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     shmctl — shared memory control


     Standard C Library (libc, -lc)


     #include <machine/param.h>
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     shmctl(int shmid, int cmd, struct shmid_ds *buf);


     Performs the action specified by cmd on the shared memory segment identified by shmid:

     IPC_STAT     Fetch the segment's struct shmid_ds, storing it in the memory pointed to by

     IPC_SET      Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the
                  segment's struct shmid_ds to match those of the struct pointed to by buf.  The
                  calling process's effective uid must match either shm_perm.uid or
                  shm_perm.cuid, or it must have superuser privileges.

     IPC_RMID     Removes the segment from the system.  The removal will not take effect until
                  all processes having attached the segment have exited; however, once the
                  IPC_RMID operation has taken place, no further processes will be allowed to
                  attach the segment.  For the operation to succeed, the calling process's
                  effective uid must match shm_perm.uid or shm_perm.cuid, or the process must
                  have superuser privileges.

     The shmid_ds structure is defined as follows:

     struct shmid_ds {
         struct ipc_perm shm_perm;   /* operation permission structure */
         size_t          shm_segsz;  /* size of segment in bytes */
         pid_t           shm_lpid;   /* process ID of last shared memory op */
         pid_t           shm_cpid;   /* process ID of creator */
         int             shm_nattch; /* number of current attaches */
         time_t          shm_atime;  /* time of last shmat() */
         time_t          shm_dtime;  /* time of last shmdt() */
         time_t          shm_ctime;  /* time of last change by shmctl() */


     The shmctl() function returns the value 0 if successful; otherwise the value -1 is returned
     and the global variable errno is set to indicate the error.


     The shmctl() system call will fail if:

     [EINVAL]           Invalid operation, or no shared memory segment was found corresponding to

     [EPERM]            The calling process's effective uid does not match the uid of the shared
                        memory segment's owner or creator.

     [EACCES]           Permission denied due to mismatch between operation and mode of shared
                        memory segment.


     shmat(2), shmdt(2), shmget(2), ftok(3)