int
shmctl(
int shmid
, int cmd
, struct shmid_ds *buf
)
)
system call performs control operations on the shared memory segment
specified by
shmid
.
Each shared memory segment has a
shmid_ds
structure associated with it which contains the following members:
struct ipc_perm shm_perm; /* operation permissions */
size_t shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* pid of last shm op */
pid_t shm_cpid; /* pid of creator */
shmatt_t 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() */
The
ipc_perm
structure used inside the
shmid_ds
structure is defined in
<sys/ipc.h
>
and contains the following members:
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; /* permission (lower 9 bits) */
The operation to be performed by
shmctl()
is specified in
cmd
and is one of:
IPC_STAT
buf
.
IPC_SET
shmid
.
The values are taken from the corresponding fields in the structure
pointed to by
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
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 segment can do this.
SHM_LOCK
shmid
in memory.
This operation can only be executed by the super-user.
SHM_UNLOCK
shmid
.
This operation can only be executed by the super-user.
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.
)
will fail if:
EACCES
]
IPC_STAT
and the caller has no read permission for this shared memory segment.
EFAULT
]
buf
specifies an invalid address.
EINVAL
]
shmid
is not a valid shared memory segment identifier.
cmd is not a valid command.
ENOMEM
]
cmd
is equal to
SHM_LOCK
and there is not enough physical memory.
EPERM
]
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 was made to increase the value of
shm_qbytes
through
IPC_SET
but the caller is not the super-user.
The
cmd
is equal to
SHM_LOCK
or
SHM_UNLOCK
and the caller is not the super-user.
AT&T
System V UNIX
.