shmctl - shared memory control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);
The shmctl() system call performs some control operations on
the shared
memory area specified by shmid.
Each shared memory segment has a data structure associated
with it, parts
of which may be altered by shmctl() and parts of which determine the actions
of shmctl().
This structure is defined as follows in <sys/shm.h>:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permissions */
int shm_segsz; /* size of segment in
bytes */
pid_t shm_lpid; /* pid of last shm op
*/
pid_t shm_cpid; /* pid of creator */
short 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() */
void *shm_internal; /* sysv stupidity */
};
The ipc_perm structure used inside the shmid_ds structure
is defined in
<sys/ipc.h> and looks like this:
struct ipc_perm {
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; /* r/w permission (see
chmod(2)) */
u_short seq; /* sequence # (to generate
unique msg/sem/shm id) */
key_t key; /* user specified
msg/sem/shm key */
};
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 superuser, 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 superuser 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
queue can do this.
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.
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate
the error.
shmctl() will fail if:
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the
caller is not
the superuser, 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 is made to increase the value of
shm_qbytes
through IPC_SET but the caller is not the superuser.
[EACCES] The command is IPC_STAT and the caller has no
read permission
for this shared memory segment.
[EINVAL] shmid is not a valid shared memory segment
identifier.
cmd is not a valid command.
[EFAULT] buf specifies an invalid address.
shmat(2), shmget(2)
OpenBSD 3.6 August 17, 1995
[ Back ] | 