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

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 disap-
       pear in a future kernel version.	 Since Linux 2.6.24, uses of this sys-
       tem call result in warnings in the kernel log.	Remove	it  from  your
       programs now; use the /proc/sys interface instead.

       This  system  call  is available only if the kernel was configured with
       the CONFIG_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 4.10 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/.



Linux				  2016-10-08			     SYSCTL(2)