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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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swapon(2) System Calls Manual swapon(2)
swapon, swapoff - start/stop swapping to file/device
Standard C library (libc, -lc)
#include <sys/swap.h>
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 path.
If the SWAP_FLAG_PREFER flag is specified in the swapon()
swapflags argument, 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
If the SWAP_FLAG_DISCARD flag is specified in the swapon()
swapflags argument, freed swap pages will be discarded before they
are reused, if the swap device supports the discard or trim
operation. (This may improve performance on some Solid State
Devices, but often it does not.) See also NOTES.
These functions may be used only by a privileged process (one
having the CAP_SYS_ADMIN capability).
Priority
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 nonnegative 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 to indicate the error.
EBUSY (for swapon()) The specified path is already being used as
a swap area.
EINVAL The file path exists, but refers neither to a regular file
nor to a block device;
EINVAL (swapon()) The indicated path does not contain a valid swap
signature or resides on an in-memory filesystem such as
tmpfs(5).
EINVAL (since Linux 3.4)
(swapon()) An invalid flag value was specified in
swapflags.
EINVAL (swapoff()) path is not currently a swap area.
ENFILE The system-wide limit on the total number of open files has
been reached.
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.
Alternatively, the maximum number of swap files are already
in use; see NOTES below.
Linux.
The 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 Linux
2.4.10, MAX_SWAPFILES has the value 8; since Linux 2.4.10, it has
the value 32. Since Linux 2.6.18, the limit is decreased by 2
(thus 30), since Linux 5.19, the limit is decreased by 3 (thus:
29) 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 migrate_pages(2)). Since Linux 2.6.32, the limit is
further decreased by 1 if the kernel is built with the
CONFIG_MEMORY_FAILURE option. Since Linux 5.14, the limit is
further decreased by 4 if the kernel is built with the
CONFIG_DEVICE_PRIVATE option. Since Linux 5.19, the limit is
further decreased by 1 if the kernel is built with the
CONFIG_PTE_MARKER option.
Discard of swap pages was introduced in Linux 2.6.29, then made
conditional on the SWAP_FLAG_DISCARD flag in Linux 2.6.36, which
still discards the entire swap area when swapon() is called, even
if that flag bit is not set.
mkswap(8), swapoff(8), swapon(8)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 swapon(2)
Pages that refer to this page: syscalls(2), systemd.exec(5), capabilities(7), swapon(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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