_sysctl

SYSCTL(2)		  Linux Programmer’s Manual		    SYSCTL(2)



NAME
       sysctl - read/write system parameters

SYNOPSIS
       #include <unistd.h>

       #include <linux/unistd.h>

       #include <linux/sysctl.h>

       _syscall1(int, _sysctl, struct __sysctl_args *, args);

       int _sysctl(struct __sysctl_args *args);

DESCRIPTION
       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.


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

       _syscall1(int, _sysctl, struct __sysctl_args *, args);
       int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
		  void *newval, size_t newlen)
       {
	       struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
	       return _sysctl(&args);
       }

       #define SIZE(x) sizeof(x)/sizeof(x[0])
       #define OSNAMESZ 100

       char osname[OSNAMESZ];
       int osnamelth;
       int name[] = { CTL_KERN, KERN_OSTYPE };

       main(){
	       osnamelth = sizeof(osname);
	       if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
		       perror("sysctl");
	       else
		       printf("This machine is running %*s\n", osnamelth, osname);
	       return 0;
       }


RETURN VALUE
       Upon  successful	 completion, _sysctl returns 0. Otherwise, a value of
       -1 is returned and errno is set to indicate the error.

ERRORS
       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.

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

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 BSD 4.4, but the declaration of the sysctl(2) function is the same
       in both.

BUGS
       The object names vary between kernel versions.  THIS MAKES THIS SYSTEM
       CALL WORTHLESS FOR APPLICATIONS.	 Use the /proc/sys interface instead.
       Not all available objects are properly documented.
       It is not yet possible  to  change  operating  system  by  writing  to
       /proc/sys/kernel/ostype.

SEE ALSO
       proc(5)



Linux 1.3.85			  1996-04-11			    SYSCTL(2)