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+# AGENT.md — Coturn
+
+Coturn is a TURN/STUN server written in C11, implementing RFC 5766, RFC 5389, and related NAT traversal protocols. It supports multiple database backends (SQLite, PostgreSQL, MySQL, Redis, MongoDB), multiple auth mechanisms, and a fuzzing harness for OSS-Fuzz.
+
+## Build
+
+```bash
+# Standard build
+mkdir build && cd build
+cmake ..
+make -j$(nproc)
+
+# Debug build
+cmake -DCMAKE_BUILD_TYPE=Debug ..
+make -j$(nproc)
+
+# Fuzzer build (requires clang or AppleClang)
+CC=clang CXX=clang++ cmake -S . -B build -DFUZZER=ON
+cmake --build build -j$(nproc)
+```
+
+Key CMake options:
+- `-DFUZZER=ON` — build OSS-Fuzz targets (requires Clang or AppleClang)
+- `-DCMAKE_BUILD_TYPE=Debug|Release`
+- `-DWITH_MYSQL=ON/OFF`, `-DWITH_PGSQL=ON/OFF`, `-DWITH_MONGO=ON/OFF`, `-DWITH_REDIS=ON/OFF`
+
+## Code style
+
+All C source must be formatted with `clang-format` using the project's [.clang-format](.clang-format):
+
+```bash
+find src -name '*.c' -o -name '*.h' | xargs clang-format -i
+```
+
+Key style rules (LLVM-based):
+- Indent: 2 spaces, no tabs
+- Column limit: 120
+- Pointer alignment: right (`int *p`)
+- Brace style: attach (K&R)
+- Zero-initialize stack buffers at declaration: `uint8_t buf[N] = {0}` or `SomeStruct s = {0}`
+
+## Tests
+
+```bash
+# Protocol conformance (RFC 5769 test vectors)
+cd examples && ./scripts/rfc5769.sh
+
+# Basic TURN relay test (run server first, then client)
+cd examples && ./scripts/basic/relay.sh
+cd examples && ./scripts/basic/udp_c2c_client.sh
+
+# Full test suite
+cd examples && ./run_tests.sh
+```
+
+See [docs/Testing.md](docs/Testing.md) for database setup and extended test scenarios.
+
+## Source layout
+
+```
+src/
+  client/          # TURN client library (C)
+  client++/        # TURN client library (C++)
+  server/          # Core TURN/STUN server logic
+  apps/
+    relay/         # turnserver main process, listeners, netengine
+    uclient/       # CLI test client
+include/turn/      # Public headers
+fuzzing/           # OSS-Fuzz targets and seed corpora
+examples/          # Test scripts and sample configs
+turndb/            # Database schema and setup scripts
+docs/              # Protocol notes and configuration docs
+```
+
+## Common patterns
+
+- **Port types**: use `uint16_t` for port fields and parameters (not `int`); port 0 means OS-assigned ephemeral
+- **Buffer initialization**: zero-initialize stack buffers at declaration (`= {0}`), not just before first use
+- **HMAC output buffers**: declare as `uint8_t buf[MAXSHASIZE] = {0}` — the buffer is written into the message before HMAC runs, so uninitialized bytes would be briefly present in the packet
+- **Uninitialized structs**: use `= {0}` for stack-allocated address structs (e.g., `ioa_addr`)
+- **Counter overflow in `turn_ports.c`**: `_turnports` uses `uint32_t low/high` counters; comparisons must be overflow-safe (use subtraction, not `>=`)
+- **Port bounds checks**: use `<= USHRT_MAX` (not `< USHRT_MAX`) when validating that an `int` holds a valid port — port 65535 is valid
+- **Error handling**: check return values of all OpenSSL/libevent calls; use `ERR_clear_error()` before HMAC operations
+- **Logging**: use `TURN_LOG_FUNC` macros, not `fprintf`/`perror`