a classy little scripting language


Wren is designed to be a scripting language that lives inside a host application, so the embedding API is as important as any of its language features. Designing this API well requires satisfying several constraints:

  1. Wren is dynamically typed, but C is not. A variable can hold a value of any type in Wren, but that’s definitely not the case in C unless you define some sort of variant type, which ultimately just kicks the problem down the road. Eventually, we have to move data across the boundary between statically and dynamically typed code.

  2. Wren uses garbage collection, but C manages memory manually. GC adds a few constraints on the API. The VM must be able to find every Wren object that is still usable, even if that object is being referenced from native C code. Otherwise, Wren could free an object that’s still in use.

    Also, we ideally don’t want to let native C code see a bare pointer to a chunk of memory managed by Wren. Many garbage collection strategies involve moving objects in memory. If we allow C code to point directly to an object, that pointer will be left dangling when the object moves. Wren’s GC doesn’t move objects today, but we would like to keep that option for the future.

  3. The embedding API needs to be fast. Users may add layers of abstraction on top of the API to make it more pleasant to work with, but the base API defines the maximum performance you can get out of the system. It’s the bottom of the stack, so there’s no way for a user to optimize around it if it’s too slow. There is no lower level alternative.

  4. We want the API to be pleasant to use. This is the last constraint because it’s the softest. Of course, we want a beautiful, usable API. But we really need to handle the above, so we’re willing to make things a bit more of a chore to reach the first three goals.

Fortunately, we aren’t the first people to tackle this. If you’re familiar with Lua’s C API, you’ll find Wren’s similar.

Performance and safety #

When code is safely snuggled within the confines of the VM, it’s pretty safe. Method calls are dynamically checked and generate runtime errors which can be caught and handled. The stack grows if it gets close to overflowing. In general, when you’re within Wren code, it tries very hard to avoid crashing and burning.

This is why you use a high level language after all—it’s safer and more productive than C. C, meanwhile, really assumes you know what you’re doing. You can cast pointers in invalid ways, misinterpret bits, use memory after freeing it, etc. What you get in return is blazing performance. Many of the reasons C is fast are because it takes all the governors and guardrails off.

Wren’s embedding API defines the border between those worlds, and takes on some of the characteristics of C. When you call any of the embedding API functions, it assumes you are calling them correctly. If you invoke a Wren method from C that expects three arguments, it trusts that you gave it three arguments.

In debug builds, Wren has assertions to check as many things as it can, but in release builds, Wren expects you to do the right thing. This means you need to take care when using the embedding API, just like you do in all C code you write. In return, you get an API that is quite fast.

Including Wren #

There are two (well, three) ways to get the Wren VM into your program:

  1. Link to the static or dynamic library. When you build Wren, it generates both shared and static libraries in lib that you can link to.

  2. Include the source directly in your application. If you want to include the source directly in your program, you don’t need to run any build steps. Just add the source files in src/vm to your project. They should compile cleanly as C99 or C++98 or anything later.

In either case, you also want to add src/include to your include path so you can find the public header for Wren:

#include "wren.h"

Wren depends only on the C standard library, so you don’t usually need to link to anything else. On some platforms (at least BSD and Linux) some of the math functions in math.h are implemented in a separate library, libm, that you have to explicitly link to.

If your program is in C++ but you are linking to the Wren library compiled as C, this header handles the differences in calling conventions between C and C++:

#include "wren.hpp"

Creating a Wren VM #

Once you’ve integrated the code into your executable, you need to create a virtual machine. To do that, you create a WrenConfiguration object and initialize it.

    WrenConfiguration config;

This gives you a basic configuration that has reasonable defaults for everything. We’ll learn more about what you can configure later, but for now we’ll just add the writeFn, so that we can print text.

First we need a function that will do something with the output that Wren sends us from System.print (or System.write). Note that it doesn’t include a newline in the output.

void writeFn(WrenVM* vm, const char* text) {
  printf("%s", text);

And then, we update the configuration to point to it.

  WrenConfiguration config;
    config.writeFn = &writeFn;

With this ready, you can create the VM:

WrenVM* vm = wrenNewVM(&config);

This allocates memory for a new VM and initializes it. The Wren C implementation has no global state, so every single bit of data Wren uses is bundled up inside a WrenVM. You can have multiple Wren VMs running independently of each other without any problems, even concurrently on different threads.

wrenNewVM() stores its own copy of the configuration, so after calling it, you can discard the WrenConfiguration struct you filled in. Now you have a live VM, waiting to run some code!

Executing Wren code #

You execute a string of Wren source code like so:

WrenInterpretResult result = wrenInterpret(
    "System.print(\"I am running in a VM!\")");

The string is a series of one or more statements separated by newlines. Wren copies the string, so you can free it after calling this. When you call wrenInterpret(), Wren first compiles your source to bytecode. If an error occurs, it returns immediately with WREN_RESULT_COMPILE_ERROR.

Otherwise, Wren spins up a new fiber and executes the code in that. Your code can in turn spawn whatever other fibers it wants. It keeps running fibers until they all complete or one suspends.

If a runtime error occurs (and another fiber doesn’t handle it), Wren aborts fibers all the way back to the main one and returns WREN_RESULT_RUNTIME_ERROR. Otherwise, when the last fiber successfully returns, it returns WREN_RESULT_SUCCESS.

All code passed to wrenInterpret() runs in a special “main” module. That way, top-level names defined in one call can be accessed in later ones. It’s similar to a REPL session.

Shutting down a VM #

Once the party is over and you’re ready to end your relationship with a VM, you need to free any memory it allocated. You do that like so:


After calling that, you obviously cannot use the WrenVM* you passed to it again. It’s dead.

Note that Wren will yell at you if you still have any live WrenHandle objects when you call this. This makes sure you haven’t lost track of any of them (which leaks memory) and you don’t try to use any of them after the VM has been freed.

A complete example #

Below is a complete example of the above. You can find this file in the example folder.

//For more details, visit https://wren.io/embedding/

#include "wren.h"

static void writeFn(WrenVM* vm, const char* text)
  printf("%s", text);

void errorFn(WrenVM* vm, WrenErrorType errorType,
             const char* module, const int line,
             const char* msg)
  switch (errorType)
      printf("[%s line %d] [Error] %s\n", module, line, msg);
    } break;
      printf("[%s line %d] in %s\n", module, line, msg);
    } break;
      printf("[Runtime Error] %s\n", msg);
    } break;

int main()

  WrenConfiguration config;
    config.writeFn = &writeFn;
    config.errorFn = &errorFn;
  WrenVM* vm = wrenNewVM(&config);

  const char* module = "main";
  const char* script = "System.print(\"I am running in a VM!\")";

  WrenInterpretResult result = wrenInterpret(vm, module, script);

  switch (result) {
      { printf("Compile Error!\n"); } break;
      { printf("Runtime Error!\n"); } break;
      { printf("Success!\n"); } break;



Next, we’ll learn to make that VM do useful stuff…

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