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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.

       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


       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

       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  filesystem  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_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

       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.


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


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