_sysctl(2)

1SYSCTL(2)                  Linux Programmer's Manual                 SYSCTL(2)
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NAME

6       sysctl - read/write system parameters
7

SYNOPSIS

9       #include <unistd.h>
10       #include <linux/sysctl.h>
11
12       int _sysctl(struct __sysctl_args *args);
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DESCRIPTION

15       Do not use this system call!  See NOTES.
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17       The _sysctl() call reads and/or writes kernel parameters.  For example,
18       the hostname, or the maximum number of open files.   The  argument  has
19       the form
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21           struct __sysctl_args {
22               int    *name;    /* integer vector describing variable */
23               int     nlen;    /* length of this vector */
24               void   *oldval;  /* 0 or address where to store old value */
25               size_t *oldlenp; /* available room for old value,
26                                   overwritten by actual size of old value */
27               void   *newval;  /* 0 or address of new value */
28               size_t  newlen;  /* size of new value */
29           };
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31       This  call  does  a  search  in a tree structure, possibly resembling a
32       directory tree under /proc/sys, and if  the  requested  item  is  found
33       calls some appropriate routine to read or modify the value.
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RETURN VALUE

36       Upon successful completion, _sysctl() returns 0.  Otherwise, a value of
37       -1 is returned and errno is set to indicate the error.
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ERRORS

40       EFAULT The invocation asked for the previous value  by  setting  oldval
41              non-NULL, but allowed zero room in oldlenp.
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43       ENOTDIR
44              name was not found.
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46       EPERM  No  search  permission for one of the encountered "directories",
47              or no read permission where oldval was nonzero, or no write per‐
48              mission where newval was nonzero.
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CONFORMING TO

51       This  call  is  Linux-specific,  and  should  not  be  used in programs
52       intended to be portable.  A sysctl() call has  been  present  in  Linux
53       since  version  1.3.57.   It  originated in 4.4BSD.  Only Linux has the
54       /proc/sys mirror, and the object naming schemes  differ  between  Linux
55       and 4.4BSD, but the declaration of the sysctl() function is the same in
56       both.
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NOTES

59       Glibc does not provide a wrapper for this system call;  call  it  using
60       syscall(2).
61
62       Or  rather... don't call it: use of this system call has long been dis‐
63       couraged, and it is so unloved that it is  likely  to  disappear  in  a
64       future  kernel  version.   Remove  it  from  your programs now; use the
65       /proc/sys interface instead.
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BUGS

68       The object names vary between kernel versions, making this system  call
69       worthless for applications.
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71       Not all available objects are properly documented.
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73       It  is  not  yet  possible  to  change  operating  system by writing to
74       /proc/sys/kernel/ostype.
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EXAMPLE

77       #define _GNU_SOURCE
78       #include <unistd.h>
79       #include <sys/syscall.h>
80       #include <string.h>
81       #include <stdio.h>
82       #include <stdlib.h>
83       #include <linux/sysctl.h>
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85       int _sysctl(struct __sysctl_args *args );
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87       #define OSNAMESZ 100
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89       int
90       main(void)
91       {
92           struct __sysctl_args args;
93           char osname[OSNAMESZ];
94           size_t osnamelth;
95           int name[] = { CTL_KERN, KERN_OSTYPE };
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97           memset(&args, 0, sizeof(struct __sysctl_args));
98           args.name = name;
99           args.nlen = sizeof(name)/sizeof(name[0]);
100           args.oldval = osname;
101           args.oldlenp = &osnamelth;
102
103           osnamelth = sizeof(osname);
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105           if (syscall(SYS__sysctl, &args) == -1) {
106               perror("_sysctl");
107               exit(EXIT_FAILURE);
108           }
109           printf("This machine is running %*s\n", osnamelth, osname);
110           exit(EXIT_SUCCESS);
111       }
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COLOPHON

117       This page is part of release 3.25 of the Linux  man-pages  project.   A
118       description  of  the project, and information about reporting bugs, can
119       be found at http://www.kernel.org/doc/man-pages/.
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123Linux                             2008-11-20                         SYSCTL(2)