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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.
p 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.
p The number of hard links as displayed by the c 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.
p The following subcommands are provided: l -tag -width truncate t Ic create Create segments with the specified paths, if they do not already exist.
p 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.
p The optional numerical .Ar mode argument specifies the initial access mode. t Ic rm Unlink the paths specified. t 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. t Ic dump Output raw bytes values from the segment to standard output. t Ic stat Print metadata for the specified path, in the format similar to the .Xr stat 1 utility. t 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
p For some commands, the following options may be provided: l -tag -width XXX t Fl h If specified, requests human-readable display of size, see .Xr humanize_number 3 . t 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 l -bullet t To show content of the shared memory segment with the path
a /1 , use the command .Dl "posixshmcontrol dump /1 | hexdump -C" t To create a segment with the path
a /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.