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.\"     @(#)core.5	8.3 (Berkeley) 12/11/93
.\" $FreeBSD$
.\"
.Dd September 2, 2015
.Dt CORE 5
.Os
.Sh NAME
.Nm core
.Nd memory image file format
.Sh SYNOPSIS
.In sys/param.h
.Sh DESCRIPTION
A small number of signals which cause abnormal termination of a process
also cause a record of the process's in-core state to be written
to disk for later examination by one of the available debuggers.
(See
.Xr sigaction 2 . )
This memory image is written to a file named by default
.Nm programname.core
in the working directory;
provided the terminated process had write permission in the directory,
and provided the abnormality did not cause
a system crash.
(In this event, the decision to save the core file is arbitrary, see
.Xr savecore 8 . )
.Pp
The maximum size of a core file is limited by
.Xr setrlimit 2 .
Files which would be larger than the limit are not created.
.Pp
The name of the file is controlled via the
.Xr sysctl 8
variable
.Va kern.corefile .
The contents of this variable describes a filename to store
the core image to.
This filename can be absolute, or relative (which
will resolve to the current working directory of the program
generating it).
.Pp
The following format specifiers may be used in the
.Va kern.corefile
sysctl to insert additional information into the resulting core file
name:
.Bl -tag -width "1234567890" -compact -offset "12345"
.It Em \&%H
Machine hostname.
.It Em \&%I
An index starting at zero until the sysctl
.Em debug.ncores
is reached.
This can be useful for limiting the number of corefiles
generated by a particular process.
.It Em \&%N
process name.
.It Em \&%P
processes PID.
.It Em \&%U
process UID.
.El
.Pp
The name defaults to
.Em \&%N.core ,
yielding the traditional
.Fx
behaviour.
.Pp
By default, a process that changes user or group credentials whether
real or effective will not create a corefile.
This behaviour can be
changed to generate a core dump by setting the
.Xr sysctl 8
variable
.Va kern.sugid_coredump
to 1.
.Pp
Corefiles can be compressed by the kernel if the following item
is included in the kernel configuration file:
.Bl -tag -width "1234567890" -compact -offset "12345"
.It options
GZIO
.El
.Pp
When the GZIO option is included, the following sysctls control whether core
files will be compressed:
.Bl -tag -width "kern.compress_user_cores_gzlevel" -compact -offset "12345"
.It Em kern.compress_user_cores_gzlevel
Gzip compression level.
Defaults to 6.
.It Em kern.compress_user_cores
Actually compress user cores.
Compressed core files will have a suffix of
.Ql .gz
appended to them.
.El
.Sh NOTES
Corefiles are written with open file descriptor information as an ELF note.
By default, file paths are packed to only use as much space as needed.
However, file paths can change at any time, including during core dump,
and this can result in truncated file descriptor data.
.Pp
All file descriptor information can be preserved by disabling packing.
This potentially wastes up to PATH_MAX bytes per open fd.
Packing is disabled with
.Dl sysctl kern.coredump_pack_fileinfo=0 .
.Sh EXAMPLES
In order to store all core images in per-user private areas under
.Pa /var/coredumps ,
the following
.Xr sysctl 8
command can be used:
.Pp
.Dl sysctl kern.corefile=/var/coredumps/\&%U/\&%N.core
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr kgdb 1 ,
.Xr setrlimit 2 ,
.Xr sigaction 2 ,
.Xr sysctl 8
.Sh HISTORY
A
.Nm
file format appeared in
.At v6 .