man pages section 2: System Calls

swapctl(2)

Name

swapctl– manage swap space

Synopsis

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

int swapctl(int cmd, void *arg);

Description

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

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.

Return Values

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.

Errors

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

EEXIST: Part of the range specified by sr_start and sr_length is already being used for swapping on the specified resource (SC_ADD).
EFAULT: Either arg, sr_name, or ste_path points to an illegal address.
EINVAL: 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).
EISDIR: The path specified for SC_ADD is a directory.
ELOOP: Too many symbolic links were encountered in translating the pathname provided to SC_ADD or SC_REMOVE.
ENAMETOOLONG: 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.
ENOENT: The pathname specified for SC_ADD or SC_REMOVE does not exist.
ENOMEM: 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.
ENOSYS: The pathname specified for SC_ADD or SC_REMOVE is not a file or block special device.
ENOTDIR: Pathname provided to SC_ADD or SC_REMOVE contained a component in the path prefix that was not a directory.
EPERM: The {PRIV_SYS_MOUNT} was not asserted in the effective set of the calling process.
EROFS: The pathname specified for SC_ADD is a read-only file system.

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

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

Examples

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 */

again:
    if ((num = swapctl(SC_GETNSWP, 0)) == -1) {
        perror("swapctl: GETNSWP");
        exit(1);
    }
    if (num == 0) {
        fprintf(stderr, "No Swap Devices Configured\n");
        exit(2);
    }
    /* 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");
        exit(3);
    }
    /* allocate num+1 string holders */
    if ((strtab = (char *)
        malloc((num + 1) * MAXSTRSIZE)) == (void *) 0) {
        fprintf(stderr, "Malloc Failed\n");
        exit(3);
    }
    /* 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) {
        perror("swapctl");
        exit(1);
    }
    if (n > num) {        /* more were added */
        free(s);
        free(strtab);
        goto again;
    }
    for (i = 0; i < n; i++)
        printf("%s %ld\n",
            s->swt_ent[i].ste_path, s->swt_ent[i].ste_pages);
}