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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
              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() 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 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-20                         SYSCTL(2)