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.\"     @(#)core.5	8.3 (Berkeley) 12/11/93
.\"
.Dd $Mdocdate: May 1 2021 $
.Dt CORE 5
.Os
.Sh NAME
.Nm core
.Nd memory image file format
.Sh SYNOPSIS
.In sys/types.h
.In sys/exec_elf.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 ) .
.Pp
By default, this memory image is written to a file named
.Pa programname.core
in the working directory, provided the terminated process had write
permission in the directory,
and the terminated process either had an effective user ID of zero or
had never changed user or group ID.
See the description of
.Dv KERN_NOSUIDCOREDUMP
in
.Xr sysctl 2
for other options.
.Pp
The maximum size of a
.Pa programname.core
file is limited by
.Xr setrlimit 2 .
Files which would be larger than the limit are not created.
.Pp
The
.Pa programname.core
file is in the ELF binary file format with an
.Va e_type
of
.Dv ET_CORE .
It has no section header table and a segment header that contains
.Dv PT_LOAD
and
.Dv PT_NOTE
segments that describe the state of the process when terminated.
.Pp
The
.Dv PT_LOAD
segments hold images of the process memory when it was terminated.
Device mappings are excluded,
as are read-only mappings that do not have private mapping changes.
Each segment's
.Va p_vaddr ,
.Va p_memsz ,
and
.Va p_flags
specify the address, size, and access protection of the memory
segment it describes.
.Pp
The
.Dv PT_NOTE
segment contains notes describing the kernel state and CPU registers
of each thread in the process.
Notes describing the process as a whole have the name
.Dq OpenBSD .
An
.Dv NT_OPENBSD_PROCINFO
note is included containing an
.Vt elfcore_procinfo
structure with information about the kernel state of the process.
An
.Dv NT_OPENBSD_AUXV
note is included containing a copy of the original process environment strings.
On sparc64, an
.Dv NT_OPENBSD_WCOOKIE
note is included containing the process's
.Sq window cookie .
.Pp
Notes describing the individual threads in the process have names
in the format
.Dq OpenBSD@%d
where the suffix is the thread ID.
For each thread, an
.Dv NT_OPENBSD_REGS
note is included containing the state of the regular registers of
the thread and, on architectures other than luna88k, an
.Dv NT_OPENBSD_FPREGS
note is also included containing the state of the floating-point
registers of the thread.
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr setrlimit 2 ,
.Xr sigaction 2 ,
.Xr sysctl 2
.Sh HISTORY
A
.Nm
file format appeared in
.At v1 .
.Ox 2.0
started using the ELF format for some architectures.
All architectures used ELF starting in
.Ox 5.4 .
.Sh CAVEATS
Programs which are started with either the set-user-ID or
set-group-ID bits set,
or which change their UID or GID after starting, will normally not
dump core.
This is to prevent sensitive information from inadvertently ending
up on disk.
This behaviour can be changed (for debugging purposes) by changing the
.Li kern.nosuidcoredump
.Xr sysctl 2
variable to the right settings.