C FFI: Getting Started

The C FFI is the low-level interface for embedding the Atelico AI Engine in any language or engine that can call C functions. Unity and Unreal use this under the hood, but you can use it directly for custom engines, Swift/Kotlin mobile apps, or any other integration.

What You'll Build

A C/C++ program that initializes the engine, loads a model, streams a chat response token-by-token, and shuts down cleanly.

By the end, you'll understand:

1. How to link the native library
2. The engine lifecycle (create / use / destroy)
3. How to make a blocking chat request
4. How to stream tokens with the poll pattern
5. Error handling and string ownership

Prerequisites

• A C or C++ compiler
• The Atelico native library (libatelico_ffi.a or atelico_ffi.dll)
• The atelico_ffi.h header file
• A downloaded model:

./atelico-asset-downloader download meta-llama/Llama-3.2-3B-Instruct-Q4_K_M

Step 1: Link the Library

Static Linking (Recommended)

# Compile with static library (macOS)
clang++ -std=c++17 main.cpp -L./lib -latelico_ffi -framework Metal -framework Foundation -o my_app

# Linux
g++ -std=c++17 main.cpp -L./lib -latelico_ffi -lpthread -ldl -lm -o my_app

Dynamic Linking

# macOS
clang++ -std=c++17 main.cpp -L./lib -latelico_ffi -o my_app
# Run with: DYLD_LIBRARY_PATH=./lib ./my_app

# Linux
g++ -std=c++17 main.cpp -L./lib -latelico_ffi -o my_app
# Run with: LD_LIBRARY_PATH=./lib ./my_app

# Windows (MSVC)
cl /EHsc main.cpp /link atelico_ffi.dll.lib
# atelico_ffi.dll must be next to the exe at runtime

Include the Header

#include "atelico_ffi.h"

Step 2: Engine Lifecycle

#include "atelico_ffi.h"
#include <cstdio>

int main()
{
    // Create the engine (NULL config = auto-detect GPU, use defaults)
    AtelicoEngine* engine = nullptr;
    int32_t rc = atelico_engine_create(nullptr, &engine);
    if (rc != ATELICO_OK || engine == nullptr)
    {
        fprintf(stderr, "Failed to create engine: %s\n", atelico_last_error());
        return 1;
    }
    printf("Engine created successfully\n");

    // ... use the engine ...

    // Shut down and free all resources
    atelico_engine_destroy(engine);
    return 0;
}

Key rules:

• atelico_engine_create must be called from the main thread
• atelico_engine_destroy is safe to call on nullptr
• Only one engine instance should exist at a time

Step 3: Load a Model

// Pre-load a model (blocking — downloads if not cached)
rc = atelico_model_load(engine, "meta-llama/Llama-3.2-3B-Instruct-Q4_K_M");
if (rc != ATELICO_OK)
{
    fprintf(stderr, "Failed to load model: %s\n", atelico_last_error());
    atelico_engine_destroy(engine);
    return 1;
}
printf("Model loaded\n");

Step 4: Blocking Chat Request

const char* request = R"({
    "model": "meta-llama/Llama-3.2-3B-Instruct-Q4_K_M",
    "messages": [
        {"role": "system", "content": "You are a friendly tavern keeper. Keep responses under 2 sentences."},
        {"role": "user", "content": "What's on the menu?"}
    ],
    "max_tokens": 100,
    "temperature": 0.7
})";

const char* response = nullptr;
rc = atelico_llm_chat(engine, request, &response);
if (rc == ATELICO_OK && response != nullptr)
{
    printf("Response: %s\n", response);
    // IMPORTANT: response is valid only until the next API call on this thread.
    // Copy it if you need to keep it.
}
else
{
    fprintf(stderr, "Chat failed: %s\n", atelico_last_error());
}

Step 5: Streaming with the Poll Pattern

Streaming is the recommended approach for real-time applications. It uses a poll loop that fits naturally into a game's frame update:

// Start a streaming request — returns immediately
uint64_t stream_id = 0;
rc = atelico_llm_chat_stream(engine, request, &stream_id);
if (rc != ATELICO_OK)
{
    fprintf(stderr, "Failed to start stream: %s\n", atelico_last_error());
    return 1;
}

// Poll for tokens (in a game, do this once per frame in your update loop)
bool done = false;
while (!done)
{
    const char* chunk_json = nullptr;
    int32_t poll_rc = atelico_stream_poll(engine, stream_id, &chunk_json);

    switch (poll_rc)
    {
    case ATELICO_OK:
        // Got a token chunk — print it
        // chunk_json is valid until the next API call
        printf("%s", chunk_json);
        fflush(stdout);
        break;

    case ATELICO_ERR_STREAM_EMPTY:
        // No data yet — in a game loop, just continue to the next frame
        // In a console app, sleep briefly to avoid busy-waiting
        break;

    case ATELICO_ERR_STREAM_DONE:
        // Stream finished
        done = true;
        break;

    default:
        // Error
        fprintf(stderr, "\nStream error: %s\n", atelico_last_error());
        done = true;
        break;
    }
}

// Clean up the stream handle
atelico_stream_destroy(engine, stream_id);
printf("\nDone.\n");

In a Game Loop

The poll pattern is designed for frame-driven engines:

// Called every frame by your engine
void OnUpdate()
{
    // Signal frame timing to the AI scheduler
    atelico_engine_on_frame(engine);

    // Poll active stream for new tokens
    if (active_stream_id != 0)
    {
        const char* chunk = nullptr;
        int32_t rc = atelico_stream_poll(engine, active_stream_id, &chunk);

        if (rc == ATELICO_OK && chunk != nullptr)
        {
            // Append token to dialogue UI
            AppendToDialogue(chunk);
        }
        else if (rc == ATELICO_ERR_STREAM_DONE)
        {
            atelico_stream_destroy(engine, active_stream_id);
            active_stream_id = 0;
            OnDialogueComplete();
        }
        // STREAM_EMPTY: no data this frame, try next frame
    }
}

Step 6: Error Handling

All functions return int32_t result codes. On failure, call atelico_last_error() for a human-readable message:

int32_t rc = atelico_llm_chat(engine, bad_request, &response);
if (rc != ATELICO_OK)
{
    const char* error = atelico_last_error();
    // error is thread-local and valid until the next API call
    fprintf(stderr, "Error (code %d): %s\n", rc, error);
}

Code	Constant	Meaning
0	ATELICO_OK	Success
-1	ATELICO_ERR_INVALID_HANDLE	NULL or invalid engine pointer
-2	ATELICO_ERR_INVALID_ARG	NULL required argument
-3	ATELICO_ERR_INIT_FAILED	Engine initialization failed
-4	ATELICO_ERR_MODEL_NOT_FOUND	Model ID not recognized
-5	ATELICO_ERR_INFERENCE_FAILED	Inference error
-6	ATELICO_ERR_STREAM_DONE	Stream completed (not an error)
-7	ATELICO_ERR_STREAM_EMPTY	No data available yet (not an error)
-8	ATELICO_ERR_JSON_PARSE	Invalid JSON in request
-9	ATELICO_ERR_STORE_NOT_FOUND	KV store not found
-10	ATELICO_ERR_IO	I/O error
-11	ATELICO_ERR_BLOCKED	Guardrail blocked content
-99	ATELICO_ERR_INTERNAL	Internal error

String Ownership

Strings you pass to the API: The API reads them during the call. You retain ownership and can free them after the call returns.

Strings returned by the API: Stored in a thread-local buffer. Valid only until the next API call on the same thread. Copy immediately if you need to keep them:

const char* response = nullptr;
atelico_llm_chat(engine, request, &response);

// Copy before making another API call
std::string saved_response = response;  // C++ copies here

// Now safe to make another call — response pointer is now invalid
atelico_llm_chat(engine, another_request, &response);

GPU Scheduling

Control GPU resource sharing at runtime:

// Scheduling modes
atelico_engine_set_scheduling_mode(engine, ATELICO_SCHEDULE_BALANCE);             // Default
atelico_engine_set_scheduling_mode(engine, ATELICO_SCHEDULE_PRIORITIZE_COMPUTE);  // Fast AI
atelico_engine_set_scheduling_mode(engine, ATELICO_SCHEDULE_PRIORITIZE_GRAPHICS); // Smooth FPS

// Resource limits
atelico_engine_set_vram_budget_mb(engine, 4096);  // Cap VRAM
atelico_engine_set_target_tps(engine, 15);        // Limit tokens/sec
atelico_engine_set_frame_time_ms(engine, 16);     // 60 FPS hint

Complete Example

#include "atelico_ffi.h"
#include <cstdio>
#include <cstring>
#include <string>

int main()
{
    // Create engine
    AtelicoEngine* engine = nullptr;
    if (atelico_engine_create(nullptr, &engine) != ATELICO_OK)
    {
        fprintf(stderr, "Init failed: %s\n", atelico_last_error());
        return 1;
    }

    // Load model
    if (atelico_model_load(engine, "meta-llama/Llama-3.2-3B-Instruct-Q4_K_M") != ATELICO_OK)
    {
        fprintf(stderr, "Model load failed: %s\n", atelico_last_error());
        atelico_engine_destroy(engine);
        return 1;
    }

    // Stream a chat response
    const char* request = R"({
        "model": "meta-llama/Llama-3.2-3B-Instruct-Q4_K_M",
        "messages": [
            {"role": "system", "content": "You are a narrator for a fantasy RPG."},
            {"role": "user", "content": "Describe the entrance to the dungeon."}
        ],
        "max_tokens": 200,
        "temperature": 0.8
    })";

    uint64_t stream = 0;
    if (atelico_llm_chat_stream(engine, request, &stream) != ATELICO_OK)
    {
        fprintf(stderr, "Stream failed: %s\n", atelico_last_error());
        atelico_engine_destroy(engine);
        return 1;
    }

    printf("NPC: ");
    bool done = false;
    while (!done)
    {
        const char* chunk = nullptr;
        int32_t rc = atelico_stream_poll(engine, stream, &chunk);
        if (rc == ATELICO_OK)
            printf("%s", chunk);
        else if (rc == ATELICO_ERR_STREAM_DONE)
            done = true;
        else if (rc != ATELICO_ERR_STREAM_EMPTY)
        {
            fprintf(stderr, "\nError: %s\n", atelico_last_error());
            done = true;
        }
    }
    printf("\n");

    atelico_stream_destroy(engine, stream);
    atelico_engine_destroy(engine);
    return 0;
}

Next Steps

• C FFI API Reference — full list of all functions, constants, and types
• Chat Completions API — detailed API reference (same JSON format)
• Structured Generation — constrain output to JSON schemas