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Package Manager C Language Target Support

Introduction

This is a proposal for adding initial package manager support for the C, C++, Objective-C, and Objective-C++ languages (henceforth, simply referred to as "C" languages). This proposal is limited in scope to only supporting targets consisting entirely of C languages; there is no provision for supporting targets which include both C and Swift sources.

Motivation

Swift has easy interoperability with C based languages through the use of the Clang modules system. We would like Swift packages to be able to include C targets which can be exposed to Swift directly as part of a single package.

This gives developers a simple mechanism for "falling back" to C when they need to access APIs which are inadequately or poorly bridged to Swift, or when they need to implement behavior which is better done in low-level C than Swift.

Proposed solution

Our proposed solution extends the convention based system to allow targets composed only of C sources. The conventions will be amended as follows:

  1. Any existing directory which defines a target will be allowed to contain a set of C sources, recognized by file extension. If a target contains any C sources, then all source code files must be C sources.

    Support for Objective-C and Objective-C++ source file extensions will be included, although those will be inherently less portable.

  2. C targets may have a special subdirectory named Includes or include (the include directory). Only one such name may be used.

    The headers in this directory are presumed to be the "exported" interface of the C target, and will be made available for use in other targets.

    The names Includes and include are somewhat unfortunate because they do not clearly communicate that these folders define public API. However, this is an established convention for organizing C language header files, and there does not seem to be a better alternative.

  3. If the include directory contains a header whose name matches that of the target, then that header will be treated as an "umbrella header" for the purposes of module map construction.

  4. If the include directory includes any file named "module.modulemap", then those will be presumed to define the modules for the target, and no module maps will be synthesized.

  5. As with Swift targets, we will use the presence of a source file named main.c (or main.cpp, etc.) to indicate an executable versus a library.

The following example layout would define a package containing a C library and a Swift target:

example/src/foo/include/foo/foo.h
example/src/foo/foo.c
example/src/foo/util.h
example/src/bar/bar.swift

In this example, the util.h would be something internal to the implementation of the foo target, while include/foo/foo.h would be the exported API to the foo library.

The package manager will naturally gain support for building these targets:

  1. The package manager will (a) construct a synthesized module map including all of the exported API headers, for use in Swift, and (b) will provide a header search path to this directory to any targets which transitively depend on this one.

    Module maps will only be synthesized for targets which either have a completely flat header layout (e.g., src/foo/include/*.h) or a single subdirectory (e.g., src/foo/include/foo/**.h). Any other structure requires the library author to provide explicit module maps. We may revisit this as we gain practical experience with it.

  2. Most packages are encouraged to include the package or target name as a subfolder of the include directory, to clarify use (e.g., src/foo/include/foo/). However, this is not required and it may be useful for legacy projects whose headers have traditionally been installed directly into /usr/include to not use this convention. This allows client code of those projects to be source compatible with versions which use the installed library.

  3. We expect C language targets to integrate with the other existing package manager features. For example, C language targets should be testable using the testing features (although such tests would initially need to be written in Swift until such time as a C language testing framework was usable by the package manager).

There are several obvious features related to C language support which are not addressed with this proposal:

  1. We anticipate the need to declare that only particular targets should have their API exported to downstream packages (for example, the package above might want to export the bar target to clients, and keep the C target as an implementation detail).

  2. No provision is made in this proposal for controlling compiler arguments. We will support the existing debug and release configurations using a fixed set of compiler flags. We expect future proposals to accomodate the need to modify those flags.

  3. We intend for the feature to be built in such a way as to support any standard compliant C compiler, but our emphasis will largely be on supporting the use of Clang as that compiler (and of course our modules support will require Clang).

  4. We believe there may be a need for targets to define their own module map. If needed, we would expect this to go into the include directory as module.modulemap. However, we intend to defer implementation of this support until after the initial feature is implemented and the use cases become clear.

Detailed design

The package manager will undertake the following additional behaviors:

  1. The project model will be extended to discover C language targets and diagnose issues (e.g., mixed C and Swift source files).

  2. The build system will be extended to compile and link each C language target. We will make use of llbuild's existing support for compiling C source code and gather GCC-compatible compiler style header dependency information for incremental rebuilds.

  3. When building a target, the package manager will automatically add an additional header search path argument to the include directory for each C language target in the transitive closure of the dependencies for the target being built.

    We should use -iquote as the header search argument for targets which are within the current package, and -I for targets which come from package dependencies. This allows projects to use #include "foo/foo.h" versus #include <foo/foo.h> syntax appropriately to distinguish between the inclusion of headers which are or are not within the package.

  4. We will synthesize module map files for each C language target with includes. Module maps will be constructed by explicitly enumerating all the headers in the include directory. To ensure deterministic behavior this will be done in lexicographic order, but the documentation will convey to users that each include header describing API is expected to be able to be included "standalone", that is, in any order.

  5. When building Swift targets, we will explicitly pass the synthesized module map for each C language target in the dependency closure to Clang (using the -fmodule-map-file=<PATH> argument). This will allow the Swift Clang importer to find those modules without needing to find them via the header search mechanism.

We explicitly have designed this support in such a way that it will be possible to support any GCC-compatible compiler for building the C target itself, not just Clang.

Impact on existing packages

There is no serious impact on existing packages, as this was previously unsupported. We will begin trying to build C family sources in existing packages, but this is likely to be desirable.

It is worth considering the impact on existing C language projects which do not follow the conventions above.

  • Most projects will not conform to these conventions. However, this is expected of any "simple" convention; we don't think that there is any other straightforward convention that would allow a significant percentage of existing C language projects to "just work".

    We do anticipate allowing certain overrides to be present in the manifest file describing a C target, to allow some projects to work with the package manager with only the addition of a correct Package.swift file. We will determine the exact overrides allowed once we are able to test options against existing C projects.

    The package manager already provides support for "system module" packages which is explicitly designed to support existing projects. The C language target support described in this proposal is targeted at new C code which is written in support of Swift projects, and believe that adopting a clean, simple convention is the best approach for supporting that goal.

  • Existing source code using existing projects (e.g., a source file using libpng) may be able to use well formed packages without modification. This is viewed as a significant advantage, as it will potentially help upstream projects ingest proper package manager support into their main tree.

Risk of non-modular headers

As part of this proposal, we will be synthesizing module maps for C language targets, for use in Swift via the Clang AST importer.

One risk with this proposal is when multiple C language targets are imported into a Swift module and those targets reference common C headers which have no defined module maps (i.e., they are "non-modular headers"). This situation can occur frequently on Linux, where the system does not typically have module maps for common headers, but can also occur on OS X if targets reference third-party headers not part of the package ecosystem.

The current compiler behavior when this occurs is hard for users to understand. The model the compiler expects is that all content is modular. However, when this model is broken, and two modules contain duplicate content, these issues (a) sometimes behave correctly, due to the subtleties of the module implementation, and (b) are not diagnosed by the compiler. This can lead to confusing failures or expose us to unintended package breaking changes in the future if the compiler becomes more strict.

Ultimately, the anticipated solution to these problem is to continue work to provide more module maps for all content used in Swift. Incorporation of this proposal may make that work a higher priority, and may expose additional issues which we need to tackle in the compiler implementation.

Alternatives considered

Avoid C language support

We could avoid supporting C language targets directly, and rely on external build systems and features to integrate them into Swift packages. We may wish to add such features independently from this proposal, but we think it is worthwhile to have some native support for C targets. This will make it easy to integrate small amounts of C code into what are otherwise Swift projects, and is in line with a long term direction of making the Swift package manager useful for non-Swift projects.

Restrictions on C language target header layout

We considered requiring targets follow the naming convention that each include directory must have a subdirectory matching the name of the target. This has the advantage that all clients of that target always include its headers using the syntax #include <foo/header.h>.

However, it reduces the usability of packages for traditional C libraries which do not install their headers in this format. It also reduces the ability of the project to impose additional organization on their own headers. For example, LLVM has a convention of laying out top-level headers into both llvm and llvm-c (for the C++ vs C API).

Since we did not have other features or areas of development where we felt it was important to restrict the layout of the headers, we felt it was best to avoid imposing unnecessary restrictions, and instead simply treat it as a recommendation.