SYSCTL(2)                  Linux Programmer's Manual                 SYSCTL(2)
NAME
       sysctl - read/write system parameters

SYNOPSIS
       #include <unistd.h>
       #include <linux/sysctl.h>

       int _sysctl(struct __sysctl_args *args);

DESCRIPTION
       The _sysctl() call reads and/or writes kernel parameters.  For example,
       the hostname, or the maximum number of open files.   The  argument  has
       the form

         struct __sysctl_args {
             int    *name;    /* integer vector describing variable */
             int     nlen;    /* length of this vector */
             void   *oldval;  /* 0 or address where to store old value */
             size_t *oldlenp; /* available room for old value,
                                 overwritten by actual size of old value */
             void   *newval;  /* 0 or address of new value */
             size_t  newlen;  /* size of new value */
         };

       This  call  does  a  search  in a tree structure, possibly resembling a
       directory tree under /proc/sys, and if  the  requested  item  is  found
       calls some appropriate routine to read or modify the value.

RETURN VALUE
       Upon successful completion, _sysctl() returns 0.  Otherwise, a value of
       -1 is returned and errno is set to indicate the error.

ERRORS
       EFAULT The invocation asked for the previous value  by  setting  oldval
              non-NULL, but allowed zero room in oldlenp.

       ENOTDIR
              name was not found.

       EPERM  No  search  permission for one of the encountered 'directories',
              or no read permission where oldval was  non-zero,  or  no  write
              permission where newval was non-zero.

CONFORMING TO
       This  call  is  Linux  specific,  and  should  not  be used in programs
       intended to be portable.  A sysctl() call has  been  present  in  Linux
       since  version  1.3.57.   It  originated in 4.4BSD.  Only Linux has the
       /proc/sys mirror, and the object naming schemes  differ  between  Linux
       and  4.4BSD,  but the declaration of the sysctl(2) function is the same
       in both.

BUGS
       The object names vary between kernel versions.  THIS MAKES THIS  SYSTEM
       CALL  WORTHLESS FOR APPLICATIONS.  Use the /proc/sys interface instead.

       Not all available objects are properly documented.

       It is not yet  possible  to  change  operating  system  by  writing  to
       /proc/sys/kernel/ostype.

EXAMPLE
       #define _GNU_SOURCE
       #include <unistd.h>
       #include <sys/syscall.h>
       #include <string.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <linux/sysctl.h>

       int _sysctl(struct __sysctl_args *args );

       #define OSNAMESZ 100

       int
       main(void)
       {
           struct __sysctl_args args;
           char osname[OSNAMESZ];
           size_t osnamelth;
           int name[] = { CTL_KERN, KERN_OSTYPE };

           memset(&args, 0, sizeof(struct __sysctl_args));
           args.name = name;
           args.nlen = sizeof(name)/sizeof(name[0]);
           args.oldval = osname;
           args.oldlenp = &osnamelth;

           osnamelth = sizeof(osname);

           if (syscall(SYS__sysctl, &args) == -1) {
               perror("_sysctl");
               exit(EXIT_FAILURE);
           }
           printf("This machine is running %*s\n", osnamelth, osname);
           exit(EXIT_SUCCESS);
       }

NOTES
       Glibc  does  not  provide a wrapper for this system call; call it using
       syscall(2).

SEE ALSO
       proc(5)

Linux 1.3.85                      2007-06-01                         SYSCTL(2)