scripts/qapi-types.py: Add dummy member to empty structs

Make sure that all generated C structs have at least one field; this
avoids potential issues with attempting to malloc space for
zero-length structs in C (g_malloc(sizeof struct) would return NULL).
It also avoids an incompatibility with C++ (where an empty struct is
size 1); that isn't important to us now but might be in future.

Generated empty structures look like this:
    struct Abort
    {
        char qapi_dummy_field_for_empty_struct;
    };

This silences clang warnings like:
./qapi-types.h:3752:1: warning: empty struct has size 0 in C, size 1 in C++ [-Wextern-c-compat]
struct Abort
^

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1419359069-16611-1-git-send-email-peter.maydell@linaro.org

1 files changed, 8 insertions, 0 deletions

diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index d2f815bca2..1eb272dd26 100644
--- a/scripts/qapi-types.py
+++ b/scripts/qapi-types.py
@@ -99,6 +99,14 @@ struct %(name)s
 
     ret += generate_struct_fields(members)
 
+    # Make sure that all structs have at least one field; this avoids
+    # potential issues with attempting to malloc space for zero-length structs
+    # in C, and also incompatibility with C++ (where an empty struct is size 1).
+    if not base and not members:
+            ret += mcgen('''
+    char qapi_dummy_field_for_empty_struct;
+''')
+
     if len(fieldname):
         fieldname = " " + fieldname
     ret += mcgen('''