[Qemu-devel] [PATCH] scripts/qapi-types.py: Add dummy member to empty structs

[Qemu-devel] [PATCH] scripts/qapi-types.py: Add dummy member to empty structs

[Peter Maydell]

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>
---
 scripts/qapi-types.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/scripts/qapi-types.py b/scripts/qapi-types.py
index d2f815b..1eb272d 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('''
-- 
2.2.0

Re: [Qemu-devel] [PATCH] scripts/qapi-types.py: Add dummy member to empty structs

[Eric Blake]

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On 12/23/2014 11:24 AM, Peter Maydell wrote:
> 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>
> ---
>  scripts/qapi-types.py | 8 ++++++++
>  1 file changed, 8 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

However, I'm a bit surprised that there is no testsuite coverage for
this; it would be nice if we had at least one test that used an empty
struct to prove that we continue to generate them and their visitors
correctly.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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