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SWAPCTL(2)                    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

           2.   If cmd is SWAP_STATS, the current statistics for swap devices
                are returned in the arg parameter.  No more than misc swap
                devices are returned.  The arg parameter should point to an
                array 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

           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
                pathname of a device to use as the dump device, should the
                system panic.

           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
     before 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
                        translating the pathname.

     [ENAMETOOLONG]     A component of a pathname exceeded {NAME_MAX}
                        characters, or an entire path name exceeded {PATH_MAX}

     [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
     <mrg@eterna.com.au>, with help from Paul Kranenburg <pk@NetBSD.org> and
     Leo Weppelman <leo@NetBSD.org>, and insights from Jason R. Thorpe

NetBSD 10.99                     May 17, 2010                     NetBSD 10.99