NAME
shmctl —
shared memory control
operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/shm.h>
int
shmctl(
int
shmid,
int cmd,
struct shmid_ds *buf);
DESCRIPTION
The
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
- Gather information about the shared memory segment and
place it in the structure pointed to by buf.
-
-
IPC_SET
- Set the value of the shm_perm.uid,
shm_perm.gid and shm_perm.mode
fields in the structure associated with 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
- Remove the shared memory segment specified by
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
- Lock the shared memory segment specified by
shmid in memory. This operation can only be executed
by the super-user.
-
-
SHM_UNLOCK
- Unlock the shared memory segment specified by
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.
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
shmctl() will fail if:
-
-
- [
EACCES
]
- The command is
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
]
- The 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.
SEE ALSO
ipcrm(1),
ipcs(1),
shmat(2),
shmget(2)
STANDARDS
The
shmctl system call conforms to
X/Open
System Interfaces and Headers Issue 5 (“XSH5”).
HISTORY
Shared memory segments appeared in the first release of
AT&T System V UNIX.