SHMCTL(2) | System Calls Manual | SHMCTL(2) |
shmctl
—
#include <sys/shm.h>
int
shmctl
(int
shmid, int cmd,
struct shmid_ds
*buf);
shmctl
() 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
IPC_SET
IPC_RMID
SHM_LOCK
SHM_UNLOCK
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.
shmctl
() will fail if:
EACCES
]IPC_STAT
and the caller has no read
permission for this shared memory segment.EFAULT
]EINVAL
]cmd is not a valid command.
ENOMEM
]SHM_LOCK
and there is not enough physical memory.EPERM
]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.
shmctl
system call conforms to
X/Open System Interfaces and Headers Issue 5
(“XSH5”).
November 25, 2006 | NetBSD 9.0 |