From 02a57ae32b08e8981b59979b80e682c9a153e94d Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Wed, 17 Feb 2016 23:48:16 -0700 Subject: qapi: Forbid empty unions and useless alternates Empty unions serve no purpose, and while we compile with gcc which permits them, strict C99 forbids them. We happen to inject a dummy 'void *data' member into the C unions that represent QAPI unions and alternates, but we want to get rid of that member (it pollutes the namespace for no good reason), which would leave us with an empty union if the user didn't provide any branches. While empty structs make sense in QAPI, empty unions don't add any expressiveness to the QMP language. So prohibit them at parse time. Update the documentation and testsuite to match. Note that the documentation already mentioned that alternates should have "two or more JSON data types"; so this also fixes the code to enforce that. However, we have existing uses of a union type with only one branch, so the 2-or-more strictness is intentionally limited to alternates. Signed-off-by: Eric Blake Message-Id: <1455778109-6278-3-git-send-email-eblake@redhat.com> Signed-off-by: Markus Armbruster --- docs/qapi-code-gen.txt | 15 ++++++++------- 1 file changed, 8 insertions(+), 7 deletions(-) (limited to 'docs') diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index 128f074..999f3b9 100644 --- a/docs/qapi-code-gen.txt +++ b/docs/qapi-code-gen.txt @@ -187,11 +187,11 @@ prevent incomplete include files. Usage: { 'struct': STRING, 'data': DICT, '*base': STRUCT-NAME } -A struct is a dictionary containing a single 'data' key whose -value is a dictionary. This corresponds to a struct in C or an Object -in JSON. Each value of the 'data' dictionary must be the name of a -type, or a one-element array containing a type name. An example of a -struct is: +A struct is a dictionary containing a single 'data' key whose value is +a dictionary; the dictionary may be empty. This corresponds to a +struct in C or an Object in JSON. Each value of the 'data' dictionary +must be the name of a type, or a one-element array containing a type +name. An example of a struct is: { 'struct': 'MyType', 'data': { 'member1': 'str', 'member2': 'int', '*member3': 'str' } } @@ -288,9 +288,10 @@ or: { 'union': STRING, 'data': DICT, 'base': STRUCT-NAME, Union types are used to let the user choose between several different variants for an object. There are two flavors: simple (no -discriminator or base), flat (both discriminator and base). A union +discriminator or base), and flat (both discriminator and base). A union type is defined using a data dictionary as explained in the following -paragraphs. +paragraphs. The data dictionary for either type of union must not +be empty. A simple union type defines a mapping from automatic discriminator values to data types like in this example: -- cgit v1.1