shmctl
SHMCTL(2) Linux Programmer's Manual SHMCTL(2)
NAME
shmctl - shared memory control
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
shmctl() allows the user to receive information on a shared memory seg-
ment, set the owner, group, and permissions of a shared memory segment,
or destroy a segment. The information about the segment identified by
shmid is returned in a shmid_ds structure:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
...
};
The highlighted fields in the member shm_perm can be set:
struct ipc_perm {
key_t key;
ushort uid; /* owner euid and egid */
ushort gid;
ushort cuid; /* creator euid and egid */
ushort cgid;
ushort mode; /* lower 9 bits of access modes */
ushort seq; /* sequence number */
};
The following cmds are available:
IPC_STAT is used to copy the information about the shared memory
segment into the buffer buf. The user must have read access
to the shared memory segment.
IPC_SET is used to apply the changes the user has made to the uid,
gid, or mode members of the shm_perms field. Only the low-
est 9 bits of mode are used. The shm_ctime member is also
updated. The user must be the owner, creator, or the
super-user.
IPC_RMID is used to mark the segment as destroyed. It will actually
be destroyed after the last detach. (I.e., when the
shm_nattch member of the associated structure shmid_ds is
zero.) The user must be the owner, creator, or the super-
user.
The user must ensure that a segment is eventually destroyed; otherwise
its pages that were faulted in will remain in memory or swap.
In addition, the super-user can prevent or allow swapping of a shared
memory segment with the following cmds: (Linux only)
SHM_LOCK prevents swapping of a shared memory segment. The user must
fault in any pages that are required to be present after
locking is enabled.
SHM_UNLOCK allows the shared memory segment to be swapped out.
The IPC_INFO, SHM_STAT and SHM_INFO control calls are used by the
ipcs(8) program to provide information on allocated resources. In the
future, these may be modified as needed or moved to a proc file system
interface.
RETURN VALUE
0 is returned on success, -1 on error.
ERRORS
On error, errno will be set to one of the following:
EACCES is returned if IPC_STAT is requested and shm_perm.modes
does not allow read access for shmid.
EFAULT The argument cmd has value IPC_SET or IPC_STAT but the
address pointed to by buf isn't accessible.
EINVAL is returned if shmid is not a valid identifier, or cmd is
not a valid command.
EIDRM is returned if shmid points to a removed identifier.
EPERM is returned if IPC_SET or IPC_RMID is attempted, and the
effective user ID of the calling process is not the creator
(as found in shm_perm.cuid), the owner (as found in
shm_perm.uid), or the super-user.
EOVERFLOW is returned if IPC_STAT is attempted, and the gid or uid
value is too large to be stored in the structure pointed to
by buf.
NOTE
Various fields in a struct shmid_ds were shorts under Linux 2.2 and
have become longs under Linux 2.4. To take advantage of this, a recom-
pilation under glibc-2.1.91 or later should suffice. (The kernel dis-
tinguishes old and new calls by a IPC_64 flag in cmd.)
CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EINVAL, ENOENT,
ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents an EIDRM error
condition.
SEE ALSO
shmget(2), shmop(2)
Linux 2.4.1 2002-01-05 SHMCTL(2)
