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.\" This documentation was written by
.\" Konstantin Belousov <kib@FreeBSD.org> under sponsorship
.\" from the FreeBSD Foundation.
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.Dd May 25, 2023
.Dt POSIXSHMCONTROL 1
.Os
.Sh NAME
.Nm posixshmcontrol
.Nd Control POSIX shared memory segments
.Sh SYNOPSIS
.Nm
.Ar create
.Op Fl l Ar pagesize
.Op Fl m Ar mode
.Op Pa path \&...
.Nm
.Ar rm
.Op Pa path \&...
.Nm
.Ar ls
.Op Fl h
.Op Fl n
.Op Fl j Ar jail
.Nm
.Ar dump
.Op Pa path \&...
.Nm
.Ar stat
.Op Fl h
.Op Fl n
.Op Pa path \&...
.Nm
.Ar truncate
.Op Fl s Ar length
.Op Pa path \&...
.Sh DESCRIPTION
The
.Nm
command manipulates the named POSIX shared memory segments.
It allows inspecting existing segments, dumping their metadata or contents,
and unlinking them.
.Pp
Unlinking removes the name from the system and, when the last process
unmaps the segment and closes file descriptor pointing to the segment,
frees underlying memory.
.Pp
The number of hard links as displayed by the
.Ic stat
subcommand, is equal to the number of references to the underlying VM
object.
It is almost always equal to the number of mappings +1, except
for transient references.
.Pp
The following subcommands are provided:
.Bl -tag -width truncate
.It Ic create
Create segments with the specified paths, if they do not already exist.
.Pp
The optional
.Ar pagesize
argument specifies the size of the virtual pages used to map the
object with
.Xr mmap 2 .
By default, the system page size is used, but on some platforms a
larger page size can be specified.
The size of an object backed by large pages must be a multiple of the
specified page size.
The
.Va hw.pagesizes
sysctl variable lists the available page sizes.
.Pp
The optional numerical
.Ar mode
argument specifies the initial access mode.
.It Ic rm
Unlink the paths specified.
.It Ic ls
List all linked named shared memory segments visible to the caller.
For each segment, the user and group owner, size, and path are displayed.
The
.Fl j
option limits the output to segments within the specified
.Ar jail
name or id.
.It Ic dump
Output raw bytes values from the segment to standard output.
.It Ic stat
Print metadata for the specified path, in the format similar to the
.Xr stat 1
utility.
.It Ic truncate
Change the length of the segments.
Argument to the
.Fl s
option specifies new length.
The human-friendly 'k', 'm', 'g' suffixes can be used, see
.Xr expand_number 3 .
If the option is not specified, assumed length is zero.
.El
.Pp
For some commands, the following options may be provided:
.Bl -tag -width XXX
.It Fl h
If specified, requests human-readable display of size, see
.Xr humanize_number 3 .
.It Fl n
Prevent translation of owner and group into symbolic names
using name-switch services, instead the raw numeric values are printed.
.El
.Sh EXIT STATUS
.Ex -std
.Sh EXAMPLES
.Bl -bullet
.It
To show content of the shared memory segment with the path
.Pa /1 ,
use the command
.Dl "posixshmcontrol dump /1 | hexdump -C"
.It
To create a segment with the path
.Pa /2
and then enlarge it to 1M, use the sequence of commands
.Dl "posixshmcontrol create /2"
.Dl "posixshmcontrol truncate -s 1m /2"
.El
.Sh SEE ALSO
.Xr hexdump 1 ,
.Xr stat 1 ,
.Xr ftruncate 2 ,
.Xr mmap 2 ,
.Xr read 2 ,
.Xr shm_open 2 ,
.Xr shm_unlink 2 ,
.Xr stat 2 ,
.Xr expand_number 3 ,
.Xr humanize_number 3 ,
.Xr shm_create_largepage 3 ,
.Xr sysctl 3
.Sh HISTORY
The
.Nm
command appeared in
.Fx 12.1 .
.Sh AUTHORS
The
.Nm
command and this manual page were written by
.An Konstantin Belousov Aq Mt kib@freebsd.org
under sponsorship from The
.Fx
Foundation.