sysctl(2) - Linux manual page (original) (raw)

sysctl(2) System Calls Manual sysctl(2)

NAME top

   sysctl - read/write system parameters

SYNOPSIS top

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

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

DESCRIPTION top

   **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;    /* number of elements 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 top

   Upon successful completion, **_sysctl**() returns 0.  Otherwise, a
   value of -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **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 top

   Linux.

HISTORY top

   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 top

   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)](../man2/syscall.2.html).

BUGS top

   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 top

   #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 top

   [proc(5)](../man5/proc.5.html)

COLOPHON top

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Linux man-pages 6.10 2024-11-17 sysctl(2)

Pages that refer to this page:syscalls(2), proc_sys(5), sctp(7)