NAME

       sysctl - read/write system parameters

SYNOPSIS

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

       [[deprecated]] int _sysctl(struct __sysctl_args *args);

DESCRIPTION

       This system call no longer exists on current kernels!  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

       EACCES, 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.

       ENOTDIR
              name was not found.

VERSIONS

       This system call first appeared in Linux 1.3.57.  It  was  removed  in  Linux  5.5;  glibc
       support was removed in glibc 2.32.

STANDARDS

       This  call  is Linux-specific, and should not be used in programs intended to be portable.
       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

       Use of this system call was long discouraged: since Linux 2.6.24, uses of this system call
       result  in  warnings  in  the  kernel  log,  and in Linux 5.5, the system call was finally
       removed.  Use the /proc/sys interface instead.

       Note that on older kernels where this system call still exists, it is  available  only  if
       the  kernel was configured with the CONFIG_SYSCTL_SYSCALL option.  Furthermore, glibc does
       not provide a wrapper for this system call, necessitating the use of syscall(2).

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.

EXAMPLES

       #define _GNU_SOURCE
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <sys/syscall.h>
       #include <unistd.h>

       #include <linux/sysctl.h>

       #define ARRAY_SIZE(arr)  (sizeof(arr) / sizeof((arr)[0]))

       int _sysctl(struct __sysctl_args *args);

       #define OSNAMESZ 100

       int
       main(void)
       {
           int                   name[] = { CTL_KERN, KERN_OSTYPE };
           char                  osname[OSNAMESZ];
           size_t                osnamelth;
           struct __sysctl_args  args;

           memset(&args, 0, sizeof(args));
           args.name = name;
           args.nlen = ARRAY_SIZE(name);
           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", (int) osnamelth, osname);
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       proc(5)