[dbohdan] 2016-09-21: The following is a minimal example how you can implement a Tcl [extension] in Swift 3.0 under Linux. Note the sure of `@_cdecl` to make the function accessible to C code under a specific symbol name.

The code has been tested with Apple's official Swift 3.0-RELEASE on Ubuntu 14.04. You can install Swift by following https://vapor.github.io/documentation/getting-started/install-swift-3-ubuntu.html%|%these instructions%|%. If you copy and paste the makefile from the be sure to replace the leading spaces with tabs in it with `sed -i 's|        |\t| Makefile'.

** Makefile **

======none
TARGET ?= libtclswiftexample.so

test: $(TARGET) bridge.h
        echo 'load $(TARGET); puts [hello]; puts [square 5]' | tclsh
$(TARGET): tclswiftexample.swift
        swiftc -emit-library -import-objc-header bridge.h $< -o $(TARGET)
clean:
        -rm $(TARGET)
.PHONY: clean test

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** bridge.h **

======c
#include "/usr/include/tcl/tcl.h"

======

** example.swift **

======none
func Hello_Cmd(cdata: ClientData?,
                      interp: UnsafeMutablePointer<Tcl_Interp>?,
                      objc: Int32,
                      objv: UnsafePointer<UnsafeMutablePointer<Tcl_Obj>?>?) -> Int32 {
    if objc != 1 {
        return TCL_ERROR
    }
    Tcl_SetObjResult(interp, Tcl_NewStringObj("Hello, World!", -1))
    return TCL_OK
}

func Square_Cmd(cdata: ClientData?,
                       interp: UnsafeMutablePointer<Tcl_Interp>?,
                       objc: Int32,
                       objv: UnsafePointer<UnsafeMutablePointer<Tcl_Obj>?>?) -> Int32 {
    if objc != 2 {
        return TCL_ERROR
    }
    var i: Int32 = 0
    let success = Tcl_GetIntFromObj(interp, objv![1], &i)
    if success != TCL_OK {
        return TCL_ERROR
    }
    Tcl_SetObjResult(interp, Tcl_NewIntObj(i*i))
    return TCL_OK
}

@_cdecl("Tclswiftexample_Init")
public func Tclswiftexample_Init(interp: UnsafeMutablePointer<Tcl_Interp>) -> CInt {
    Tcl_CreateObjCommand(interp, "::hello", Hello_Cmd, nil, nil);
    Tcl_CreateObjCommand(interp, "::square", Square_Cmd, nil, nil);
    return TCL_OK
}

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