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

       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 prior-
       ities, 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

       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 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.

       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).

       There is an upper limit on the number of swap files that may be used, defined by the kernel constant  MAX_SWAP-
       FILES.   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 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 ker-
       nel 2.6.32, the limit is decreased by 1 if the kernel is built with the CONFIG_MEMORY_FAILURE option.

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2007-06-22                         SWAPON(2)