swapon() - Unix, Linux System Call
swapon, swapoff - start/stop swapping to file/device
#include <asm/page.h> /* to find PAGE_SIZE */
int swapon(const char *path, int swapflags);
int swapoff(const char *path);
swapon() sets the swap area to the file or block device specified by
path. swapoff() stops swapping to the file or block device specified by
swapon() takes a
swapflags has the
SWAP_FLAG_PREFER bit turned on, the new swap area will have a higher priority than default.
The priority is encoded within
(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
These functions may only be used by a privileged process (one having the
Each swap area has a priority, either high or low.
The default priority is low.
Within the low-priority areas,
newer areas are even lower priority than older areas.
All priorities set with
swapflags are high-priority, higher than default.
They may have any non-negative value chosen by the caller.
Higher numbers mean higher priority.
Swap pages are allocated from areas in priority order,
highest priority first.
For areas with different priorities,
a higher-priority area is exhausted before using a lower-priority area.
If two or more areas have the same priority,
and it is the highest priority available,
pages are allocated on a round-robin basis between them.
As of Linux 1.3.6, the kernel usually follows these rules,
but there are exceptions.
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
swapon()) The specified
path is already being used as a swap area.
path exists, but refers neither to a regular file nor to a block device;
swapon(), the indicated path does not contain a valid swap signature;
swapoff(), path is not currently a swap area.
The system limit on the total number of open files has been reached.
path does not exist.
The system has insufficient memory to start swapping.
The caller does not have the
CAP_SYS_ADMIN capability or the maximal number of swap files has been reached.
(The maximum is
CONFIG_MIGRATION is enabled,
In Linux kernel versions prior to 2.4.10, the maximum was always
MAX_SWAPFILES, which was 8.
These functions are Linux specific and should not be used in programs
intended to be portable.
swapflags argument was introduced in Linux 1.3.2.
The partition or path must be prepared with