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