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SWAPCTL(2)		    BSD	System Calls Manual		    SWAPCTL(2)

     swapctl --	modify swap configuration

     Standard C	Library	(libc, -lc)

     #include <unistd.h>
     #include <sys/swap.h>

     swapctl(int cmd, void *arg, int misc);

     The swapctl function is used to add and delete swap devices, and modify
     their configuration.

     The cmd parameter specifies the operation to be performed.	 The arg and
     misc parameters have different meanings, depending	on the cmd parameter.

	   1.	If cmd is SWAP_NSWAP, the current number of swap devices in
		the system is returned.	 The arg and misc parameters are ig-

	   2.	If cmd is SWAP_STATS, the current statistics for swap devices
		are returned in	the arg	parameter.  No more than misc swap de-
		vices are returned.  The arg parameter should point to an ar-
		ray of at least	misc struct swapent structures:

		struct swapent {
			dev_t	se_dev;			/* device id */
			int	se_flags;		/* entry flags */
			int	se_nblks;		/* total blocks	*/
			int	se_inuse;		/* blocks in use */
			int	se_priority;		/* priority */
			char	se_path[PATH_MAX+1];	/* path	to entry */

		The flags are defined as

			SWF_INUSE	in use:	we have	swapped	here
			SWF_ENABLE	enabled: we can	swap here
			SWF_BUSY	busy: I/O happening here
			SWF_FAKE	fake: still being built

	   3.	If cmd is SWAP_ON, the arg parameter is	used as	a pathname of
		a file to enable swapping to.  The misc	parameter is used to
		set the	priority of this swap device.

	   4.	If cmd is SWAP_OFF, the	arg parameter is used as the pathname
		of a file to disable swapping from.  The misc parameter	is ig-

	   5.	If cmd is SWAP_CTL, the	arg and	misc parameters	have the same
		function as for	the SWAP_ON case, except that they change the
		priority of a currently	enabled	swap device.

	   6.	If cmd is SWAP_DUMPDEV,	the arg	parameter is used as the path-
		name of	a device to use	as the dump device, should the system

	   7.	If cmd is SWAP_GETDUMPDEV, the arg parameter points to a
		dev_t, which is	filled in by the current dump device.

     When swapping is enabled on a block device, the first portion of the disk
     is	left unused to prevent any disklabel present from being	overwritten.
     This space	is allocated from the swap device when the SWAP_ON command is

     The priority of a swap device can be used to fill faster swap devices be-
     fore slower ones.	A priority of 0	is the highest,	with larger numbers
     having lower priority.  For a fuller discussion on	swap priority, see the
     SWAP PRIORITY section in swapctl(8).

     If	the cmd	parameter is SWAP_NSWAP	or SWAP_STATS, swapctl() returns the
     number of swap devices, if	successful.  The SWAP_NSWAP command is always
     successful.  Otherwise it returns 0 on success and	-1 on failure, setting
     the global	variable errno to indicate the error.

     swapctl() succeeds	unless:

     [EACCES]		Search permission is denied for	a component of the
			path prefix.

     [EBUSY]		The device specified by	arg has	already	been made
			available for swapping.

     [EFAULT]		arg points outside the process'	allocated address

     [EINVAL]		The device configured by arg has no associated size,
			or the cmd was unknown.

     [EIO]		An I/O error occurred while opening the	swap device.

     [ELOOP]		Too many symbolic links	were encountered in translat-
			ing the	pathname.

     [ENAMETOOLONG]	A component of a pathname exceeded {NAME_MAX} charac-
			ters, or an entire path	name exceeded {PATH_MAX} char-

     [ENOENT]		The named device does not exist.  For the SWAP_CTL
			command, the named device is not currently enabled for

     [ENOTDIR]		A component of the path	prefix is not a	directory.

     [ENXIO]		The major device number	of arg is out of range (this
			indicates no device driver exists for the associated
			hardware); or the block	device specified by arg	is not
			marked as a swap partition in the disklabel.

     [EPERM]		The caller is not the super-user.


     The swapctl() function call appeared in NetBSD 1.3.  The se_path member
     was added to struct swapent in NetBSD 1.4,	when the header	file was also
     moved from	<vm/vm_swap.h> to its current location in <sys/swap.h>.

     The current swap system was designed and implemented by Matthew Green
     <>, with help from Paul Kranenburg <> and
     Leo Weppelman <>, and insights from Jason R.	Thorpe

BSD				 May 17, 2010				   BSD


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