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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/qapi-code-gen.txt | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/docs/devel/qapi-code-gen.txt b/docs/devel/qapi-code-gen.txt index b517b0c..e8ec8ac 100644 --- a/docs/devel/qapi-code-gen.txt +++ b/docs/devel/qapi-code-gen.txt @@ -719,6 +719,34 @@ any non-empty complex type (struct, union, or alternate), and a pointer to that QAPI type is passed as a single argument. +=== Features === + +Sometimes, the behaviour of QEMU changes compatibly, but without a +change in the QMP syntax (usually by allowing values or operations that +previously resulted in an error). QMP clients may still need to know +whether the extension is available. + +For this purpose, a list of features can be specified for a struct type. +This is exposed to the client as a list of string, where each string +signals that this build of QEMU shows a certain behaviour. + +In the schema, features can be specified as simple strings, for example: + +{ 'struct': 'TestType', + 'data': { 'number': 'int' }, + 'features': [ 'allow-negative-numbers' ] } + +Another option is to specify features as dictionaries, where the key +'name' specifies the feature string to be exposed to clients: + +{ 'struct': 'TestType', + 'data': { 'number': 'int' }, + 'features': [ { 'name': 'allow-negative-numbers' } ] } + +This expanded form is necessary if you want to make the feature +conditional (see below in "Configuring the schema"). + + === Downstream extensions === QAPI schema names that are externally visible, say in the Client JSON @@ -771,6 +799,16 @@ Example: a conditional 'bar' enum member. [ 'foo', { 'name' : 'bar', 'if': 'defined(IFCOND)' } ] } +Similarly, features can be specified as a dictionary with a 'name' and +an 'if' key. + +Example: a conditional 'allow-negative-numbers' feature + +{ 'struct': 'TestType', + 'data': { 'number': 'int' }, + 'features': [ { 'name': 'allow-negative-numbers', + 'if' 'defined(IFCOND)' } ] } + Please note that you are responsible to ensure that the C code will compile with an arbitrary combination of conditions, since the generators are unable to check it at this point. |