a.cc failed: (Exit 1): clang-16 failed: error executing command /tmp/Rel/bin/clang-16 -U_FORTIFY_SOURCE -fstack-protector -Wall -Wthread-safety -Wself-assign -Wunused-but-set-parameter -Wno-free-nonheap-object -fcolor-diagnostics -fno-omit-frame-pointer '-std=c++0x' ... (remaining 29 arguments skipped)

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
In file included from a.cc:1:
./a.h:1:10: error: module //:a does not depend on a module exporting 'c.h'
#include "c.h"
         ^
1 error generated.

The relevant Clang driver options are: '-fmodule-name=//:a' '-fmodule-map-file=bazel-out/k8-fastbuild/bin/a.cppmap' -fmodules-strict-decluse -Wprivate-header '-fmodule-map-file=external/local_config_cc/module.modulemap' '-fmodule-map-file=bazel-out/k8-fastbuild/bin/b.cppmap'. Note that c.cppmap is not loaded as :c is not a direct dependency.

Here are the generated .cppmap module maps:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

% cat bazel-out/k8-fastbuild/bin/a.cppmap
module "//:a" {
  export *
  private textual header "../../../a.h"
  use "//:b"
  use "@bazel_tools//tools/cpp:malloc"
  use "crosstool"
}
extern module "//:b" "../../../bazel-out/k8-fastbuild/bin/b.cppmap"
extern module "@bazel_tools//tools/cpp:malloc" "../../../bazel-out/k8-fastbuild/bin/external/bazel_tools/tools/cpp/malloc.cppmap"
extern module "crosstool" "../../../external/local_config_cc/module.modulemap"
% cat bazel-out/k8-fastbuild/bin/b.cppmap
module "//:b" {
  export *
  textual header "../../../b.h"
  use "//:c"
  use "crosstool"
}
extern module "//:c" "../../../bazel-out/k8-fastbuild/bin/c.cppmap"
extern module "crosstool" "../../../external/local_config_cc/module.modulemap"
% cat bazel-out/k8-fastbuild/bin/c.cppmap
module "//:c" {
  export *
  textual header "../../../c.h"
  use "crosstool"
}
extern module "crosstool" "../../../external/local_config_cc/module.modulemap"

external/local_config_cc/module.modulemap contains files in Clang's default include paths to make -fmodules-strict-decluse happy.

parse_headers

The feature parse_headers ensures header files are self-contained. When the feature is configured and enabled, Bazel parses non-textual headers in srcs and hdrs attributes by compiling them as main files using -fsyntax-only. Such headers do not require other headers or macros as a precondition. A source file including the such a header can freely reorder it among included files.

Some problems detected by parse_headers can be detected by layering_check as well. I.e. even if parse_headers is disabled, layering_check can detect the problems.

When layering_check is enabled, it applies to the header compiles due to parse_headers. Enabling both can detect more problems.