shmctl(2)							System Calls Manual							 shmctl(2)

NAME

       shmctl - Performs shared memory control operations

SYNOPSIS

       #include <sys/shm.h>

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

       Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/shm.h> if programs
       are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO	or  X/Open
       standards, but may be required on other vendors' systems that conform to these standards.

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       shmctl(): XSH4.2

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       Specifies  the  ID  of  the  shared memory region.  Specifies the type of command.  The possible commands are: IPC_STAT, IPC_SET, IPC_RMID,
       SHM_LOCK, and SHM_UNLOCK.  Specifies the address of a shmid_ds structure.

DESCRIPTION

       The shmctl() function provides a variety of shared memory control operations as specified by the cmd parameter.

       The cmd values and their operations are as follows: Queries the shared memory region ID by copying the contents of its associated  shmid_ds
       data  structure	into  the buf structure.  Sets the shared memory region ID by copying values found in the buf structure into corresponding
       fields in the shmid_ds structure associated with the shared memory region ID.  The fields are set as follows: The shm_perm.uid field is set
       to the owner's user ID.	The shm_perm.gid field is set to the owner's group ID.	The shm_perm.mode field is set to the access modes for the
       shared memory region.  Only the low-order nine bits are set.  Removes the shared memory region ID and deallocates its  associated  shmid_ds
       structure.   [Tru64  UNIX]  Locks  the shared memory segment specified by shmid in memory.  [Tru64 UNIX]  Unlocks the shared memory segment
       specified by shmid in memory.

       The shmid_ds structure is used only with the IPC_STAT and IPC_SET commands. In either case, the calling process	must  have  allocated  the
       structure before making the call.

       [Tru64  UNIX]  The  SHM_LOCK  and  SHM_UNLOCK  commands can be used to lock (wire) down a shared segment in memory to prevent it from being
       paged out.

NOTES

       [Tru64 UNIX]  When using the SHM_LOCK command, make sure that the system has enough physical memory available for the shared segment to	be
       wired without exceeding the system-wide limit or otherwise severely impacting system performance.

       [Tru64  UNIX]  The SHM_LOCK operation wires all the pages in a global shared segment and prevents the pageout daemon from reclaiming any of
       the pages.  This can potentially lead to thrashing.

RESTRICTIONS

       The following restrictions apply to the shared memory commands: For the IPC_SET and IPC_RMID commands, the effective user ID of the calling
       process	must  be  equal  to that of superuser or equal to the value of shm_perm.cuid or shm_perm.uid in the associated shmid_ds structure.
       [Tru64 UNIX]  For the SHM_LOCK and SHM_UNLOCK commands, the effective user ID of the calling process must be equal to that of superuser.

RETURN VALUES

       Upon successful completion, a value of 0 (zero) is returned. If the shmctl() function fails, a value of -1 is returned and errno is set	to
       indicate the error.

ERRORS

       The  shmctl()  function	sets  errno  to  the specified values for the following conditions: The cmd parameter is IPC_STAT, but the calling
       process does not have read permission.  [Tru64 UNIX]  The cmd parameter is SHM_LOCK, and the system-wide  wire  limit  has  been  exceeded.
       [Tru64  UNIX]  The  cmd	parameter is IPC_STAT or IPC_SET.  An error occurred in accessing the buf structure.  The shmid parameter does not
       specify a valid shared memory region ID, or cmd is not a valid command.

	      [Tru64 UNIX]  The cmd parameter is SHM_UNLOCK, and the segment was not locked.  The cmd parameter is equal  to  either  IPC_RMID	or
	      IPC_SET, and the calling process does not have appropriate privilege.

RELATED INFORMATION

       Functions: shmat(2), shmdt(2), shmget(2)

       Data structures: shmid_ds(4)

       Standards: standards(5) delim off

																	 shmctl(2)