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);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       Do not use this system call!  See NOTES.

       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.

       EACCES, EPERM
              No search permission for one of the encountered "directories", or no  read  permis‐
              sion where oldval was nonzero, or no write permission where newval was nonzero.

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() function is the same in both.

NOTES
       Glibc does not provide a wrapper for this system  call;  call  it  using  syscall(2).   Or
       rather...   don't call it: use of this system call has long been discouraged, and it is so
       unloved that it is likely to disappear in a future kernel version.   Since  Linux  2.6.24,
       uses  of  this system call result in warnings in the kernel log.  Remove it from your pro‐
       grams now; use the /proc/sys interface instead.

       This  system  call  is  available  only  if  the  kernel  was  configured  with  the  CON‐
       FIG_SYSCTL_SYSCALL option.

BUGS
       The  object  names  vary  between  kernel  versions, making this system call worthless for
       applications.

       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);
       }

SEE ALSO
       proc(5)

COLOPHON
       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.



Linux                                       2012-12-22                                  SYSCTL(2)