SWAPON(2) Linux Programmer's Manual SWAPON(2)
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
swapon() takes a swapflags argument. If 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 swapflags as:
(prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
These functions may only be used by a privileged process (one having
the CAP_SYS_ADMIN capability).
Each swap area has a priority, either high or low. The default prior-
ity 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
On success, zero is returned. On error, -1 is returned, and errno is
EBUSY (for swapon()) The specified path is already being used as a
EINVAL The file path exists, but refers neither to a regular file nor
to a block device; or, for swapon(), the indicated path does not
contain a valid swap signature or resides on an in-memory file
system like tmpfs; or, for swapoff(), path is not currently a
ENFILE The system limit on the total number of open files has been
ENOENT The file path does not exist.
ENOMEM The system has insufficient memory to start swapping.
EPERM The caller does not have the CAP_SYS_ADMIN capability. Alterna-
tively, the maximum number of swap files are already in use; see
These functions are Linux-specific and should not be used in programs
intended to be portable. The second swapflags argument was introduced
in Linux 1.3.2.
The partition or path must be prepared with mkswap(8).
There is an upper limit on the number of swap files that may be used,
defined by the kernel constant MAX_SWAPFILES. Before kernel 2.4.10,
MAX_SWAPFILES has the value 8; since kernel 2.4.10, it has the value
32. Since kernel 2.6.18, the limit is decreased by 2 (thus: 30) if the
kernel is built with the CONFIG_MIGRATION option (which reserves two
swap table entries for the page migration features of mbind(2) and
mkswap(8), swapoff(8), swapon(8)
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-06-22 SWAPON(2)