1SYSCTL(2)                  Linux Programmer's Manual                 SYSCTL(2)
2
3
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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);
13

DESCRIPTION

15       Do not use this system call!  See NOTES.
16
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
20
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           };
30
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.
34

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.
38

ERRORS

40       EFAULT The invocation asked for the previous value  by  setting  oldval
41              non-NULL, but allowed zero room in oldlenp.
42
43       ENOTDIR
44              name was not found.
45
46       EPERM  No  search  permission for one of the encountered "directories",
47              or no read permission where oldval was  non-zero,  or  no  write
48              permission where newval was non-zero.
49

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.
57

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.
66

BUGS

68       The object names vary between kernel versions, making this system  call
69       worthless for applications.
70
71       Not all available objects are properly documented.
72
73       It  is  not  yet  possible  to  change  operating  system by writing to
74       /proc/sys/kernel/ostype.
75

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>
84
85       int _sysctl(struct __sysctl_args *args );
86
87       #define OSNAMESZ 100
88
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 };
96
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);
104
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       }
112

SEE ALSO

114       proc(5)
115

COLOPHON

117       This page is part of release 3.22 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)
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