man2/sysctl.2

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