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  prior-
       ity  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.

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, 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_SWAP-
              FILES (32) otherwise.  In Linux kernel versions prior to 2.4.10,
              the maximum was always MAX_SWAPFILES, which was 8.

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).

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

Linux 2.6.7                       2004-10-10                         SWAPON(2)