NAME

       sysctl - read/write system parameters

SYNOPSIS

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

       int _sysctl(struct __sysctl_args *args);

DESCRIPTION

       This system call no longer exists on current kernels!  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

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

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

       ENOTDIR
              name was not found.

VERSIONS

       This system call first appeared in Linux 1.3.57.  It was removed in Linux 5.5.

CONFORMING TO

       This call is Linux-specific, and should not be used in programs intended to  be  portable.
       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

       Use of this system call was long discouraged: since Linux 2.6.24, uses of this system call
       result in warnings in the kernel log, and in  Linux  5.5,  the  system  call  was  finally
       removed.  Use the /proc/sys interface instead.

       Note  that  on  older kernels where this system call still exists, it is available only if
       the kernel was configured with the CONFIG_SYSCTL_SYSCALL option.  Furthermore, glibc  does
       not provide a wrapper for this system call, necessitating the use of syscall(2).

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 5.05 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 https://www.kernel.org/doc/man-pages/.