MarSe32m/swift-library-linking

swift-library-linking

This repository contains documentation on building Swift libraries using only swiftc. There are examples presented in the repository. Below is some basic documentation.

Let's say we have a library consisting of files mylib.swift, mylib2.swift, lib.h and lib.c. We'll build these into a library and module both statically and dynamically.

Dynamic library

To build a dynamic Swift library run the following command on macOS, Linux and Windows

swiftc -parse-as-library -emit-module -emit-library -module-name NAME_OF_SWIFT_MODULE mylib.swift mylib2.swift

To build a dynamic C Library run the following command on macOS (.dylib), Linux (.so) and Windows (.dll)

clang -shared lib.c -o NAME_OF_C_MODULE(.dll/.so/.dylib)

For C modules you need to create a modulemap file, for example module.modulemap

module NAME_OF_C_MODULE {
    header "lib.h"
    export *
}

To use the libraries, you need to import them in the Swift file by

import NAME_OF_SWIFT_MODULE
import NAME_OF_C_MODULE

and compile the file with

swiftc -I. -L. -lNAME_OF_SWIFT_MODULE -lNAME_OF_C_MODULE main.swift

On Linux if you get an error "Cannot open shared object file: No such file or directory" when running the program, copy the dynamic libraries into /usr/local/lib and run sudo ldconfig and it should fix it.

Static library

On Windows, at the moment, you have to specify the linker separately

swiftc -parse-as-library -use-ld=lld-link -emit-module -emit-library -static -module-name NAME_OF_SWIFT_MODULE mylib.swift mylib2.swift -o NAME_OF_SWIFT_MODULE.lib

clang -static -fuse-ld=llvm-lib lib.c -o NAME_OF_C_MODULE.lib

On Linux and macOS you have to explicity link the static library

swiftc -parse-as-library -emit-module -emit-library -static -module-name <NAME_OF_MODULE> mylib.swift mylib2.swift

clang -c -fPIC lib.c -o lib.o
ar r NAME_OF_C_MODULE.a lib.o

For C modules you need to create a modulemap file, for example

module NAME_OF_C_MODULE {
    header "lib.h"
    export *
}

To use the libraries, you need to import them by

import NAME_OF_SWIFT_MODULE
import NAME_OF_C_MODULE

and compile the file with

swiftc -I. -L. -lNAME_OF_SWIFT_MODULE -lNAME_OF_C_MODULE main.swift

also the methods need to be annotated by __declspec(dllexport), so you could do something like

#if _WIN32
#define SEBBU_LIB_EXPORT __declspec(dllexport)
#else
#define SEBBU_LIB_EXPORT __attribute__((visibility(default)))
#endif

Calling Swift code from C

You can also use Swift functions marked as @_cdecl from C:

swiftc -parse-as-library -emit-library -emit-module -module-name NAME_OF_SWIFT_MODULE library.swift

Static Swift libraries used by C-code only work on Linux at the moment (i.e. flag -static works only on Linux). Then as per usual compile the C executable with

clang -I. -L. -lNAME_OF_MODULE main.c -o main.exe

On linux you can use

readelf -a <file> | grep "Shared library"

(grep Shared is also enough) to find all the needed shared libraries of an executable or .so file.