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SWAPON(2) FreeBSD System Calls Manual SWAPON(2)

swapon, swapoff
control devices for interleaved paging/swapping

Standard C Library (libc, -lc)

#include <unistd.h>

swapon(const char *special);

swapoff(const char *special, u_int flags);

The swapon() system call makes the block device special available to the system for allocation for paging and swapping. The names of potentially available devices are known to the system and defined at system configuration time. The size of the swap area on special is calculated at the time the device is first made available for swapping.

The swapoff() system call disables paging and swapping on the given device. All associated swap metadata are deallocated, and the device is made available for other purposes.

The special argument points to the name of the device or file used for swapping. flags argument takes the following flags:

Overrides a very conservative check that prevents swapoff if the total amount of free memory and remaining swap devices space might be unsufficient for the system to continue operating.

If an error has occurred, a value of -1 is returned and errno is set to indicate the error.

Both swapon() and swapoff() can fail if:
A component of the path prefix is not a directory.
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
The named device does not exist.
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 super-user.
The special argument points outside the process's allocated address space.

Additionally, swapon() can fail for the following reasons:

The special argument is not a block device.
The device specified by special has already been made available for swapping
The major device number of special is out of range (this indicates no device driver exists for the associated hardware).
An I/O error occurred while opening the swap device.
Corrupted data was detected while reading from the file system to open the swap device.

Lastly, swapoff() can fail if:

The system is not currently swapping to special.
Not enough virtual memory is available to safely disable paging and swapping to the given device.

config(8), swapon(8), sysctl(8)

The swapon() system call appeared in 4.0BSD. The swapoff() system call appeared in FreeBSD 5.0.
December 8, 2021 FreeBSD 13.1-RELEASE

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