How to print "hello world" from your Hook!
The Hook API provides a set of functions in the namespace trace which write output to the xrpld log file when xrpld is configured with the trace log-level. These functions, generally speaking, allow you to see the value of variables, buffers and otherwise trace the execution and state of a Hook at runtime.
📘 Hint
At time of writing there is no interactive Hook Debugger. You must use the trace functions.
The following trace functions are available in the Hooks API
Print a utf-8 message, followed by a user-specified buffer (this last optionally as hex.)
Print a utf-8 message, followed by an integer.
Print a utf-8 message, followed by an XFL Floating point number.
trace_slot
Print a utf-8 message, followed by the serialized contents of a slot.
The following code will print a single trace line then accept the Originating Transaction.
An example of the log-line produced by xahaud when a payment is sent out of or into the Hook Account:
The above appears in the log as all-one-line, but split here for visibility.
👍 Use testnet
The Xahau Testnet is the perfect place to test your Hooks.
A breakdown of the log format appears in the table below
2021-Apr-13 13:59:11.083700726 UTC View:TRC
xahaud's prefix to the log line
1
HookTrace
This is a trace initiated by the Hook itself not some other information about the Hook. Other information is available on tags HookError, HookEmit and HookInfo.
2
[rHb9CJAWyB4rj91VRWn96DkukG4bwdtyTh
The first account in the square brackets is the Hook Account.
3
-rE3SfnjwfzZFL3JK9cLVfJuy8Ar1XnCqPw]:
The second account in the square brackets is the Originating Account.
4
A number
This is the message the Hook was told to deliver before the trace payload
5
10
This is the trace payload
6
🚧 Tip
Xahaudproduces a lot of output. It is therefore generally advisible to grep logs for the account/s you are interested in. For example use:tail -f log | grep HookTrace | grep <account>