redhat man page for 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 = SIZE(osname);
	       if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
		       perror("sysctl");
	       else
		       printf("This machine is running %*s
", 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 permis-
	      sion 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)