5 files changed, 283 insertions, 0 deletions
diff --git a/examples/using_union_messages/Makefile b/examples/using_union_messages/Makefile
new file mode 100644
index 0000000..66396a0
--- /dev/null
+++ b/examples/using_union_messages/Makefile
@@ -0,0 +1,20 @@
+# Include the nanopb provided Makefile rules
+include ../../extra/nanopb.mk
+
+# Compiler flags to enable all warnings & debug info
+CFLAGS = -ansi -Wall -Werror -g -O0
+CFLAGS += -I$(NANOPB_DIR)
+
+all: encode decode
+	./encode 1 | ./decode
+	./encode 2 | ./decode
+	./encode 3 | ./decode
+
+.SUFFIXES:
+
+clean:
+	rm -f encode unionproto.pb.h unionproto.pb.c
+
+%: %.c unionproto.pb.c
+	$(CC) $(CFLAGS) -o $@ $^ $(NANOPB_CORE)
+
diff --git a/examples/using_union_messages/README.txt b/examples/using_union_messages/README.txt
new file mode 100644
index 0000000..7a1e75d
--- /dev/null
+++ b/examples/using_union_messages/README.txt
@@ -0,0 +1,52 @@
+Nanopb example "using_union_messages"
+=====================================
+
+Union messages is a common technique in Google Protocol Buffers used to
+represent a group of messages, only one of which is passed at a time.
+It is described in Google's documentation:
+https://developers.google.com/protocol-buffers/docs/techniques#union
+
+This directory contains an example on how to encode and decode union messages
+with minimal memory usage. Usually, nanopb would allocate space to store
+all of the possible messages at the same time, even though at most one of
+them will be used at a time.
+
+By using some of the lower level nanopb APIs, we can manually generate the
+top level message, so that we only need to allocate the one submessage that
+we actually want. Similarly when decoding, we can manually read the tag of
+the top level message, and only then allocate the memory for the submessage
+after we already know its type.
+
+
+Example usage
+-------------
+
+Type `make` to run the example. It will build it and run commands like
+following:
+
+./encode 1 | ./decode
+Got MsgType1: 42
+./encode 2 | ./decode
+Got MsgType2: true
+./encode 3 | ./decode
+Got MsgType3: 3 1415
+
+This simply demonstrates that the "decode" program has correctly identified
+the type of the received message, and managed to decode it.
+
+
+Details of implementation
+-------------------------
+
+unionproto.proto contains the protocol used in the example. It consists of
+three messages: MsgType1, MsgType2 and MsgType3, which are collected together
+into UnionMessage.
+
+encode.c takes one command line argument, which should be a number 1-3. It
+then fills in and encodes the corresponding message, and writes it to stdout.
+
+decode.c reads a UnionMessage from stdin. Then it calls the function
+decode_unionmessage_type() to determine the type of the message. After that,
+the corresponding message is decoded and the contents of it printed to the
+screen.
+
diff --git a/examples/using_union_messages/decode.c b/examples/using_union_messages/decode.c
new file mode 100644
index 0000000..b9f4af5
--- /dev/null
+++ b/examples/using_union_messages/decode.c
@@ -0,0 +1,96 @@
+/* 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;
+    uint32_t 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_istream_t substream;
+    bool status;
+    if (!pb_make_string_substream(stream, &substream))
+        return false;
+    
+    status = pb_decode(&substream, fields, dest_struct);
+    pb_close_string_substream(stream, &substream);
+    return status;
+}
+
+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("Decode failed: %s\n", PB_GET_ERROR(&stream));
+        return 1;
+    }
+    
+    return 0;
+}
+
+
+
diff --git a/examples/using_union_messages/encode.c b/examples/using_union_messages/encode.c
new file mode 100644
index 0000000..e124bf9
--- /dev/null
+++ b/examples/using_union_messages/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 */
+    }
+}
+
+
diff --git a/examples/using_union_messages/unionproto.proto b/examples/using_union_messages/unionproto.proto
new file mode 100644
index 0000000..d7c9de2
--- /dev/null
+++ b/examples/using_union_messages/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;
+}
+