sysctl(2)

NAME

   sysctl - read/write system parameters

SYNOPSIS

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

   int _sysctl(struct __sysctl_args *args);

   Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

   Do not use this system call!  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.

CONFORMING TO

   This  call  is  Linux-specific,  and  should  not  be  used in programs
   intended to be portable.  A sysctl() call has  been  present  in  Linux
   since  version  1.3.57.   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

   Glibc does not provide a wrapper for this system call;  call  it  using
   syscall(2).   Or  rather...  don't call it: use of this system call has
   long been discouraged, and it is  so  unloved  that  it  is  likely  to
   disappear in a future kernel version.  Since Linux 2.6.24, uses of this
   system call result in warnings in the kernel log.  Remove it from  your
   programs now; use the /proc/sys interface instead.

   This  system  call  is available only if the kernel was configured with
   the CONFIG_SYSCTL_SYSCALL option.

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 4.09 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/.

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