Add topic on MAVLink C library #defines

en/SUMMARY.md

@@ -11,6 +11,7 @@
 * [Using MAVLink Libraries](getting_started/use_libraries.md)
   * [C (mavgen)](mavgen_c/README.md)
     * [Message Signing](mavgen_c/message_signing_c.md)
+    * [C Defines](mavgen_c/defines.md)
     * [Examples](mavgen_c/examples.md)
       * [UART Interface (C)](mavgen_c/example_c_uart.md)
       * [UDP Example (C)](mavgen_c/example_c_udp.md)

en/mavgen_c/defines.md

@@ -0,0 +1,31 @@
+# MAVLink C Library #defines
+
+The following C `#defines` can be set in code in order to tune the setup for use on different platforms.
+
+- `MAVLINK_USE_CONVENIENCE_FUNCTIONS` ([protocol.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/protocol.h)): Causes convenience functions to be defined, including: `_mav_finalize_message_chan_send()`, `_mavlink_send_uart()`, `_mavlink_resend_uart()`. To use, add `#define MAVLINK_USE_CONVENIENCE_FUNCTIONS` to your code.
+- `MAVLINK_COMM_NUM_BUFFERS`: Sets the maximum number of comms buffers to be used (comms channels).
+  By default this is set to 16 on Linux, Windows and macOS, and to 4 on other platforms.
+  You might set `#define MAVLINK_COMM_NUM_BUFFERS 2` on an embedded system that would only ever have two channels, in order to reduce memory overheads.
+  A stack overrun will occur if there are more buffers used than allocated.
+  See [FAQ > How can I further reduce the generated C library size?](../about/faq.md#developers).
+- `MAVLINK_SIGNING_FLAG_SIGN_OUTGOING` ([mavlink_types.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/mavlink_types.h)): Enable/disable outgoing signing.
+  See [Enabling Signing on a Channel > Enabling Signing on a Channel](../mavgen_c/message_signing_c.md#enabling_signing_channel)
+- `MAVLINK_EXTERNAL_RX_STATUS` ([mavlink_helpers.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/mavlink_helpers.h)): `mavlink_status_t* mavlink_get_channel_status(uint8_t chan)` returns the status of a particular channel using a preallocated `static` array.
+  Define `MAVLINK_EXTERNAL_RX_STATUS` if you want to use your own external array (named `m_mavlink_status`) of `mavlink_status_t` objects.
+- `MAVLINK_EXTERNAL_RX_BUFFER` ([mavlink_helpers.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/mavlink_helpers.h)): Set if you want to define your own channel comms buffers on the stack or heap, rather than using the pre-allocated `static` buffers.
+  If you define this you will need to declare an array of `mavlink_message_t` (one for each channel) named `m_mavlink_buffer`.
+- `NATIVE_BIG_ENDIAN` ([protocol.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/protocol.h)): Enable if using MAVLink on a system that is native big-endian (MAVLINK_LITTLE_ENDIAN is true by default).
+- `MAVLINK_SEPARATE_HELPERS` ([protocol.h](https://github.com/ArduPilot/pymavlink/blob/master/generator/C/include_v2.0/protocol.h)): Set to remove all the helpers declared `mavlink_helpers.h`, allowing you to provide an alternative implementation.
+- `MAVLINK_MAX_PAYLOAD_LEN`: Override the maximum payload length.
+  The default maximum payload length is 255 bytes. With care you can override this to reduce memory usage.
+  For example, to reduce the maximum payload to the smallest size supported by a dialect you could do:
+
+  ```c
+  #include <version.h>
+  #define MAVLINK_MAX_PAYLOAD_LEN MAVLINK_MAX_DIALECT_PAYLOAD_SIZE
+  ```
+
+  See also [FAQ > How can I further reduce the generated C library size?](../about/faq.md#developers).
+
+
+There are a number of other internal defines that should not be modified.