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NAME

       sysctl - read/write system parameters

SYNOPSIS

       #include <unistd.h>
       #include <linux/unistd.h>
       #include <linux/sysctl.h>
       #include <errno.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

       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(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)