swapon(2)

1SWAPON(2)                  Linux Programmer's Manual                 SWAPON(2)
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NAME

6       swapon, swapoff - start/stop swapping to file/device
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SYNOPSIS

9       #include <unistd.h>
10       #include <asm/page.h> /* to find PAGE_SIZE */
11       #include <sys/swap.h>
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13       int swapon(const char *path, int swapflags);
14       int swapoff(const char *path);
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DESCRIPTION

17       swapon()  sets  the  swap area to the file or block device specified by
18       path.  swapoff() stops swapping to the file or block  device  specified
19       by path.
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21       swapon()   takes   a   swapflags   argument.    If  swapflags  has  the
22       SWAP_FLAG_PREFER bit turned on, the new swap area will  have  a  higher
23       priority than default.  The priority is encoded within swapflags as:
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25           (prio << SWAP_FLAG_PRIO_SHIFT) & SWAP_FLAG_PRIO_MASK
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27       These  functions  may  only be used by a privileged process (one having
28       the CAP_SYS_ADMIN capability).
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PRIORITY

31       Each swap area has a priority, either high or low.  The default  prior‐
32       ity  is low.  Within the low-priority areas, newer areas are even lower
33       priority than older areas.
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35       All priorities  set  with  swapflags  are  high-priority,  higher  than
36       default.   They  may  have any non-negative value chosen by the caller.
37       Higher numbers mean higher priority.
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39       Swap pages are allocated from areas in priority order, highest priority
40       first.   For areas with different priorities, a higher-priority area is
41       exhausted before using a lower-priority area.  If  two  or  more  areas
42       have the same priority, and it is the highest priority available, pages
43       are allocated on a round-robin basis between them.
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45       As of Linux 1.3.6, the kernel usually follows these  rules,  but  there
46       are exceptions.
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RETURN VALUE

49       On  success,  zero is returned.  On error, -1 is returned, and errno is
50       set appropriately.
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ERRORS

53       EBUSY  (for swapon()) The specified path is already  being  used  as  a
54              swap area.
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56       EINVAL The  file  path exists, but refers neither to a regular file nor
57              to a block device; or, for swapon(), the indicated path does not
58              contain  a  valid swap signature; or, for swapoff(), path is not
59              currently a swap area.
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61       ENFILE The system limit on the total number  of  open  files  has  been
62              reached.
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64       ENOENT The file path does not exist.
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66       ENOMEM The system has insufficient memory to start swapping.
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68       EPERM  The  caller  does not have the CAP_SYS_ADMIN capability.  If all
69              swapfiles are in use.  (If CONFIG_MIGRATION is enabled then  the
70              maximal  number of swap files is MAX_SWAPFILES-2 (30 since linux
71              2.4.10), otherwise MAX_SWAPFILES (32 since Linux 2.4.10, 8  ear‐
72              lier) swap files could be used)
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CONFORMING TO

75       These  functions  are Linux specific and should not be used in programs
76       intended to be portable.  The second swapflags argument was  introduced
77       in Linux 1.3.2.
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NOTES

80       The partition or path must be prepared with mkswap(8).
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