SHMCTL(2) BSD Programmer's Manual SHMCTL(2)
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 ac-
tions 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 exe-
cuted 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 des-
troy the data associated with it. Only the superuser or a pro-
cess 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 deter-
mined 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 permis-
sion 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)
MirBSD #10-current August 17, 1995 1