POSIXSHMCONTROL(1)      FreeBSD General Commands Manual     POSIXSHMCONTROL(1)

NAME
     posixshmcontrol - Control POSIX shared memory segments

SYNOPSIS
     posixshmcontrol create [-m mode] [path ...]
     posixshmcontrol rm [path ...]
     posixshmcontrol ls [-h] [-n] [-j jail]
     posixshmcontrol dump [path ...]
     posixshmcontrol stat [-h] [-n] [path ...]
     posixshmcontrol truncate [-s length] [path ...]

DESCRIPTION
     The posixshmcontrol command manipulates the named POSIX shared memory
     segments.  It allows inspecting existing segments, dumping their metadata
     or contents, and unlinking them.

     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.

     The number of hard links as displayed by the 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.

     The following subcommands are provided:

     create    Create segments with the specified paths, if not exist.  The
               mode optional numerical argument specifies initial access mode.

     rm        Unlink the paths specified.

     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 -j option limits the output to
               segments within the specified jail name or id.

     dump      Output raw bytes values from the segment to standard output.

     stat      Print metadata for the specified path, in the format similar to
               the stat(1) utility.

     truncate  Change the length of the segments.  Argument to the -s option
               specifies new length.  The human-friendly 'k', 'm', 'g'
               suffixes can be used, see expand_number(3).  If the option is
               not specified, assumed length is zero.

     For some commands, the following options may be provided:

     -h   If specified, requests human-readable display of size, see
          humanize_number(3).

     -n   Prevent translation of owner and group into symbolic names using
          name-switch services, instead the raw numeric values are printed.

EXIT STATUS
     The posixshmcontrol utility exits 0 on success, and >0 if an error
     occurs.

EXAMPLES
     •   To show content of the shared memory segment with the path /1, use
         the command
               posixshmcontrol dump /1 | hexdump -C

     •   To create a segment with the path /2 and then enlarge it to 1M, use
         the sequence of commands
               posixshmcontrol create /2
               posixshmcontrol truncate -s 1m /2

SEE ALSO
     hexdump(1), stat(1), ftruncate(2), read(2), shm_open(2), shm_unlink(2),
     stat(2), expand_number(3), humanize_number(3), sysctl(3)

HISTORY
     The posixshmcontrol command appeared in FreeBSD 12.1.

AUTHORS
     The posixshmcontrol command and this manual page were written by
     Konstantin Belousov <kib@freebsd.org> under sponsorship from The FreeBSD
     Foundation.

FreeBSD 13.1-RELEASE-p6        February 26, 2022       FreeBSD 13.1-RELEASE-p6