Implement support for oneofs (C unions).

Basic test included, should probably add an oneof to the AllTypes test also.

Update issue 131
Status: Started

tests/oneof/decode_oneof.c

@@ -0,0 +1,72 @@
+/* Decode a message using oneof fields */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <pb_decode.h>
+#include "oneof.pb.h"
+#include "test_helpers.h"
+#include "unittests.h"
+
+int main(int argc, char **argv)
+{
+    uint8_t buffer[OneOfMessage_size];
+    OneOfMessage msg = OneOfMessage_init_zero;
+    pb_istream_t stream;
+    size_t count;
+    int option;
+
+    if (argc != 2)
+    {
+        fprintf(stderr, "Usage: encode_oneof [number]\n");
+        return 1;
+    }
+    option = atoi(argv[1]);
+
+    SET_BINARY_MODE(stdin);
+    count = fread(buffer, 1, sizeof(buffer), stdin);
+
+    if (!feof(stdin))
+    {
+        printf("Message does not fit in buffer\n");
+        return 1;
+    }
+
+    stream = pb_istream_from_buffer(buffer, count);
+
+    if (!pb_decode(&stream, OneOfMessage_fields, &msg))
+    {
+        printf("Decoding failed: %s\n", PB_GET_ERROR(&stream));
+        return 1;
+    }
+
+    {
+        int status = 0;
+
+        /* Check that the basic fields work normally */
+        TEST(msg.prefix == 123);
+        TEST(msg.suffix == 321);
+
+        /* Check that we got the right oneof according to command line */
+        if (option == 1)
+        {
+            TEST(msg.which_values == OneOfMessage_first_tag);
+            TEST(msg.values.first == 999);
+        }
+        else if (option == 2)
+        {
+            TEST(msg.which_values == OneOfMessage_second_tag);
+            TEST(strcmp(msg.values.second, "abcd") == 0);
+        }
+        else if (option == 3)
+        {
+            TEST(msg.which_values == OneOfMessage_third_tag);
+            TEST(msg.values.third.array[0] == 1);
+            TEST(msg.values.third.array[1] == 2);
+            TEST(msg.values.third.array[2] == 3);
+            TEST(msg.values.third.array[3] == 4);
+            TEST(msg.values.third.array[4] == 5);
+        }
+
+        return status;
+    }
+}