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
       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.

       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.

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.  Remove it from your programs now; use the /proc/sys interface instead.

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 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-11-20                                                  SYSCTL(2)