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 segment, set the owner, group, and permis
sions 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 following are private */
unsigned short shm_npages; /* size of segment (pages) */
unsigned long *shm_pages;
struct shm_desc *attaches; /* descriptors for attaches */
};
The 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 mem
ory segment.
IPC_SET is used to apply the changes the user has made
to the uid, gid, or mode members of the
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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.
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EFAULT The argument cmd has value IPC_SET or IPC_STAT
but the address pointed to by buf isn't acces
sible.
EINVAL is returned if shmid is not a valid identi
fier, or cmd is not a valid command.
EIDRM is returned if shmid points to a removed iden
tifier.
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
shmget(2), shmop(2)
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