diff options
| -rw-r--r-- | docs/writing-qmp-commands.txt | 47 |
1 files changed, 27 insertions, 20 deletions
diff --git a/docs/writing-qmp-commands.txt b/docs/writing-qmp-commands.txt index 0ad51aa22a..8349dec8af 100644 --- a/docs/writing-qmp-commands.txt +++ b/docs/writing-qmp-commands.txt @@ -210,19 +210,17 @@ if you don't see these strings, then something went wrong. === Errors === QMP commands should use the error interface exported by the error.h header -file. The basic function used to set an error is the error_set() one. +file. Basically, errors are set by calling the error_set() function. Let's say we don't accept the string "message" to contain the word "love". If -it does contain it, we want the "hello-world" command to the return the -InvalidParameter error. - -Only one change is required, and it's in the C implementation: +it does contain it, we want the "hello-world" command to return an error: void qmp_hello_world(bool has_message, const char *message, Error **errp) { if (has_message) { if (strstr(message, "love")) { - error_set(errp, QERR_INVALID_PARAMETER, "message"); + error_set(errp, ERROR_CLASS_GENERIC_ERROR, + "the word 'love' is not allowed"); return; } printf("%s\n", message); @@ -231,30 +229,40 @@ void qmp_hello_world(bool has_message, const char *message, Error **errp) } } -Let's test it. Build qemu, run it as defined in the "Testing" section, and -then issue the following command: +The first argument to the error_set() function is the Error pointer to pointer, +which is passed to all QMP functions. The second argument is a ErrorClass +value, which should be ERROR_CLASS_GENERIC_ERROR most of the time (more +details about error classes are given below). The third argument is a human +description of the error, this is a free-form printf-like string. + +Let's test the example above. Build qemu, run it as defined in the "Testing" +section, and then issue the following command: -{ "execute": "hello-world", "arguments": { "message": "we love qemu" } } +{ "execute": "hello-world", "arguments": { "message": "all you need is love" } } The QMP server's response should be: { "error": { - "class": "InvalidParameter", - "desc": "Invalid parameter 'message'", - "data": { - "name": "message" - } + "class": "GenericError", + "desc": "the word 'love' is not allowed" } } -Which is the InvalidParameter error. +As a general rule, all QMP errors should use ERROR_CLASS_GENERIC_ERROR. There +are two exceptions to this rule: + + 1. A non-generic ErrorClass value exists* for the failure you want to report + (eg. DeviceNotFound) + + 2. Management applications have to take special action on the failure you + want to report, hence you have to add a new ErrorClass value so that they + can check for it -When you have to return an error but you're unsure what error to return or -which arguments an error takes, you should look at the qerror.h file. Note -that you might be required to add new errors if needed. +If the failure you want to report doesn't fall in one of the two cases above, +just report ERROR_CLASS_GENERIC_ERROR. -FIXME: describe better the error API and how to add new errors. + * All existing ErrorClass values are defined in the qapi-schema.json file === Command Documentation === @@ -275,7 +283,6 @@ here goes "hello-world"'s new entry for the qapi-schema.json file: # @message: #optional string to be printed # # Returns: Nothing on success. -# If @message contains "love", InvalidParameter # # Notes: if @message is not provided, the "Hello, world" string will # be printed instead |