NAME

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

SYNOPSIS

       #include <unistd.h>
       #include <sys/swap.h>

       int swapon(const char *path, int swapflags);
       int swapoff(const char *path);

DESCRIPTION

       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.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       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.

CONFORMING TO

       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.

NOTES

       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  migrate_pages(2)).  Since kernel 2.6.32, the limit is further decreased by 1 if the kernel
       is built with the CONFIG_MEMORY_FAILURE option.

       Discard of swap pages was introduced in kernel 2.6.29, then made  conditional  on  the  SWAP_FLAG_DISCARD
       flag  in  kernel  2.6.36, which still discards the entire swap area when swapon() is called, even if that
       flag bit is not set.

SEE ALSO

       mkswap(8), swapoff(8), swapon(8)

COLOPHON

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