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NAME

       sysctl - read/write system parameters

SYNOPSIS

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

       int _sysctl(struct __sysctl_args *args);

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

       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  nonzero,  or  no  write permission where newval was
              nonzero.

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 disappear in a future kernel version.  Remove it from your
       programs now; use the /proc/sys interface instead.

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

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