path: root/man2/shmctl.2

'\" t
.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
.\" and Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified 1993-07-28, Rik Faith <faith@cs.unc.edu>
.\" Modified 1993-11-28, Giorgio Ciucci <giorgio@crcc.it>
.\" Modified 1997-01-31, Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-02-18, Andries Brouwer <aeb@cwi.nl>
.\" Modified 2002-01-05, 2004-05-27, 2004-06-17,
.\"    Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified 2004-10-11, aeb
.\" Modified, Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Language and formatting clean-ups
.\"	Updated shmid_ds structure definitions
.\"	Added information on SHM_DEST and SHM_LOCKED flags
.\"	Noted that CAP_IPC_LOCK is not required for SHM_UNLOCK
.\"		since Linux 2.6.9
.\" Modified, 2004-11-25, mtk, notes on 2.6.9 RLIMIT_MEMLOCK changes
.\" 2005-04-25, mtk -- noted aberrant Linux behavior w.r.t. new
.\"	attaches to a segment that has already been marked for deletion.
.\" 2005-08-02, mtk: Added IPC_INFO, SHM_INFO, SHM_STAT descriptions.
.\" 2018-03-20, dbueso: Added SHM_STAT_ANY description.
.\"
.TH shmctl 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
shmctl \- System V shared memory control
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/shm.h>
.PP
.BI "int shmctl(int " shmid ", int " cmd ", struct shmid_ds *" buf );
.fi
.SH DESCRIPTION
.BR shmctl ()
performs the control operation specified by
.I cmd
on the System\ V shared memory segment whose identifier is given in
.IR shmid .
.PP
The
.I buf
argument is a pointer to a \fIshmid_ds\fP structure,
defined in \fI<sys/shm.h>\fP as follows:
.PP
.in +4n
.EX
struct shmid_ds {
    struct ipc_perm shm_perm;    /* Ownership and permissions */
    size_t          shm_segsz;   /* Size of segment (bytes) */
    time_t          shm_atime;   /* Last attach time */
    time_t          shm_dtime;   /* Last detach time */
    time_t          shm_ctime;   /* Creation time/time of last
                                    modification via shmctl() */
    pid_t           shm_cpid;    /* PID of creator */
    pid_t           shm_lpid;    /* PID of last shmat(2)/shmdt(2) */
    shmatt_t        shm_nattch;  /* No. of current attaches */
    ...
};
.EE
.in
.PP
The fields of the
.I shmid_ds
structure are as follows:
.TP 12
.I shm_perm
This is an
.I ipc_perm
structure (see below) that specifies the access permissions
on the shared memory segment.
.TP
.I shm_segsz
Size in bytes of the shared memory segment.
.TP
.I shm_atime
Time of the last
.BR shmat (2)
system call that attached this segment.
.TP
.I shm_dtime
Time of the last
.BR shmdt (2)
system call that detached tgis segment.
.TP
.I shm_ctime
Time of creation of segment or time of the last
.BR shmctl ()
.B IPC_SET
operation.
.TP
.I shm_cpid
ID of the process that created the shared memory segment.
.TP
.I shm_lpid
ID of the last process that executed a
.BR shmat (2)
or
.BR shmdt (2)
system call on this segment.
.TP
.I shm_nattch
Number of processes that have this segment attached.
.PP
The
.I ipc_perm
structure is defined as follows
(the highlighted fields are settable using
.BR IPC_SET ):
.PP
.in +4n
.EX
struct ipc_perm {
    key_t          __key;    /* Key supplied to shmget(2) */
    uid_t          \fBuid\fP;      /* Effective UID of owner */
    gid_t          \fBgid\fP;      /* Effective GID of owner */
    uid_t          cuid;     /* Effective UID of creator */
    gid_t          cgid;     /* Effective GID of creator */
    unsigned short \fBmode\fP;     /* \fBPermissions\fP + SHM_DEST and
                                SHM_LOCKED flags */
    unsigned short __seq;    /* Sequence number */
};
.EE
.in
.PP
The least significant 9 bits of the
.I mode
field of the
.I ipc_perm
structure define the access permissions for the shared memory segment.
The permission bits are as follows:
.TS
l l.
0400	Read by user
0200	Write by user
0040	Read by group
0020	Write by group
0004	Read by others
0002	Write by others
.TE
.PP
Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
(It is not necessary to have execute permission on a segment
in order to perform a
.BR shmat (2)
call with the
.B SHM_EXEC
flag.)
.PP
Valid values for
.I cmd
are:
.TP
.B IPC_STAT
Copy information from the kernel data structure associated with
.I shmid
into the
.I shmid_ds
structure pointed to by \fIbuf\fP.
The caller must have read permission on the
shared memory segment.
.TP
.B IPC_SET
Write the values of some members of the
.I shmid_ds
structure pointed to by
.I buf
to the kernel data structure associated with this shared memory segment,
updating also its
.I shm_ctime
member.
.IP
The following fields are updated:
\fIshm_perm.uid\fP, \fIshm_perm.gid\fP,
and (the least significant 9 bits of) \fIshm_perm.mode\fP.
.IP
The effective UID of the calling process must match the owner
.RI ( shm_perm.uid )
or creator
.RI ( shm_perm.cuid )
of the shared memory segment, or the caller must be privileged.
.TP
.B IPC_RMID
Mark the segment to be destroyed.
The segment will actually be destroyed
only after the last process detaches it (i.e., when the
.I shm_nattch
member of the associated structure
.I shmid_ds
is zero).
The caller must be the owner or creator of the segment, or be privileged.
The
.I buf
argument is ignored.
.IP
If a segment has been marked for destruction, then the (nonstandard)
.B SHM_DEST
flag of the
.I shm_perm.mode
field in the associated data structure retrieved by
.B IPC_STAT
will be set.
.IP
The caller \fImust\fP ensure that a segment is eventually destroyed;
otherwise its pages that were faulted in will remain in memory or swap.
.IP
See also the description of
.I  /proc/sys/kernel/shm_rmid_forced
in
.BR proc (5).
.TP
.BR IPC_INFO " (Linux-specific)"
Return information about system-wide shared memory limits and
parameters in the structure pointed to by
.IR buf .
This structure is of type
.I shminfo
(thus, a cast is required),
defined in
.I <sys/shm.h>
if the
.B _GNU_SOURCE
feature test macro is defined:
.IP
.in +4n
.EX
struct shminfo {
    unsigned long shmmax; /* Maximum segment size */
    unsigned long shmmin; /* Minimum segment size;
                             always 1 */
    unsigned long shmmni; /* Maximum number of segments */
    unsigned long shmseg; /* Maximum number of segments
                             that a process can attach;
                             unused within kernel */
    unsigned long shmall; /* Maximum number of pages of
                             shared memory, system\-wide */
};
.EE
.in
.IP
The
.IR shmmni ,
.IR shmmax ,
and
.I shmall
settings can be changed via
.I /proc
files of the same name; see
.BR proc (5)
for details.
.TP
.BR SHM_INFO " (Linux-specific)"
Return a
.I shm_info
structure whose fields contain information
about system resources consumed by shared memory.
This structure is defined in
.I <sys/shm.h>
if the
.B _GNU_SOURCE
feature test macro is defined:
.IP
.in +4n
.EX
struct shm_info {
    int           used_ids; /* # of currently existing
                               segments */
    unsigned long shm_tot;  /* Total number of shared
                               memory pages */
    unsigned long shm_rss;  /* # of resident shared
                               memory pages */
    unsigned long shm_swp;  /* # of swapped shared
                               memory pages */
    unsigned long swap_attempts;
                            /* Unused since Linux 2.4 */
    unsigned long swap_successes;
                            /* Unused since Linux 2.4 */
};
.EE
.in
.TP
.BR SHM_STAT " (Linux-specific)"
Return a
.I shmid_ds
structure as for
.BR IPC_STAT .
However, the
.I shmid
argument is not a segment identifier, but instead an index into
the kernel's internal array that maintains information about
all shared memory segments on the system.
.TP
.BR SHM_STAT_ANY " (Linux-specific, since Linux 4.17)"
Return a
.I shmid_ds
structure as for
.BR SHM_STAT .
However,
.I shm_perm.mode
is not checked for read access for
.IR shmid ,
meaning that any user can employ this operation (just as any user may read
.I /proc/sysvipc/shm
to obtain the same information).
.PP
The caller can prevent or allow swapping of a shared
memory segment with the following \fIcmd\fP values:
.TP
.BR SHM_LOCK " (Linux-specific)"
Prevent swapping of the shared memory segment.
The caller must fault in
any pages that are required to be present after locking is enabled.
If a segment has been locked, then the (nonstandard)
.B SHM_LOCKED
flag of the
.I shm_perm.mode
field in the associated data structure retrieved by
.B IPC_STAT
will be set.
.TP
.BR SHM_UNLOCK " (Linux-specific)"
Unlock the segment, allowing it to be swapped out.
.PP
Before Linux 2.6.10, only a privileged process
could employ
.B SHM_LOCK
and
.BR SHM_UNLOCK .
Since Linux 2.6.10, an unprivileged process can employ these operations
if its effective UID matches the owner or creator UID of the segment, and
(for
.BR SHM_LOCK )
the amount of memory to be locked falls within the
.B RLIMIT_MEMLOCK
resource limit (see
.BR setrlimit (2)).
.\" There was some weirdness in Linux 2.6.9: SHM_LOCK and SHM_UNLOCK could
.\" be applied to a segment, regardless of ownership of the segment.
.\" This was a botch-up in the move to RLIMIT_MEMLOCK, and was fixed
.\" in Linux 2.6.10.  MTK, May 2005
.SH RETURN VALUE
A successful
.B IPC_INFO
or
.B SHM_INFO
operation returns the index of the highest used entry in the
kernel's internal array recording information about all
shared memory segments.
(This information can be used with repeated
.B SHM_STAT
or
.B SHM_STAT_ANY
operations to obtain information about all shared memory segments
on the system.)
A successful
.B SHM_STAT
operation returns the identifier of the shared memory segment
whose index was given in
.IR shmid .
Other operations return 0 on success.
.PP
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
\fBIPC_STAT\fP or \fBSHM_STAT\fP is requested and
\fIshm_perm.mode\fP does not allow read access for
.IR shmid ,
and the calling process does not have the
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EFAULT
The argument
.I cmd
has value
.B IPC_SET
or
.B IPC_STAT
but the address pointed to by
.I buf
isn't accessible.
.TP
.B EIDRM
\fIshmid\fP points to a removed identifier.
.TP
.B EINVAL
\fIshmid\fP is not a valid identifier, or \fIcmd\fP
is not a valid command.
Or: for a
.B SHM_STAT
or
.B SHM_STAT_ANY
operation, the index value specified in
.I shmid
referred to an array slot that is currently unused.
.TP
.B ENOMEM
(Since Linux 2.6.9),
.B SHM_LOCK
was specified and the size of the to-be-locked segment would mean
that the total bytes in locked shared memory segments would exceed
the limit for the real user ID of the calling process.
This limit is defined by the
.B RLIMIT_MEMLOCK
soft resource limit (see
.BR setrlimit (2)).
.TP
.B EOVERFLOW
\fBIPC_STAT\fP is attempted, and the GID or UID value
is too large to be stored in the structure pointed to by
.IR buf .
.TP
.B EPERM
\fBIPC_SET\fP or \fBIPC_RMID\fP is attempted, and the
effective user ID of the calling process is not that of the creator
(found in
.IR shm_perm.cuid ),
or the owner
(found in
.IR shm_perm.uid ),
and the process was not privileged (Linux: did not have the
.B CAP_SYS_ADMIN
capability).
.IP
Or (before Linux 2.6.9),
.B SHM_LOCK
or
.B SHM_UNLOCK
was specified, but the process was not privileged
(Linux: did not have the
.B CAP_IPC_LOCK
capability).
(Since Linux 2.6.9, this error can also occur if the
.B RLIMIT_MEMLOCK
is 0 and the caller is not privileged.)
.SH VERSIONS
Linux permits a process to attach
.RB ( shmat (2))
a shared memory segment that has already been marked for deletion
using
.IR shmctl(IPC_RMID) .
This feature is not available on other UNIX implementations;
portable applications should avoid relying on it.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.\" SVr4 documents additional error conditions EINVAL,
.\" ENOENT, ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor SVID documents
.\" an EIDRM error condition.
.PP
Various fields in a \fIstruct shmid_ds\fP were typed as
.I short
under Linux 2.2
and have become
.I long
under Linux 2.4.
To take advantage of this,
a recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by an
.B IPC_64
flag in
.IR cmd .)
.SH NOTES
The
.BR IPC_INFO ,
.BR SHM_STAT ,
and
.B SHM_INFO
operations are used by the
.BR ipcs (1)
program to provide information on allocated resources.
In the future, these may modified or moved to a
.I /proc
filesystem interface.
.SH SEE ALSO
.BR mlock (2),
.BR setrlimit (2),
.BR shmget (2),
.BR shmop (2),
.BR capabilities (7),
.BR sysvipc (7)