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       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 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.


       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

       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_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


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


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