Mapping Strings from C – tutorial

Working with C strings

There is no dedicated type in C language for strings. A developer knows from a method signature or the documentation, whether a given char * means a C string in the context. Strings in the C language are null-terminated, a trailing zero character \0 is added at the end of a bytes sequence to mark a string termination. Usually, UTF-8 encoded strings are used. The UTF-8 encoding uses variable width characters, and it is backward compatible with ASCII. Kotlin/Native uses UTF-8 character encoding by default.

The best way to understand the mapping between C and Kotlin languages is to try it out on a small example. Create a small library headers for that. First, create a lib.h file with the following declaration of functions that deal with the C strings:

In the example, you see the most popular ways to pass or receive a string in the C language. Take the return of return_string with care. In general, it is best to make sure you use the right function to dispose the returned char* with the right free(..) function call.

Kotlin/Native comes with the cinterop tool; the tool generates bindings between the C language and Kotlin. It uses a .def file to specify a C library to import. More details on this are in the Interop with C Libraries tutorial. The quickest way to try out C API mapping is to have all C declarations in the interop.def file, without creating any .h of .c files at all. Then place the C declarations in a interop.def file after the special --- separator line:

The interop.def file is enough to compile and run the application or open it in an IDE. Now it is time to create project files, open the project in IntelliJ IDEA and run it.

Inspect generated Kotlin APIs for a C library

While it is possible to use the command line, either directly or by combining it with a script file (such as .sh or .bat file), this approach doesn't scale well for big projects that have hundreds of files and libraries. It is then better to use the Kotlin/Native compiler with a build system, as it helps to download and cache the Kotlin/Native compiler binaries and libraries with transitive dependencies and run the compiler and tests. Kotlin/Native can use the Gradle build system through the kotlin-multiplatform plugin.

We covered the basics of setting up an IDE compatible project with Gradle in the A Basic Kotlin/Native Application tutorial. Please check it out if you are looking for detailed first steps and instructions on how to start a new Kotlin/Native project and open it in IntelliJ IDEA. In this tutorial, we'll look at the advanced C interop related usages of Kotlin/Native and multiplatform builds with Gradle.

First, create a project folder. All the paths in this tutorial will be relative to this folder. Sometimes the missing directories will have to be created before any new files can be added.

Use the following build.gradle(.kts) Gradle build file:

The project file configures the C interop as an additional step of the build. Let's move the interop.def file to the src/nativeInterop/cinterop directory. Gradle recommends using conventions instead of configurations, for example, the source files are expected to be in the src/nativeMain/kotlin folder. By default, all the symbols from C are imported to the interop package, you may want to import the whole package in our .kt files. Check out the Multiplatform Gradle DSL reference to learn about all the different ways you could configure it.

Let's create a src/nativeMain/kotlin/hello.kt stub file with the following content to see how C string declarations are visible from Kotlin:

Now you are ready to open the project in IntelliJ IDEA and to see how to fix the example project. While doing that, see how C strings are mapped into Kotlin/Native.

Strings in Kotlin

With the help of IntelliJ IDEA's Go to | Declaration or compiler errors, you see the following generated API for the C functions:

These declarations look clear. All char * pointers are turned into str: CValuesRef<ByteVar>? for parameters and to CPointer<ByteVar>? in return types. Kotlin turns char type into kotlin.Byte type, as it is usually an 8-bit signed value.

In the generated Kotlin declarations, you see that str is represented as CValuesRef<ByteVar/>?. The type is nullable, and you can simply pass Kotlin null as the parameter value.

Pass Kotlin string to C

Let's try to use the API from Kotlin. Call pass_string first:

Passing a Kotlin string to C is easy, thanks to the fact that there is String.cstr extension property in Kotlin for it. There is also String.wcstr for cases when you need UTF-16 wide characters.

Read C Strings in Kotlin

This time you'll take a returned char * from the return_string function and turn it into a Kotlin string. For that, do the following in Kotlin:

This code uses the toKString() extension function above. Please do not miss out the toString() function. The toKString() has two overloaded extension functions in Kotlin:

The first extension takes a char * as a UTF-8 string and turns it into a String. The second function does the same but for wide UTF-16 strings.

Receive C string bytes from Kotlin

This time we will ask a C function to write us a C string to a given buffer. The function is called copy_string. It takes a pointer to the location writing characters and the allowed buffer size. The function returns something to indicate if it has succeeded or failed. Let's assume 0 means it succeeded, and the supplied buffer was big enough:

First of all, you need to have a native pointer to pass to the C function. Use the usePinned extension function to temporarily pin the native memory address of the byte array. The C function fills in the byte array with data. Use another extension function ByteArray.decodeToString() to turn the byte array into a Kotlin String, assuming UTF-8 encoding.

Fix the Code

You've now seen all the definitions and it is time to fix the code. Run the runDebugExecutableNative Gradle task in the IDE or use the following command to run the code:

The code in the final hello.kt file may look like this:

Next steps

Continue to explore more C language types and their representation in Kotlin/Native in our other tutorials:

• Mapping primitive data types from C

• Mapping struct and union types from C

• Mapping function pointers from C

The C Interop documentation documentation covers more advanced scenarios of the interop.