SNAP(6) SNAP(6)
NAME
snap - process snapshots
DESCRIPTION
Process snapshots are used to save a process image for
debugging on another machine or at another time. They are
like old Unix core dumps but can hold multiple process
images and are smaller.
The first line of a snapshot begins with the prefix
``process snapshot'' and often contains other information as
well, such as creation time, user name, system name, cpu
type, and kernel type. This information is intended for
humans, not programs. Programs reading snapshots should
only check that this line begins with the specified prefix.
Throughout the rest of the snapshot, decimal strings are
always right-justified, blank-padded to at least 11
characters, and followed by a single space character.
The rest of the snapshot is one or more records, each of
which begins with a one-line header. This header is a
decimal process id followed by an identification string,
which denotes the type of data in the record.
Records of type fd, fpregs, kregs, noteid, ns, proc, regs,
segment, and status are all formatted as a decimal number n
followed by n bytes of data. This data is the contents of
the file of the same name found in /proc.
The format of the mem and text sections is not as simple.
These sections contain one or more page descriptions. Each
describes a one kilobyte page of data. If the section is
not a multiple of a kilobyte in size, the last page will be
shorter. Each description begins with a one-byte flag. If
the flag is r, then it is followed by a page of binary data.
If the flag is z, then the data is understood to be zeros,
and is omitted. If the flag is m or t, then it is followed
by two decimal strings p and o, indicating that this page is
the same as the page at offset o of the memory or text seg-
ment for process p. This data must have been previously
described in the snapshot, and the offset must be a multiple
of a kilobyte.
It is not guaranteed that any of the sections described
above be in a process snapshot, although the snapshot
quickly becomes useless when too much is missing.
Memory and text images may be incomplete. The memory or
text file for a given process may be split across multiple
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SNAP(6) SNAP(6)
disjoint sections in the snapshot.
SEE ALSO
proc(3), snap(4).
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