man2/sysctl.2

   1 .\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
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  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 1996-04-11 "Linux 1.3.85" "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/unistd.h>
  36 .br
  37 .B #include <linux/sysctl.h>
  38 .br
  39 .B #include <errno.h>
  40 .sp
  41 .B _syscall1(int, _sysctl, struct __sysctl_args *, args)
  42           /* Using \fBsyscall\fP(2) may be preferable; see \fBintro\fP(2) */
  43 .sp
  44 .BI "int _sysctl(struct __sysctl_args *" args );
  45 .fi
  46 .SH DESCRIPTION
  47 The
  48 .BR _sysctl ()
  49 call reads and/or writes kernel parameters.
  50 For example, the hostname,
  51 or the maximum number of open files.
  52 The argument has the form
  53 .PP
  54 .nf
  55 struct __sysctl_args {
  56     int *name;        /* integer vector describing variable */
  57     int nlen;         /* length of this vector */
  58     void *oldval;     /* 0 or address where to store old value */
  59     size_t *oldlenp;  /* available room for old value,
  60                          overwritten by actual size of old value */
  61     void *newval;     /* 0 or address of new value */
  62     size_t newlen;    /* size of new value */
  63 };
  64 .fi
  65 .PP
  66 This call does a search in a tree structure, possibly resembling
  67 a directory tree under
  68 .IR /proc/sys ,
  69 and if the requested item is found calls some appropriate routine
  70 to read or modify the value.
  71 .SH EXAMPLE
  72 .nf
  73 #include <linux/unistd.h>
  74 #include <linux/types.h>
  75 #include <linux/sysctl.h>
  76
  77 _syscall1(int, _sysctl, struct __sysctl_args *, args);
  78 int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
  79            void *newval, size_t newlen)
  80 {
  81     struct __sysctl_args args = { name, nlen, oldval, oldlenp,
  82                                   newval, newlen };
  83     return _sysctl(&args);
  84 }
  85
  86 #define SIZE(x) sizeof(x)/sizeof(x[0])
  87 #define OSNAMESZ 100
  88
  89 char osname[OSNAMESZ];
  90 int osnamelth;
  91 int name[] = { CTL_KERN, KERN_OSTYPE };
  92
  93 int
  94 main(void)
  95 {
  96     osnamelth = sizeof(osname);
  97     if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
  98         perror("sysctl");
  99     else
 100         printf("This machine is running %*s\en", osnamelth, osname);
 101     return 0;
 102 }
 103 .fi
 104 .SH "RETURN VALUE"
 105 Upon successful completion,
 106 .BR _sysctl ()
 107 returns 0.
 108 Otherwise, a value of \-1 is returned and
 109 .I errno
 110 is set to indicate the error.
 111 .SH ERRORS
 112 .TP
 113 .B EFAULT
 114 The invocation asked for the previous value by setting
 115 .I oldval
 116 non-NULL, but allowed zero room in
 117 .IR oldlenp .
 118 .TP
 119 .B ENOTDIR
 120 .I name
 121 was not found.
 122 .TP
 123 .B EPERM
 124 No search permission for one of the encountered `directories',
 125 or no read permission where
 126 .I oldval
 127 was non-zero, or no write permission where
 128 .I newval
 129 was non-zero.
 130 .SH "CONFORMING TO"
 131 This call is Linux specific, and should not be used in programs
 132 intended to be portable.
 133 A
 134 .BR sysctl ()
 135 call has been present in Linux since version 1.3.57.
 136 It originated in
 137 4.4BSD.
 138 Only Linux has the
 139 .I /proc/sys
 140 mirror, and the object naming schemes differ between Linux and 4.4BSD,
 141 but the declaration of the
 142 .BR sysctl (2)
 143 function is the same in both.
 144 .SH BUGS
 145 The object names vary between kernel versions.
 146 THIS MAKES THIS SYSTEM CALL WORTHLESS FOR APPLICATIONS.
 147 Use the
 148 .I /proc/sys
 149 interface instead.
 150 .br
 151 Not all available objects are properly documented.
 152 .br
 153 It is not yet possible to change operating system by writing to
 154 .IR /proc/sys/kernel/ostype .
 155 .SH "SEE ALSO"
 156 .BR proc (5)