SWAPCTL(2) System Calls Manual SWAPCTL(2)

swapctl — modify swap configuration

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

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

The () function is used to add and delete swap devices, modify their configuration or query their statistics.

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

  • If cmd is SWAP_NSWAP, the current number of swap devices in the system is returned. The arg and misc parameters are ignored.
  • 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];	/* 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

    All but SWAP_NSWAP and SWAP_STATS require superuser privileges.

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

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 used.

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:

[]
A component of the path prefix is not a directory.
[]
A component of a pathname exceeded NAME_MAX characters, or an entire pathname (including the terminating NUL) exceeded PATH_MAX bytes.
[]
The named device does not exist. For the SWAP_CTL command, the named device is not currently enabled for swapping.
[]
Search permission is denied for a component of the path prefix.
[]
Too many symbolic links were encountered in translating the pathname.
[]
The caller is not the superuser.
[]
The device specified by arg has already been made available for swapping.
[]
The device configured by arg has insufficient size, or the cmd was unknown.
[]
The major device number of arg is out of range (this indicates no device driver exists for the associated hardware).
[]
An I/O error occurred while opening the swap device.
[]
arg points outside the process' allocated address space.
[]
The device specified by arg cannot be used for swapping.

config(8), swapctl(8)

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>.

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 <thorpej@NetBSD.ORG>.

June 24, 2024 OpenBSD 7.6