_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/unistd.h>
11       #include <linux/sysctl.h>
12       #include <errno.h>
13
14       _syscall1(int, _sysctl, struct __sysctl_args *, args)
15                 /* Using syscall(2) may be preferable; see intro(2) */
16
17       int _sysctl(struct __sysctl_args *args);
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DESCRIPTION

20       The  _sysctl() call reads and/or writes kernel parameters. For example,
21       the hostname, or the maximum number of open files. The argument has the
22       form
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24       struct __sysctl_args {
25               int *name;        /* integer vector describing variable */
26               int nlen;         /* length of this vector */
27               void *oldval;     /* 0 or address where to store old value */
28               size_t *oldlenp;  /* available room for old value,
29                                    overwritten by actual size of old value */
30               void *newval;     /* 0 or address of new value */
31               size_t newlen;    /* size of new value */
32       };
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34       This  call  does  a  search  in a tree structure, possibly resembling a
35       directory tree under /proc/sys, and if  the  requested  item  is  found
36       calls some appropriate routine to read or modify the value.
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38

EXAMPLE

40       #include <linux/unistd.h>
41       #include <linux/types.h>
42       #include <linux/sysctl.h>
43
44       _syscall1(int, _sysctl, struct __sysctl_args *, args);
45       int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp,
46                  void *newval, size_t newlen)
47       {
48               struct __sysctl_args args={name,nlen,oldval,oldlenp,newval,newlen};
49               return _sysctl(&args);
50       }
51
52       #define SIZE(x) sizeof(x)/sizeof(x[0])
53       #define OSNAMESZ 100
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55       char osname[OSNAMESZ];
56       int osnamelth;
57       int name[] = { CTL_KERN, KERN_OSTYPE };
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59       main(){
60               osnamelth = sizeof(osname);
61               if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0))
62                       perror("sysctl");
63               else
64                       printf("This machine is running %*s\n", osnamelth, osname);
65               return 0;
66       }
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RETURN VALUE

70       Upon  successful completion, _sysctl() returns 0. Otherwise, a value of
71       -1 is returned and errno is set to indicate the error.
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ERRORS

74       EFAULT The invocation asked for the previous value  by  setting  oldval
75              non-NULL, but allowed zero room in oldlenp.
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77       ENOTDIR
78              name was not found.
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80       EPERM  No  search  permission for one of the encountered `directories',
81              or no read permission where oldval was  non-zero,  or  no  write
82              permission where newval was non-zero.
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CONFORMING TO

85       This  call  is  Linux  specific,  and  should  not  be used in programs
86       intended to be portable.  A sysctl() call has  been  present  in  Linux
87       since  version  1.3.57.  It  originated  in  4.4BSD. Only Linux has the
88       /proc/sys mirror, and the object naming schemes  differ  between  Linux
89       and  4.4BSD,  but the declaration of the sysctl(2) function is the same
90       in both.
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BUGS

93       The object names vary between kernel versions.  THIS MAKES THIS  SYSTEM
94       CALL WORTHLESS FOR APPLICATIONS.  Use the /proc/sys interface instead.
95       Not all available objects are properly documented.
96       It  is  not  yet  possible  to  change  operating  system by writing to
97       /proc/sys/kernel/ostype.
98