enhance to generate encode routine to be more robust to encode NULL

compiler/capnpc-c.c

@@ -1256,7 +1256,11 @@ void mk_struct_ptr_encoder(capnp_ctx_t *ctx, struct node *n) {
            n->name.str, n->name.str, buf);
   str_addf(&(ctx->SRC), "\t%s_ptr ptr;\n", n->name.str);
   str_addf(&(ctx->SRC), "\tstruct %s d;\n", n->name.str);
+  str_addf(&(ctx->SRC), "\t%s zero_ = {0}; \n", buf);
   str_addf(&(ctx->SRC), "\tptr = new_%s(cs);\n", n->name.str);
+  str_addf(&(ctx->SRC), "\tif (s == NULL) {\n");
+  str_addf(&(ctx->SRC), "\t\ts = &zero_;\n");
+  str_addf(&(ctx->SRC), "\t}\n");
   str_addf(&(ctx->SRC), "\tencode_%s(cs, &d, s);\n", n->name.str);
   str_addf(&(ctx->SRC), "\twrite_%s(&d, ptr);\n", n->name.str);
   str_addf(&(ctx->SRC), "\t(*p) = ptr;\n");

examples/book/book.capnp

@@ -15,11 +15,17 @@ struct Publish $C.mapname("publish_t") {
   isbn  @0: UInt64;
   year  @1: UInt32;
 }
+
+struct Nulldata $C.mapname("nulldata_t") {
+  null  @0: UInt32 $C.mapname("null_");
+}
+
 struct Book $C.mapname("book_t") {
   title   @0: Text;
   authors @1: List(Text) $C.mapname("authors") $C.maplistcount("n_authors");
   chapters @5: List(Chapter) $C.mapname("chapters_") $C.maplistcount("n_chapters");
   publish  @6: Publish;
+  nulldata @7: Nulldata;
   magic1  @2: List(UInt32) $C.mapname("magic_1") $C.maplistcount("n_magic1");
   acquire :union $C.mapuniontag("acquire_method") {
     buy   @3: Text;

examples/book/book.h

@@ -16,6 +16,10 @@ typedef struct {
     uint32_t year;
 } publish_t;
     
+typedef struct {
+    int null_;
+} nulldata_t;
+
 typedef struct {
     char *title;
     int   n_authors;
@@ -23,6 +27,7 @@ typedef struct {
     int   n_chapters;
     chapter_t *chapters_;
     publish_t *publish;
+    nulldata_t *nulldata;
     int   n_magic1;
     uint32_t *magic_1;
     int   acquire_method;

examples/book/test.c

@@ -45,6 +45,7 @@ int encode() {
     book.n_chapters = 3;
     book.chapters_ = &chapters[0];
     book.publish = &publish ;
+    book.nulldata = NULL;
     book.n_magic1 = 2;
     book.magic_1 = &magic1[0];
     book.acquire_method = Book_acquire_buy;