SWAPCTL(2) System Calls SWAPCTL(2)


swapctl - manage swap space


#include <sys/stat.h>
#include <sys/swap.h>

int swapctl(int cmd, void *arg);


The swapctl() function adds, deletes, or returns information about swap
resources. cmd specifies one of the following options contained in

SC_ADD /* add a resource for swapping */
SC_LIST /* list the resources for swapping */
SC_REMOVE /* remove a resource for swapping */
SC_GETNSWP /* return number of swap resources */

When SC_ADD or SC_REMOVE is specified, arg is a pointer to a swapres
structure containing the following members:

char *sr_name; /* pathname of resource */
off_t sr_start; /* offset to start of swap area */
off_t sr_length; /* length of swap area */

The sr_start and sr_length members are specified in 512-byte blocks. A
swap resource can only be removed by specifying the same values for the
sr_start and sr_length members as were specified when it was added. Swap
resources need not be removed in the order in which they were added.

When SC_LIST is specified, arg is a pointer to a swaptable structure
containing the following members:

int swt_n; /* number of swapents following */
struct swapent swt_ent[]; /* array of swt_n swapents */

A swapent structure contains the following members:

char *ste_path; /* name of the swap file */
off_t ste_start; /* starting block for swapping */
off_t ste_length; /* length of swap area */
long ste_pages; /* number of pages for swapping */
long ste_free; /* number of ste_pages free */
long ste_flags; /* ST_INDEL bit set if swap file */
/* is now being deleted */

The SC_LIST function causes swapctl() to return at most swt_n entries.
The return value of swapctl() is the number actually returned. The
ST_INDEL bit is turned on in ste_flags if the swap file is in the process
of being deleted.

When SC_GETNSWP is specified, swapctl() returns as its value the number
of swap resources in use. arg is ignored for this operation.

The SC_ADD and SC_REMOVE functions will fail if calling process does not
have appropriate privileges.


Upon successful completion, the function swapctl() returns a value of 0
for SC_ADD or SC_REMOVE, the number of struct swapent entries actually
returned for SC_LIST, or the number of swap resources in use for
SC_GETNSWP. Upon failure, the function swapctl() returns a value of -1
and sets errno to indicate an error.


Under the following conditions, the function swapctl() fails and sets
errno to:

Part of the range specified by sr_start and sr_length is
already being used for swapping on the specified resource

Either arg, sr_name, or ste_path points to an illegal

The specified function value is not valid, the path
specified is not a swap resource (SC_REMOVE), part of the
range specified by sr_start and sr_length lies outside
the resource specified (SC_ADD), or the specified swap
area is less than one page (SC_ADD).

The path specified for SC_ADD is a directory.

Too many symbolic links were encountered in translating
the pathname provided to SC_ADD or SC_REMOVE.

The length of a component of the path specified for
SC_ADD or SC_REMOVE exceeds NAME_MAX characters or the
length of the path exceeds PATH_MAX characters and
_POSIX_NO_TRUNC is in effect.

The pathname specified for SC_ADD or SC_REMOVE does not

An insufficient number of struct swapent structures were
provided to SC_LIST, or there were insufficient system
storage resources available during an SC_ADD or
SC_REMOVE, or the system would not have enough swap space
after an SC_REMOVE.

The pathname specified for SC_ADD or SC_REMOVE is not a
file or block special device.

Pathname provided to SC_ADD or SC_REMOVE contained a
component in the path prefix that was not a directory.

The {PRIV_SYS_MOUNT} was not asserted in the effective
set of the calling process.

The pathname specified for SC_ADD is a read-only file

Additionally, the swapctl() function will fail for 32-bit interfaces if:

The amount of swap space configured on the machine is too
large to be represented by a 32-bit quantity.


Example 1: The usage of the SC_GETNSWP and SC_LIST commands.

The following example demonstrates the usage of the SC_GETNSWP and
SC_LIST commands.

#include <sys/stat.h>
#include <sys/swap.h>
#include <stdio.h>

#define MAXSTRSIZE 80

main(argc, argv)
int argc;
char *argv[];
swaptbl_t *s;
int i, n, num;
char *strtab; /* string table for path names */

if ((num = swapctl(SC_GETNSWP, 0)) == -1) {
perror("swapctl: GETNSWP");
if (num == 0) {
fprintf(stderr, "No Swap Devices Configured\n");
/* allocate swaptable for num+1 entries */
if ((s = (swaptbl_t *)
malloc(num * sizeof(swapent_t) +
sizeof(struct swaptable))) ==
(void *) 0) {
fprintf(stderr, "Malloc Failed\n");
/* allocate num+1 string holders */
if ((strtab = (char *)
malloc((num + 1) * MAXSTRSIZE)) == (void *) 0) {
fprintf(stderr, "Malloc Failed\n");
/* initialize string pointers */
for (i = 0; i < (num + 1); i++) {
s->swt_ent[i].ste_path = strtab + (i * MAXSTRSIZE);

s->swt_n = num + 1;
if ((n = swapctl(SC_LIST, s)) < 0) {
if (n > num) { /* more were added */
goto again;
for (i = 0; i < n; i++)
printf("%s %ld\n",
s->swt_ent[i].ste_path, s->swt_ent[i].ste_pages);



September 25, 1997 SWAPCTL(2)