SWAPON(2)                                           Linux Programmer's Manual                                          SWAPON(2)



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

SYNOPSIS
       #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);

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.

       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).

   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; 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 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.  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.

SEE ALSO
       mkswap(8), swapoff(8), swapon(8)

COLOPHON
       This  page  is  part of release 3.25 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                                                      2010-06-15                                                  SWAPON(2)