man2/sysctl.2

   1 .\" Copyright (C) 1996 Andries Brouwer (aeb@cwi.nl)
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  25 .\" Written 11 April 1996 by Andries Brouwer <aeb@cwi.nl>
  26 .\" 960412: Added comments from Stephen Tweedie
  27 .\" Modified Tue Oct 22 22:28:41 1996 by Eric S. Raymond <esr@thyrsus.com>
  28 .\" Modified Mon Jan  5 20:31:04 1998 by aeb.
  29 .\"
  30 .TH SYSCTL 2 2019-03-06 "Linux" "Linux Programmer's Manual"
  31 .SH NAME
  32 sysctl \- read/write system parameters
  33 .SH SYNOPSIS
  34 .nf
  35 .B #include <unistd.h>
  36 .B #include <linux/sysctl.h>
  37 .PP
  38 .BI "int _sysctl(struct __sysctl_args *" args );
  39 .fi
  40 .PP
  41 .IR Note :
  42 There is no glibc wrapper for this system call; see NOTES.
  43 .SH DESCRIPTION
  44 .B Do not use this system call!
  45 See NOTES.
  46 .PP
  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 .in +4n
  55 .EX
  56 struct __sysctl_args {
  57     int    *name;    /* integer vector describing variable */
  58     int     nlen;    /* length of this vector */
  59     void   *oldval;  /* 0 or address where to store old value */
  60     size_t *oldlenp; /* available room for old value,
  61                         overwritten by actual size of old value */
  62     void   *newval;  /* 0 or address of new value */
  63     size_t  newlen;  /* size of new value */
  64 };
  65 .EE
  66 .in
  67 .PP
  68 This call does a search in a tree structure, possibly resembling
  69 a directory tree under
  70 .IR /proc/sys ,
  71 and if the requested item is found calls some appropriate routine
  72 to read or modify the value.
  73 .SH RETURN VALUE
  74 Upon successful completion,
  75 .BR _sysctl ()
  76 returns 0.
  77 Otherwise, a value of \-1 is returned and
  78 .I errno
  79 is set to indicate the error.
  80 .SH ERRORS
  81 .TP
  82 .BR EACCES ", " EPERM
  83 No search permission for one of the encountered "directories",
  84 or no read permission where
  85 .I oldval
  86 was nonzero, or no write permission where
  87 .I newval
  88 was nonzero.
  89 .TP
  90 .B EFAULT
  91 The invocation asked for the previous value by setting
  92 .I oldval
  93 non-NULL, but allowed zero room in
  94 .IR oldlenp .
  95 .TP
  96 .B ENOTDIR
  97 .I name
  98 was not found.
  99 .SH CONFORMING TO
 100 This call is Linux-specific, and should not be used in programs
 101 intended to be portable.
 102 A
 103 .BR sysctl ()
 104 call has been present in Linux since version 1.3.57.
 105 It originated in
 106 4.4BSD.
 107 Only Linux has the
 108 .I /proc/sys
 109 mirror, and the object naming schemes differ between Linux and 4.4BSD,
 110 but the declaration of the
 111 .BR sysctl ()
 112 function is the same in both.
 113 .SH NOTES
 114 Glibc does not provide a wrapper for this system call; call it using
 115 .BR syscall (2).
 116 Or rather...
 117 .I don't
 118 call it:
 119 use of this system call has long been discouraged,
 120 and it is so unloved that
 121 \fBit is likely to disappear in a future kernel version\fP.
 122 .\" See http://lwn.net/Articles/247243/
 123 Since Linux 2.6.24,
 124 uses of this system call result in warnings in the kernel log.
 125 .\" Though comments in suggest that it is needed by old glibc binaries,
 126 .\" so maybe it's not going away.
 127 Remove it from your programs now; use the
 128 .I /proc/sys
 129 interface instead.
 130 .PP
 131 This system call is available only if the kernel was configured with the
 132 .B CONFIG_SYSCTL_SYSCALL
 133 option.
 134 .SH BUGS
 135 The object names vary between kernel versions,
 136 making this system call worthless for applications.
 137 .PP
 138 Not all available objects are properly documented.
 139 .PP
 140 It is not yet possible to change operating system by writing to
 141 .IR /proc/sys/kernel/ostype .
 142 .SH EXAMPLE
 143 .EX
 144 #define _GNU_SOURCE
 145 #include <unistd.h>
 146 #include <sys/syscall.h>
 147 #include <string.h>
 148 #include <stdio.h>
 149 #include <stdlib.h>
 150 #include <linux/sysctl.h>
 151
 152 int _sysctl(struct __sysctl_args *args );
 153
 154 #define OSNAMESZ 100
 155
 156 int
 157 main(void)
 158 {
 159     struct __sysctl_args args;
 160     char osname[OSNAMESZ];
 161     size_t osnamelth;
 162     int name[] = { CTL_KERN, KERN_OSTYPE };
 163
 164     memset(&args, 0, sizeof(struct __sysctl_args));
 165     args.name = name;
 166     args.nlen = sizeof(name)/sizeof(name[0]);
 167     args.oldval = osname;
 168     args.oldlenp = &osnamelth;
 169
 170     osnamelth = sizeof(osname);
 171
 172     if (syscall(SYS__sysctl, &args) == \-1) {
 173         perror("_sysctl");
 174         exit(EXIT_FAILURE);
 175     }
 176     printf("This machine is running %*s\en", osnamelth, osname);
 177     exit(EXIT_SUCCESS);
 178 }
 179 .EE
 180 .SH SEE ALSO
 181 .BR proc (5)