1SYSCTL(2) Linux Programmer's Manual SYSCTL(2)
2
6 sysctl - read/write system parameters
7
9 #include <unistd.h>
10 #include <linux/sysctl.h>
11 int _sysctl(struct __sysctl_args *args);
13 Note: There is no glibc wrapper for this system call; see NOTES.
15
17 Do not use this system call! See NOTES.
18 The _sysctl() call reads and/or writes kernel parameters. For example,
20 the hostname, or the maximum number of open files. The argument has
21 the form
22 struct __sysctl_args {
24 int *name; /* integer vector describing variable */
25 int nlen; /* length of this vector */
26 void *oldval; /* 0 or address where to store old value */
27 size_t *oldlenp; /* available room for old value,
28 overwritten by actual size of old value */
29 void *newval; /* 0 or address of new value */
30 size_t newlen; /* size of new value */
31 };
32 This call does a search in a tree structure, possibly resembling a
34 directory tree under /proc/sys, and if the requested item is found
35 calls some appropriate routine to read or modify the value.
36
38 Upon successful completion, _sysctl() returns 0. Otherwise, a value of
39 -1 is returned and errno is set to indicate the error.
40
42 EACCES, EPERM
43 No search permission for one of the encountered "directories",
44 or no read permission where oldval was nonzero, or no write per‐
45 mission where newval was nonzero.
46 EFAULT The invocation asked for the previous value by setting oldval
48 non-NULL, but allowed zero room in oldlenp.
49 ENOTDIR
51 name was not found.
52
54 This call is Linux-specific, and should not be used in programs
55 intended to be portable. A sysctl() call has been present in Linux
56 since version 1.3.57. It originated in 4.4BSD. Only Linux has the
57 /proc/sys mirror, and the object naming schemes differ between Linux
58 and 4.4BSD, but the declaration of the sysctl() function is the same in
59 both.
60
62 Glibc does not provide a wrapper for this system call; call it using
63 syscall(2). Or rather... don't call it: use of this system call has
64 long been discouraged, and it is so unloved that it is likely to disap‐
65 pear in a future kernel version. Since Linux 2.6.24, uses of this sys‐
66 tem call result in warnings in the kernel log. Remove it from your
67 programs now; use the /proc/sys interface instead.
68 This system call is available only if the kernel was configured with
70 the CONFIG_SYSCTL_SYSCALL option.
71
73 The object names vary between kernel versions, making this system call
74 worthless for applications.
75 Not all available objects are properly documented.
77 It is not yet possible to change operating system by writing to
79 /proc/sys/kernel/ostype.
80
82 #define _GNU_SOURCE
83 #include <unistd.h>
84 #include <sys/syscall.h>
85 #include <string.h>
86 #include <stdio.h>
87 #include <stdlib.h>
88 #include <linux/sysctl.h>
89 int _sysctl(struct __sysctl_args *args );
91 #define OSNAMESZ 100
93 int
95 main(void)
96 {
97 struct __sysctl_args args;
98 char osname[OSNAMESZ];
99 size_t osnamelth;
100 int name[] = { CTL_KERN, KERN_OSTYPE };
101 memset(&args, 0, sizeof(struct __sysctl_args));
103 args.name = name;
104 args.nlen = sizeof(name)/sizeof(name[0]);
105 args.oldval = osname;
106 args.oldlenp = &osnamelth;
107 osnamelth = sizeof(osname);
109 if (syscall(SYS__sysctl, &args) == -1) {
111 perror("_sysctl");
112 exit(EXIT_FAILURE);
113 }
114 printf("This machine is running %*s\n", osnamelth, osname);
115 exit(EXIT_SUCCESS);
116 }
117
119 proc(5)
120
122 This page is part of release 5.04 of the Linux man-pages project. A
123 description of the project, information about reporting bugs, and the
124 latest version of this page, can be found at
125 https://www.kernel.org/doc/man-pages/.
126Linux 2019-03-06 SYSCTL(2)