swapon() - Unix, Linux System Call
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swapon() - Unix, Linux System Call

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swapon, swapoff - start/stop swapping to file/device


#include <unistd.h>
#include <asm/page.h> /* to find PAGE_SIZE */
#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.

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:


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 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.


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; or, for swapon(), the indicated path does not contain a valid swap signature; or, for swapoff(), path is not currently a swap area.
ENFILE The system 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 or the maximal number of swap files has been reached. (The maximum is MAX_SWAPFILES-2(30) if CONFIG_MIGRATION is enabled, MAX_SWAPFILES (32) otherwise. 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. The second swapflags argument was introduced in Linux 1.3.2.


The partition or path must be prepared with mkswap(8).


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