SHMCTL
Section: System Calls (2)
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BSD mandoc
NetBSD
NAME
shmctl
- shared memory control operations
SYNOPSIS
Fd #include <sys/types.h>
Fd #include <sys/ipc.h>
Fd #include <sys/msg.h>
Ft int
Fn shmctl int shmid int cmd struct shmid_ds *buf
DESCRIPTION
The
Fn shmctl
system call performs some control operations on the shared memory area
specified by
Fa shmid .
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 .
This structure is defined as follows in
Aq Pa sys/shm.h :
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 */
};
The
Bf -literal
ipc_perm
Ef structure used inside the
Bf -literal
shmid_ds
Ef structure is defined in
Aq Pa sys/ipc.h
and looks like this:
struct ipc_perm {
ushort cuid; /* creator user id */
ushort cgid; /* creator group id */
ushort uid; /* user id */
ushort gid; /* group id */
ushort mode; /* r/w permission (see chmod(2)) */
ushort seq; /* sequence # (to generate unique msg/sem/shm id) */
key_t key; /* user specified msg/sem/shm key */
};
The operation to be performed by
Fn shmctl
is specified in
Fa cmd
and is one of:
- IPC_STAT
-
Gather information about the shared memory segment and place it in the
structure pointed to by
Fa buf .
- IPC_SET
-
Set the value of the
shm_perm.uid
shm_perm.gid
and
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 super-user, or a process that
has an effective user id equal to either
shm_perm.cuid
or
shm_perm.uid
in the data structure associated with the shared memory segment.
- IPC_RMID
-
Remove the shared memory segment specified by
Fa shmid
and destroy the data associated with it. Only the super-user or a process
with an effective uid equal to the
shm_perm.cuid
or
shm_perm.uid
values in the data structure associated with the queue can do this.
The read and write permissions on a shared memory identifier
are determined by the
shm_perm.mode
field in the same way as is
done with files (see
chmod(2)),
but the effective uid can match either the
shm_perm.cuid
field or the
shm_perm.uid
field, and the
effective gid can match either
shm_perm.cgid
or
shm_perm.gid
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable
errno
is set to indicate the error.
ERRORS
Fn shmctl
will fail if:
- Bq Er EPERM
-
Fa cmd
is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does
the effective uid match either the
shm_perm.uid
or
shm_perm.cuid
fields of the data structure associated with the shared memory segment.
An attempt is made to increase the value of
shm_qbytes
through IPC_SET
but the caller is not the super-user.
- Bq Er EACCESS
-
The command is IPC_STAT
and the caller has no read permission for this shared memory segment.
- Bq Er EINVAL
-
Fa shmid
is not a valid shared memory segment identifier.
cmd
is not a valid command.
- Bq Er EFAULT
-
Fa buf
specifies an invalid address.
SEE ALSO
shmat(2),
shmdt(2),
shmget(2)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUES
-
- ERRORS
-
- SEE ALSO
-
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Time: 16:28:59 GMT, April 18, 2022