Added example on how to handle unions.

example_unions/Makefile

@@ -0,0 +1,17 @@
+CFLAGS=-ansi -Wall -Werror -I .. -g -O0
+DEPS=../pb_decode.c ../pb_decode.h ../pb_encode.c ../pb_encode.h ../pb.h
+
+all: encode decode
+	./encode 1 | ./decode
+	./encode 2 | ./decode
+	./encode 3 | ./decode
+
+clean:
+	rm -f encode unionproto.pb.h unionproto.pb.c
+
+%: %.c $(DEPS) unionproto.pb.h unionproto.pb.c
+	$(CC) $(CFLAGS) -o $@ $< ../pb_decode.c ../pb_encode.c unionproto.pb.c
+
+unionproto.pb.h unionproto.pb.c: unionproto.proto ../generator/nanopb_generator.py
+	protoc -I. -I../generator -I/usr/include -ounionproto.pb $<
+	python ../generator/nanopb_generator.py unionproto.pb

example_unions/decode.c

@@ -0,0 +1,92 @@
+/* This program reads a message from stdin, detects its type and decodes it.
+ */
+ 
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+#include <pb_decode.h>
+#include "unionproto.pb.h"
+
+/* This function reads manually the first tag from the stream and finds the
+ * corresponding message type. It doesn't yet decode the actual message.
+ *
+ * Returns a pointer to the MsgType_fields array, as an identifier for the
+ * message type. Returns null if the tag is of unknown type or an error occurs.
+ */
+const pb_field_t* decode_unionmessage_type(pb_istream_t *stream)
+{
+    pb_wire_type_t wire_type;
+    int tag;
+    bool eof;
+
+    while (pb_decode_tag(stream, &wire_type, &tag, &eof))
+    {
+        if (wire_type == PB_WT_STRING)
+        {
+            const pb_field_t *field;
+            for (field = UnionMessage_fields; field->tag != 0; field++)
+            {
+                if (field->tag == tag && (field->type & PB_LTYPE_SUBMESSAGE))
+                {
+                    /* Found our field. */
+                    return field->ptr;
+                }
+            }
+        }
+        
+        /* Wasn't our field.. */
+        pb_skip_field(stream, wire_type);
+    }
+    
+    return NULL;
+}
+
+bool decode_unionmessage_contents(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct)
+{
+    pb_field_t field = {}; /* NB: Could get rid of this wrapper by fixing issue #2. */
+    field.ptr = fields;
+    
+    return pb_dec_submessage(stream, &field, dest_struct);
+}
+
+int main()
+{
+    /* Read the data into buffer */
+    uint8_t buffer[512];
+    size_t count = fread(buffer, 1, sizeof(buffer), stdin);
+    pb_istream_t stream = pb_istream_from_buffer(buffer, count);
+    
+    const pb_field_t *type = decode_unionmessage_type(&stream);
+    bool status = false;
+    
+    if (type == MsgType1_fields)
+    {
+        MsgType1 msg = {};
+        status = decode_unionmessage_contents(&stream, MsgType1_fields, &msg);
+        printf("Got MsgType1: %d\n", msg.value);
+    }
+    else if (type == MsgType2_fields)
+    {
+        MsgType2 msg = {};
+        status = decode_unionmessage_contents(&stream, MsgType2_fields, &msg);
+        printf("Got MsgType2: %s\n", msg.value ? "true" : "false");
+    }
+    else if (type == MsgType3_fields)
+    {
+        MsgType3 msg = {};
+        status = decode_unionmessage_contents(&stream, MsgType3_fields, &msg);
+        printf("Got MsgType3: %d %d\n", msg.value1, msg.value2);    
+    }
+    
+    if (!status)
+    {
+        printf("Decoding failed.\n");
+        return 1;
+    }
+    
+    return 0;
+}
+
+
+

example_unions/encode.c

@@ -0,0 +1,85 @@
+/* This program takes a command line argument and encodes a message in
+ * one of MsgType1, MsgType2 or MsgType3.
+ */
+ 
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+#include <pb_encode.h>
+#include "unionproto.pb.h"
+
+/* This function is the core of the union encoding process. It handles
+ * the top-level pb_field_t array manually, in order to encode a correct
+ * field tag before the message. The pointer to MsgType_fields array is
+ * used as an unique identifier for the message type.
+ */
+bool encode_unionmessage(pb_ostream_t *stream, const pb_field_t messagetype[], const void *message)
+{
+    const pb_field_t *field;
+    for (field = UnionMessage_fields; field->tag != 0; field++)
+    {
+        if (field->ptr == messagetype)
+        {
+            /* This is our field, encode the message using it. */
+            if (!pb_encode_tag_for_field(stream, field))
+                return false;
+            
+            return pb_encode_submessage(stream, messagetype, message);
+        }
+    }
+    
+    /* Didn't find the field for messagetype */
+    return false;
+}
+
+int main(int argc, char **argv)
+{
+    if (argc != 2)
+    {
+        fprintf(stderr, "Usage: %s (1|2|3)\n", argv[0]);
+        return 1;
+    }
+    
+    uint8_t buffer[512];
+    pb_ostream_t stream = pb_ostream_from_buffer(buffer, sizeof(buffer));
+    
+    bool status = false;
+    int msgtype = atoi(argv[1]);
+    if (msgtype == 1)
+    {
+        /* Send message of type 1 */
+        MsgType1 msg = {42};
+        status = encode_unionmessage(&stream, MsgType1_fields, &msg);
+    }
+    else if (msgtype == 2)
+    {
+        /* Send message of type 2 */
+        MsgType2 msg = {true};
+        status = encode_unionmessage(&stream, MsgType2_fields, &msg);
+    }
+    else if (msgtype == 3)
+    {
+        /* Send message of type 3 */
+        MsgType3 msg = {3, 1415};
+        status = encode_unionmessage(&stream, MsgType3_fields, &msg);
+    }
+    else
+    {
+        fprintf(stderr, "Unknown message type: %d\n", msgtype);
+        return 2;
+    }
+    
+    if (!status)
+    {
+        fprintf(stderr, "Encoding failed!\n");
+        return 3;
+    }
+    else
+    {
+        fwrite(buffer, 1, stream.bytes_written, stdout);
+        return 0; /* Success */
+    }
+}
+
+

example_unions/unionproto.proto

@@ -0,0 +1,30 @@
+// This is an example of how to handle 'union' style messages
+// with nanopb, without allocating memory for all the message types.
+//
+// There is no official type in Protocol Buffers for describing unions,
+// but they are commonly implemented by filling out exactly one of
+// several optional fields.
+
+message MsgType1
+{
+    required int32 value = 1;
+}
+
+message MsgType2
+{
+    required bool value = 1;
+}
+
+message MsgType3
+{
+    required int32 value1 = 1;
+    required int32 value2 = 2;
+}
+
+message UnionMessage
+{
+    optional MsgType1 msg1 = 1;
+    optional MsgType2 msg2 = 2;
+    optional MsgType3 msg3 = 3;
+}
+