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Files in the BNU spool directories
that have filenames beginning with the characters P.
are checkpoint files.
A checkpoint file is created when processing terminates abnormally.
Unlike the TM.pid.ddd file, it is not removed.
Therefore, when the transfer session is re-established, the length
of the checkpoint file serves as an appropriate place to restart the
transfer of the file, instead of restarting from the beginning.
When checkpointing is specified for a file being transferred from another
computer, the checkpoint file is used instead of a TM file.
checkpoint files are created in the spool directory.
Checkpointing occurs only between two systems that have the BNU
enhancements introduced in UNIX System V Release 4.
The names of the checkpoint files have the following format:
P.systmxxxxyyy
where systm is the first five characters in the name of the remote computer, xxxx is a four-digit job sequence number assigned by uucp. The four-digit job sequence number may be followed by a sub-sequence number, yyy, which is used when there are several P. files created for a work (C.) file. When the entire file is received, the P.systmxxxxyyy file is moved to the pathname specified in the C.sysnxxxx file (discussed shortly) that caused the transmission.