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.

STANDARDS

       Linux.

HISTORY

       Linux 1.3.57.  Removed in Linux 5.5, glibc 2.32.

       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)

Linux man-pages 6.7                                2023-10-31                                          sysctl(2)