documentation of tw_output from the wiki

Author: gonsie

_posts/2019-09-25-lp-printf.md

@@ -0,0 +1,30 @@
+---
+title: "LP Printing with tw_output"
+category: feature
+author: "Justin LaPre"
+---
+
+LP-level printing is available from the forward event handler.
+Models simply need to attach an output buffer to the event being processed, using the `tw_output` function.
+If this event is eventually committed, all attached output will be sent to standard-out.
+
+## Example
+
+    void event_handler (state *s, tw_bf *bf, message *in_msg, tw_lp *lp) {
+        // this output is associated with in_msg
+        tw_output(lp, "this has printf functionality: %d %f %s\n", 1, 2.0, "three");
+    }
+
+
+
+Argument 1: The lp you're on (4th argument for your event handler).
+
+Argument 2: Printf-style string, with specifiers (%-escaped codes).
+
+A variable number of additional arguments are needed to pass data for each specifier.
+
+### Notes
+
+1. The `tw_output()` calls are associated with the event being processed.
+2. Multiple outputs can be attached to the same event.
+3. Output will always be in-order from an LP-perspective though the messages may be out-of-order from a global perspective.  You may wish to include a timestamp for sorting.