libcurl and Kotlin/Native on Windows

Building libcurl on Windows and using it from Kotlin/Native

Once upon a time, I decided to answer a question on StackOverflow to explain how to use libcurl with Kotlin/Native. Now it turned out I will cover a more detailed instruction here. How @shanselman once told in his talk - if a reply is big enough it should go to a documentation of a blog post.

Building Curl and Libcurl on Windows

The first step is to open the page and read it. Let’s download sources from In my case, I downloaded the There is also a sources mirror on GitHub:

Of course, we need a Windows machine or a VM to build it and use it from Windows.

Let’s extract the downloaded sources. There will be the curl-7.61.1 folder. Let’s assume the extracted curl sources directory is the current working directory.

I open the VS2015 x64 Native Command Prompt shell on my machine. That one comes from Microsoft Visual Studio 2015. There might be another shortcut to run the shell from a newer or older version of Visual Studio.

It is a useful file under winbuild/BUILD.WINDOWS.txt

Execute the command

cd winbuild
nmake /f mode=dll

The command above build curl and libcurl for us. The build generates static library that uses some default dependencies from the system to run. For example, it uses Windows SSPI as the implementation of the SSL and TLS. Check out more options on curl building in the winbuild/BUILD.WINDOWS.txt.

The compiled binaries are found under the builds folder. The contest should be as follows:

dir builds
10/29/2018  09:48 PM    <DIR>          .
10/29/2018  09:48 PM    <DIR>          ..
10/29/2018  09:48 PM    <DIR>          libcurl-vc-x64-release-dll-ipv6-sspi-winssl
10/29/2018  09:48 PM    <DIR>          libcurl-vc-x64-release-dll-ipv6-sspi-winssl-obj-curl
10/29/2018  09:48 PM    <DIR>          libcurl-vc-x64-release-dll-ipv6-sspi-winssl-obj-lib

We have our dynamic curl library and include files directory under the builds/libcurl-vc-x64-release-dll-ipv6-sspi-winssl folder.

Importing libcurl into Kotlin/Native

Let’s use the curl sample and the libcurl sample from the Kotlin/Native repository and adapt them to work on Windows too.

The first step is to create the .def file to import the native libcurl library into Kotlin. More details on that are explained in the Interop with C Libraries tutorial.

Let’s add the following libcurl.def file:

headers = curl/curl.h
headerFilter = curl/*

Now we may call the cinteop tool from Kotlin/Native from a parent folder, where we extracted curl sources:

cinterop -def libcurl.def -compilerOpts -Icurl-7.61.1\builds\libcurl-vc-x64-release-dll-ipv6-sspi-winssl\include -o libcurl

We assume here that we have Kotlin/Native compiler installed and registered in the system PATH. For more information, please refer to the A Basic Kotlin Application. Let’s assume, we have a console, where the kotlinc-native, cinterop, and klib commands are available.

The call to cinterop generates the libcurl.klib file for us. Let’s use it from the Kotlin/Native sources!

Using libcurl from Kotlin/Native

It is time to follow the example from the curl sample and create our first curl application in Windows.

Let’s create the file curl.kt with the following contents:

import libcurl.*
import kotlinx.cinterop.*

fun main(args: Array<String>) {
    val curl = curl_easy_init()
    if (curl != null) {
        curl_easy_setopt(curl, CURLOPT_URL, "")
        curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L)
        val res = curl_easy_perform(curl)
        if (res != CURLE_OK) {
            println("curl_easy_perform() failed ${curl_easy_strerror(res)?.toKString()}")

Now we call the command to build the executable with Kotlin/Native:

kotlinc-native curl.kt -l libcurl -linker-options "curl-7.61.1\builds\libcurl-vc-x64-release-dll-ipv6-sspi-winssl\lib\libcurl.lib"  -o kurl

The application kurl.exe is ready to go. The only thing it requires is the libcurl.dll. The file you may find under the curl-7.61.1\builds\libcurl-vc-x64-release-dll-ipv6-sspi-winssl\bin\libcurl.dll path. You may try running it.

Static Linking

It is not too nice to have a requirement to have the libcurl.dll included and located near the executable we create. Static linking, in theory, allows having only one solid binary, that contains everything. One may easily change the curl build to mode=static. The result will be the static library libcurl_a.lib. Linking it with Kotlin/Native binary turned out to be tricky. To start, Kotlin/Native uses MinGW environment and the libcurl_a.lib is compiled to use dynamic MSVCRT. Theoretically, it should be possible to make it work, if you know the answer - just let me know!

It is also a good comment for the original StackOverflow thread to use MinGW specific build of curl.

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