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 2007-06-01 "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 +0.25i
  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 (2)
 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 .SH BUGS
 110 The object names vary between kernel versions.
 111 .B This makes this system call worthless for applications.
 112 Use the
 113 .I /proc/sys
 114 interface instead.
 115 .PP
 116 Not all available objects are properly documented.
 117 .PP
 118 It is not yet possible to change operating system by writing to
 119 .IR /proc/sys/kernel/ostype .
 120 .SH EXAMPLE
 121 .nf
 122 #define _GNU_SOURCE
 123 #include <unistd.h>
 124 #include <sys/syscall.h>
 125 #include <string.h>
 126 #include <stdio.h>
 127 #include <stdlib.h>
 128 #include <linux/sysctl.h>
 129
 130 int _sysctl(struct __sysctl_args *args );
 131
 132 #define OSNAMESZ 100
 133
 134 int
 135 main(void)
 136 {
 137     struct __sysctl_args args;
 138     char osname[OSNAMESZ];
 139     size_t osnamelth;
 140     int name[] = { CTL_KERN, KERN_OSTYPE };
 141
 142     memset(&args, 0, sizeof(struct __sysctl_args));
 143     args.name = name;
 144     args.nlen = sizeof(name)/sizeof(name[0]);
 145     args.oldval = osname;
 146     args.oldlenp = &osnamelth;
 147
 148     osnamelth = sizeof(osname);
 149
 150     if (syscall(SYS__sysctl, &args) == \-1) {
 151         perror("_sysctl");
 152         exit(EXIT_FAILURE);
 153     }
 154     printf("This machine is running %*s\\n", osnamelth, osname);
 155     exit(EXIT_SUCCESS);
 156 }
 157 .fi
 158 .SH "SEE ALSO"
 159 .BR proc (5)