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RedHat 9 (Linux i386) - man page for sysctl (redhat section 2)

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 = SIZE(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 permis-
	      sion 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)


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