| SHMCTL(2) | System Calls Manual | SHMCTL(2) | 
shmctl —
#include <sys/shm.h>
int
  
  shmctl(int
    shmid, int cmd,
    struct shmid_ds
  *buf);
shmctl() system call performs control operations on
  the shared memory segment specified by shmid.
Each shared memory segment has a shmid_ds structure associated with it which contains the following members:
    struct ipc_perm shm_perm;   /* operation permissions */
    size_t          shm_segsz;  /* size of segment in bytes */
    pid_t           shm_lpid;   /* pid of last shm op */
    pid_t           shm_cpid;   /* pid of creator */
    shmatt_t        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() */
The ipc_perm structure used inside the
    shmid_ds structure is defined in
    <sys/ipc.h> and contains the
    following members:
    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; /* permission (lower 9 bits) */
The operation to be performed by shmctl()
    is specified in cmd and is one of:
IPC_STATIPC_SETIPC_RMIDSHM_LOCKSHM_UNLOCKThe 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.
shmctl() will fail if:
EACCES]IPC_STAT and the caller has no read
      permission for this shared memory segment.EFAULT]EINVAL]cmd is not a valid command.
ENOMEM]SHM_LOCK
      and there is not enough physical memory.EPERM]IPC_SET or
      IPC_RMID and the caller is not the super-user, 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 was made to increase the value of
        shm_qbytes through IPC_SET
        but the caller is not the super-user.
The cmd is equal to
        SHM_LOCK or SHM_UNLOCK
        and the caller is not the super-user.
shmctl system call conforms to
  X/Open System Interfaces and Headers Issue 5
  (“XSH5”).
| November 25, 2006 | NetBSD 9.4 |