shmid_ds(4) [osf1 man page]

shmid_ds(4)						     Kernel Interfaces Manual						       shmid_ds(4)

NAME

       shmid_ds - Defines a shared memory region

SYNOPSIS

       #include <sys/shm.h>
       struct  shmid_ds{      struct ipc_perm shm_perm;      int shm_segsz;	 u_short shm_lpid;	u_short shm_cpid;      u_short shm_nattch;
	    time_t shm_atime;	   time_t shm_dtime;	  time_t shm_ctime; };

DESCRIPTION

       The shmid_ds structure defines a shared memory region associated with a shared memory region ID.  There is one shared memory region per ID.
       Collectively,  the shared memory regions are maintained in a shared memory table, with the shared memory region IDs identifying the entries
       in the table.

       The IPC permissions for the shared memory regions are implemented in a separate, but associated, ipc_perm structure.

       A shared memory region is created indirectly via the shmget() call.  If shmget() is called with a non-existent shared memory region ID, the
       kernel allocates a new shmid_ds structure, initializes it, and returns the ID that is to be associated with the region.

       The  kernel  allocates  actual  memory of shm_segsz bytes only when a process attaches a region to its address space.  Attached regions are
       maintained in a separate region table.  The entries in the shared memory table point to the associated attached regions in the  region  ta-
       ble.  The same shared memory region can be attached multiple times, by the same or different processes.	Each attachment of the region cre-
       ates a new entry in the region table.

       After a process attaches a shared memory region, the region becomes part of the process's virtual address space.  Processes  access  shared
       memory regions by using the same machine instructions used to access any virtual address.

FIELDS

       The  ipc_perm structure that defines permissions for shared memory operations.  See NOTES.  The size of the shared memory region, in bytes.
       The process ID of the process that created the shared memory region ID.	The process ID of the last process that  performed  a  shmat()	or
       shmdt()	operation  on  the  shared memory region.  The number of processes that currently have this region attached.  The time of the last
       shmat() operation.  The time of the last shmdt() operation.  The time of the last shmctl() operation that changed a member of the  shmid_ds
       structure.

NOTES

       The  shm_perm  field  identifies the associated ipc_perm structure that defines the permissions for operations on the shared memory region.
       The ipc_perm structure (from the sys/ipc.h header file) is shown here.

       struct ipc_perm {
	       ushort	 uid;	     /* owner's user id 	   */
	       ushort	 gid;	     /* owner's group id	   */
	       ushort	 cuid;	     /* creator's user id	   */
	       ushort	 cgid;	     /* creator's group id	   */
	       ushort	 mode;	     /* access modes		   */
	       ushort	 seq;	     /* slot usage sequence number */
	       key_t	 key;	     /* key			   */ };

       The mode field is a nine-bit field that contains the permissions for shared memory operations.  The first three bits identify owner permis-
       sions;  the second three bits identify group permissions; and the last three bits identify other permissions.  In each group, the first bit
       indicates read permission; the second bit indicates write permission; and the third bit is not used.

RELATED INFORMATION

       Functions: shmat(2), shmdt(2), shmctl(2), shmget(2) delim off

																       shmid_ds(4)

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SHMCTL(2)						      BSD System Calls Manual							 SHMCTL(2)

NAME

     shmctl -- shared memory control

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     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 buf.

     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 supe-
		  ruser 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() */
     };

RETURN VALUES

     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.

ERRORS

     The shmctl() system call will fail if:

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

     [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.

SEE ALSO

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

BSD
								   July 17, 1995							       BSD

Linux and UNIX Man Pages

shmid_ds(4) [osf1 man page]

Check Out this Related Man Page