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| author | Eric Blake | 2016-02-18 07:48:16 +0100 |
|---|---|---|
| committer | Markus Armbruster | 2016-02-19 11:08:56 +0100 |
| commit | 02a57ae32b08e8981b59979b80e682c9a153e94d (patch) | |
| tree | 418d1597e1ea5ff8e3f119fe453e990746299fe3 /docs | |
| parent | qapi: Simplify excess input reporting in input visitors (diff) | |
| download | qemu-02a57ae32b08e8981b59979b80e682c9a153e94d.tar.gz qemu-02a57ae32b08e8981b59979b80e682c9a153e94d.tar.xz qemu-02a57ae32b08e8981b59979b80e682c9a153e94d.zip | |
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 <eblake@redhat.com>
Message-Id: <1455778109-6278-3-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Diffstat (limited to 'docs')
| -rw-r--r-- | docs/qapi-code-gen.txt | 15 |
1 files changed, 8 insertions, 7 deletions
diff --git a/docs/qapi-code-gen.txt b/docs/qapi-code-gen.txt index 128f074a2d..999f3b98f0 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: |