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
15 This system call no longer exists on current kernels! See NOTES.
16 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 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 This call does a search in a tree structure, possibly resembling a di‐
32 rectory tree under /proc/sys, and if the requested item is found calls
33 some appropriate routine to read or modify the value.
34
36 Upon successful completion, _sysctl() returns 0. Otherwise, a value of
37 -1 is returned and errno is set to indicate the error.
38
40 EACCES, EPERM
41 No search permission for one of the encountered "directories",
42 or no read permission where oldval was nonzero, or no write per‐
43 mission where newval was nonzero.
44 EFAULT The invocation asked for the previous value by setting oldval
46 non-NULL, but allowed zero room in oldlenp.
47 ENOTDIR
49 name was not found.
50
52 This system call first appeared in Linux 1.3.57. It was removed in
53 Linux 5.5; glibc support was removed in version 2.32.
54
56 This call is Linux-specific, and should not be used in programs in‐
57 tended to be portable. It originated in 4.4BSD. Only Linux has the
58 /proc/sys mirror, and the object naming schemes differ between Linux
59 and 4.4BSD, but the declaration of the sysctl() function is the same in
60 both.
61
63 Use of this system call was long discouraged: since Linux 2.6.24, uses
64 of this system call result in warnings in the kernel log, and in Linux
65 5.5, the system call was finally removed. Use the /proc/sys interface
66 instead.
67 Note that on older kernels where this system call still exists, it is
69 available only if the kernel was configured with the CON‐
70 FIG_SYSCTL_SYSCALL option. Furthermore, glibc does not provide a wrap‐
71 per for this system call, necessitating the use of syscall(2).
72
74 The object names vary between kernel versions, making this system call
75 worthless for applications.
76 Not all available objects are properly documented.
78 It is not yet possible to change operating system by writing to
80 /proc/sys/kernel/ostype.
81
83 #define _GNU_SOURCE
84 #include <unistd.h>
85 #include <sys/syscall.h>
86 #include <string.h>
87 #include <stdio.h>
88 #include <stdlib.h>
89 #include <linux/sysctl.h>
90 int _sysctl(struct __sysctl_args *args );
92 #define OSNAMESZ 100
94 int
96 main(void)
97 {
98 struct __sysctl_args args;
99 char osname[OSNAMESZ];
100 size_t osnamelth;
101 int name[] = { CTL_KERN, KERN_OSTYPE };
102 memset(&args, 0, sizeof(args));
104 args.name = name;
105 args.nlen = sizeof(name)/sizeof(name[0]);
106 args.oldval = osname;
107 args.oldlenp = &osnamelth;
108 osnamelth = sizeof(osname);
110 if (syscall(SYS__sysctl, &args) == -1) {
112 perror("_sysctl");
113 exit(EXIT_FAILURE);
114 }
115 printf("This machine is running %*s\n", osnamelth, osname);
116 exit(EXIT_SUCCESS);
117 }
118
120 proc(5)
121
123 This page is part of release 5.12 of the Linux man-pages project. A
124 description of the project, information about reporting bugs, and the
125 latest version of this page, can be found at
126 https://www.kernel.org/doc/man-pages/.
127Linux 2021-03-22 SYSCTL(2)