SHMCTL (2)
shared memory control
SYNOPSIS
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#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid , int cmd , struct shmid_ds * buf );
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DESCRIPTION
shmctl()
allows the user to receive information on a shared memory segment,
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:
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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 following are private */
unsigned short shm_npages; /* size of segment (pages) */
unsigned long *shm_pages;
struct shm_desc *attaches; /* descriptors for attaches */
};
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The fields in the member shm_perm can be set:
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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 lowest 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 man be modified as needed or moved to a proc file system interface.
SYSTEM CALLS
fork()
After a
fork()
the child inherits the attached shared memory segments.
exec()
After an
exec()
all attached shared memory segments are detached (not destroyed).
exit()
Upon
exit()
all attached shared memory segments are detached (not destroyed).
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
msqid .
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
user is not the creator, the owner, or the super-user, and the user
does not have permission granted to their group or to the world.
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
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