sysctl(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

SYSCTL(2)               Linux Programmer's Manual              SYSCTL(2)

NAME         top

       sysctl - read/write system parameters

SYNOPSIS         top

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

       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;    /* 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         top

       Upon successful completion, _sysctl() returns 0.  Otherwise, a
       value of -1 is returned and errno 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.

VERSIONS         top

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

CONFORMING TO         top

       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         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).

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 <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(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         top

       proc(5)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-11-01                      SYSCTL(2)

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