A command returns OK on completion or ACK some error on failure. These denote the end of command execution.
The nature of the error can be gleaned from the information that follows the ACK. ACK lines are of the form:
ACK [error@command_listNum] {current_command} message_text\n
These responses are generated by a call to
commandError. They contain four separate
terms. Let's look at each of them:
error: numeric value of one
of the ACK_ERROR constants defined
in ack.h.
command_listNum: offset of the command that caused the error in a Command List. An error will always cause a command list to terminate at the command that causes the error.
current_command: name of the command, in a Command List, that was executing when the error occurred.
message_text: some (hopefully) informative text that describes the nature of the error.
Example 1.1. foo
An example might help. Consider the following sequence sent from the client to the server.
command_list_begin
volume 86
play 10240
status
command_list_end
The server responds with: ACK [50@1] {play} song doesn't exist: "10240"
This tells us that the play command, which was the
second in the list (the first or only command is
numbered 0), failed with error 50. The number 50
translates to ACK_ERROR_NO_EXIST--the
song doesn't exist. This is reiterated by the message text
which also tells us which song doesn't exist.