man2/sysctl.2

   1 .\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
   2 .\"
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
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  22 .\"
  23 .\" Written 11 April 1996 by Andries Brouwer <aeb@cwi.nl>
  24 .\" 960412: Added comments from Stephen Tweedie
  25 .\" Modified Tue Oct 22 22:28:41 1996 by Eric S. Raymond <esr@thyrsus.com>
  26 .\" Modified Mon Jan  5 20:31:04 1998 by aeb.
  27 .\"
  28 .TH SYSCTL 2 2007-09-22 "Linux" "Linux Programmer's Manual"
  29 .SH NAME
  30 sysctl \- read/write system parameters
  31 .SH SYNOPSIS
  32 .nf
  33 .B #include <unistd.h>
  34 .br
  35 .B #include <linux/sysctl.h>
  36 .sp
  37 .BI "int _sysctl(struct __sysctl_args *" args );
  38 .fi
  39 .SH DESCRIPTION
  40 The
  41 .BR _sysctl ()
  42 call reads and/or writes kernel parameters.
  43 For example, the hostname,
  44 or the maximum number of open files.
  45 The argument has the form
  46 .PP
  47 .in +4n
  48 .nf
  49 struct __sysctl_args {
  50     int    *name;    /* integer vector describing variable */
  51     int     nlen;    /* length of this vector */
  52     void   *oldval;  /* 0 or address where to store old value */
  53     size_t *oldlenp; /* available room for old value,
  54                         overwritten by actual size of old value */
  55     void   *newval;  /* 0 or address of new value */
  56     size_t  newlen;  /* size of new value */
  57 };
  58 .fi
  59 .in
  60 .PP
  61 This call does a search in a tree structure, possibly resembling
  62 a directory tree under
  63 .IR /proc/sys ,
  64 and if the requested item is found calls some appropriate routine
  65 to read or modify the value.
  66 .SH "RETURN VALUE"
  67 Upon successful completion,
  68 .BR _sysctl ()
  69 returns 0.
  70 Otherwise, a value of \-1 is returned and
  71 .I errno
  72 is set to indicate the error.
  73 .SH ERRORS
  74 .TP
  75 .B EFAULT
  76 The invocation asked for the previous value by setting
  77 .I oldval
  78 non-NULL, but allowed zero room in
  79 .IR oldlenp .
  80 .TP
  81 .B ENOTDIR
  82 .I name
  83 was not found.
  84 .TP
  85 .B EPERM
  86 No search permission for one of the encountered "directories",
  87 or no read permission where
  88 .I oldval
  89 was non-zero, or no write permission where
  90 .I newval
  91 was non-zero.
  92 .SH "CONFORMING TO"
  93 This call is Linux specific, and should not be used in programs
  94 intended to be portable.
  95 A
  96 .BR sysctl ()
  97 call has been present in Linux since version 1.3.57.
  98 It originated in
  99 4.4BSD.
 100 Only Linux has the
 101 .I /proc/sys
 102 mirror, and the object naming schemes differ between Linux and 4.4BSD,
 103 but the declaration of the
 104 .BR sysctl ()
 105 function is the same in both.
 106 .SH NOTES
 107 Glibc does not provide a wrapper for this system call; call it using
 108 .BR syscall (2).
 109
 110 Or rather... don't call it:
 111 use of this system call has long been discouraged,
 112 and it is so unloved that
 113 \fBit is likely to disappear in a future kernel version\fP.
 114 .\" See http://lwn.net/Articles/247243/
 115 Remove it from your programs now; use the
 116 .I /proc/sys
 117 interface instead.
 118 .SH BUGS
 119 The object names vary between kernel versions,
 120 making this system call worthless for applications.
 121 .PP
 122 Not all available objects are properly documented.
 123 .PP
 124 It is not yet possible to change operating system by writing to
 125 .IR /proc/sys/kernel/ostype .
 126 .SH EXAMPLE
 127 .nf
 128 #define _GNU_SOURCE
 129 #include <unistd.h>
 130 #include <sys/syscall.h>
 131 #include <string.h>
 132 #include <stdio.h>
 133 #include <stdlib.h>
 134 #include <linux/sysctl.h>
 135
 136 int _sysctl(struct __sysctl_args *args );
 137
 138 #define OSNAMESZ 100
 139
 140 int
 141 main(void)
 142 {
 143     struct __sysctl_args args;
 144     char osname[OSNAMESZ];
 145     size_t osnamelth;
 146     int name[] = { CTL_KERN, KERN_OSTYPE };
 147
 148     memset(&args, 0, sizeof(struct __sysctl_args));
 149     args.name = name;
 150     args.nlen = sizeof(name)/sizeof(name[0]);
 151     args.oldval = osname;
 152     args.oldlenp = &osnamelth;
 153
 154     osnamelth = sizeof(osname);
 155
 156     if (syscall(SYS__sysctl, &args) == \-1) {
 157         perror("_sysctl");
 158         exit(EXIT_FAILURE);
 159     }
 160     printf("This machine is running %*s\\n", osnamelth, osname);
 161     exit(EXIT_SUCCESS);
 162 }
 163 .fi
 164 .SH "SEE ALSO"
 165 .BR proc (5)