libc/sys/shmctl.2

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.\" Copyright (c) 1995 Frank van der Linden
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.Dd $Mdocdate: December 10 2014 $
.Dt SHMCTL 2
.Os
.Sh NAME
.Nm shmctl
.Nd shared memory control operations
.Sh SYNOPSIS
.In sys/shm.h
.Ft int
.Fn shmctl "int shmid" "int cmd" "struct shmid_ds *buf"
.Sh DESCRIPTION
The
.Fn shmctl
system call performs some control operations on the shared memory area
specified by
.Fa shmid .
.Pp
Each shared memory segment has a data structure associated with it,
parts of which may be altered by
.Fn shmctl
and parts of which determine the actions of
.Fn shmctl .
.Pp
This structure is defined as follows in
.In sys/shm.h :
.Bd -literal
struct shmid_ds {
	struct ipc_perm shm_perm;     /* operation permissions */
	int             shm_segsz;    /* size of segment in bytes */
	pid_t           shm_lpid;     /* pid of last shm op */
	pid_t           shm_cpid;     /* pid of creator */
	short           shm_nattch;   /* # of current attaches */
	time_t          shm_atime;    /* last shmat() time*/
	time_t          shm_dtime;    /* last shmdt() time */
	time_t          shm_ctime;    /* last change by shmctl() */
	void		*shm_internal; /* sysv stupidity */
};
.Ed
.Pp
The
.Bf -literal
ipc_perm
.Ef
structure used inside the
.Bf -literal
shmid_ds
.Ef
structure is defined in
.In sys/ipc.h
and looks like this:
.Bd -literal
struct ipc_perm {
	uid_t		cuid;	/* creator user id */
	gid_t		cgid;	/* creator group id */
	uid_t		uid;	/* user id */
	gid_t		gid;	/* group id */
	mode_t		mode;	/* r/w permission (see chmod(2)) */
	u_short		seq;	/* sequence # (to generate unique msg/sem/shm id) */
	key_t		key;	/* user specified msg/sem/shm key */
};
.Ed
.Pp
The operation to be performed by
.Fn shmctl
is specified in
.Fa cmd
and is one of:
.Bl -tag -width IPC_RMIDX
.It Dv IPC_STAT
Gather information about the shared memory segment and place it in the
structure pointed to by
.Fa buf .
.It Dv IPC_SET
Set the value of the
.Va shm_perm.uid ,
.Va shm_perm.gid
and
.Va shm_perm.mode
fields in the structure associated with
.Fa shmid .
The values are taken from the corresponding fields in the structure
pointed to by
.Fa buf .
This operation can only be executed by the superuser, or a process that
has an effective user ID equal to either
.Va shm_perm.cuid
or
.Va shm_perm.uid
in the data structure associated with the shared memory segment.
.It Dv IPC_RMID
Mark the shared memory segment specified by
.Fa shmid
for removal when it is no longer in use by any process.
When it is removed, all data associated with it will be destroyed too.
Only the superuser or a process with an effective UID equal to the
.Va shm_perm.cuid
or
.Va shm_perm.uid
values in the data structure associated with the queue can do this.
.El
.Pp
The read and write permissions on a shared memory identifier
are determined by the
.Va shm_perm.mode
field in the same way as is
done with files (see
.Xr chmod 2 ) ,
but the effective UID can match either the
.Va shm_perm.cuid
field or the
.Va shm_perm.uid
field, and the
effective GID can match either
.Va shm_perm.cgid
or
.Va shm_perm.gid .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
.Fn shmctl
will fail if:
.Bl -tag -width Er
.It Bq Er EPERM
.Fa cmd
is equal to
.Dv IPC_SET
or
.Dv IPC_RMID
and the caller is not the superuser, nor does
the effective UID match either the
.Va shm_perm.uid
or
.Va shm_perm.cuid
fields of the data structure associated with the shared memory segment.
.Pp
An attempt is made to increase the value of
.Va shm_qbytes
through
.Dv IPC_SET
but the caller is not the superuser.
.It Bq Er EACCES
The command is
.Dv IPC_STAT
and the caller has no read permission for this shared memory segment.
.It Bq Er EINVAL
.Fa shmid
is not a valid shared memory segment identifier.
.Pp
.Va cmd
is not a valid command.
.It Bq Er EFAULT
.Fa buf
specifies an invalid address.
.El
.Sh SEE ALSO
.Xr shmat 2 ,
.Xr shmget 2
.Sh STANDARDS
Segments which are marked for removal (but not yet removed
since they are still in use) can be attached to by new callers
using
.Xr shmat 2 .
This is permitted as an extension beyond the standards.