Specifications

3-10
Guide to Printers and Printing
Spooler Data Flow (enq Command)
The commands lp, lpr, qprt, and enq can be used to submit a job to the spooler for
processing. The enq command is the true entry point to the spooler; lp, lpr, and qprt all
parse their own arguments and compose a call to enq. This can be demonstrated by
executing the following steps as the root user at a shell prompt:
1. Enter mount /bin/echo /bin/enq.
2. Enter qprt –Pasc –fp –z1 –p12 –s courier –C –N 3 /etc/motd.
3. Enter umount /bin/enq.
The qprt command in step 2 attempts to submit a print job to the spooler and have it placed
on the queue named asc, requesting three copies of the message–of–the–day in a 12–point
Courier font rotated 90 degrees. qprt parses its command line arguments and builds an
argument vector to pass to enq. When the qprt command tries to invoke enq with the
argument vector, it instead invokes the echo command, which is mounted over the enq
command. Thus the argument vector generated by the qprt command is passed to the
echo command, which in turn simply echoes the argument vector to your display. This
procedure will work with lp and lpr as well. Aside from demonstrating that qprt really is a
front end to enq, this technique is also useful when you are trying to figure out how to get
unsupported flags into the spooler. See Filters on page 4-150 for further information.
Execution of the qprt command in step 2 should cause the following output to be written to
the display element specified by your TERM environment variable:
–P asc –o –o –f –o p –z –o 1 –o –p –o 12 –o –s courier –C –N 3 /etc/motd
This is the argument vector generated by this specific instance of the qprt command. If
echo had not been mounted over enq, the following job submission command would have
been executed:
enq –P asc –o –f –o p –o –z –o 1 –o –p –o 12 –o –s courier –C –N 3
/etc/motd
A job submission command must end with the name of one or more real files that exist in a
file system accessible by the base operating system. This is true even when the queue is
set up to handle jobs other than print jobs.
Note: Make sure that you execute step 3. Otherwise, the spooler will be disabled.
When the enq command is executed, either directly or by lp, lpr, or qprt, it assigns a job
number to the job. By default, lp will return the job number. lpr and qprt will not return the
job number unless you specifically request it with a flag.
enq creates a JDF and places it in /var/spool/lpd/qdir, then writes the name of the JDF to
a message queue and signals the qdaemon (by sending it a SIGUSR2) that a new JDF
exists. The qdaemon then reads the name of the JDF from the message queue, accesses
the JDF directly, and reads the data contained in the JDF into an internal data structure it
maintains to track all the jobs currently in the spooler. At this point in time, the job has been
accepted by the spooler.
A JDF is created for all spooling system operations other than a queue status query; the
structure of a JDF differs between print requests versus job cancellation requests versus
queue control requests, and so on, but a JDF is created nevertheless. Commands with the
same function as lpstat still call enq to do their work, but neither is a JDF created nor is the
qdaemon involved.
When the qdaemon determines that the device upon which the job is queued is available,
the qdaemon invokes the backend for the queue, passing it arguments specified by the
JDF. The backend processes the job.