User ID, as a symbol, associated with the current handle.

q).z.u
`demo

For

• handle 0 (console) returns the userid under which the process is running.
• handles > 0 returns either:
  • on the server end of a connection, the userid as passed to hopen by the client
  • on the client end of a connection, the null symbol `

q).z.u                  / console is handle 0
`charlie
q)0".z.u"               / explicitly using handle 0
`charlie
q)h:hopen`:localhost:5000:geoffrey:geffspasswd
q)h".z.u"               / server side .z.u is as passed by the client to hopen
`geoffrey
q)h({.z.w".z.u"};::)    / client side returns null symbol
`