sysctl

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

NAME
       sysctl - read/write system parameters

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

       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 di-
       rectory 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 per-
              mission 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.

CONFORMING TO
       This call is Linux-specific, and should not be  used  in  programs  in-
       tended  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   CON-
       FIG_SYSCTL_SYSCALL option.  Furthermore, glibc does not provide a wrap-
       per 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.

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
       This page is part of release 5.05 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                             2020-02-09                         SYSCTL(2)