sysctl(2)

1sysctl(2)                     System Calls Manual                    sysctl(2)
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NAME

6       sysctl - read/write system parameters
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SYNOPSIS

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

15       This system call no longer exists on current kernels!  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 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.
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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       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.
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45       EFAULT The invocation asked for the previous value  by  setting  oldval
46              non-NULL, but allowed zero room in oldlenp.
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48       ENOTDIR
49              name was not found.
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STANDARDS

52       Linux.
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HISTORY

55       Linux 1.3.57.  Removed in Linux 5.5, glibc 2.32.
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57       It  originated in 4.4BSD.  Only Linux has the /proc/sys mirror, and the
58       object naming schemes differ between Linux and 4.4BSD, but the declara‐
59       tion of the sysctl() function is the same in both.
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NOTES

62       Use  of this system call was long discouraged: since Linux 2.6.24, uses
63       of this system call result in warnings in the kernel log, and in  Linux
64       5.5,  the system call was finally removed.  Use the /proc/sys interface
65       instead.
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67       Note that on older kernels where this system call still exists,  it  is
68       available   only   if   the   kernel   was  configured  with  the  CON‐
69       FIG_SYSCTL_SYSCALL option.  Furthermore, glibc does not provide a wrap‐
70       per for this system call, necessitating the use of syscall(2).
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BUGS

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

82       #define _GNU_SOURCE
83       #include <stdio.h>
84       #include <stdlib.h>
85       #include <string.h>
86       #include <sys/syscall.h>
87       #include <unistd.h>
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89       #include <linux/sysctl.h>
90
91       #define ARRAY_SIZE(arr)  (sizeof(arr) / sizeof((arr)[0]))
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93       int _sysctl(struct __sysctl_args *args);
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95       #define OSNAMESZ 100
96
97       int
98       main(void)
99       {
100           int                   name[] = { CTL_KERN, KERN_OSTYPE };
101           char                  osname[OSNAMESZ];
102           size_t                osnamelth;
103           struct __sysctl_args  args;
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105           memset(&args, 0, sizeof(args));
106           args.name = name;
107           args.nlen = ARRAY_SIZE(name);
108           args.oldval = osname;
109           args.oldlenp = &osnamelth;
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111           osnamelth = sizeof(osname);
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113           if (syscall(SYS__sysctl, &args) == -1) {
114               perror("_sysctl");
115               exit(EXIT_FAILURE);
116           }
117           printf("This machine is running %*s\n", (int) osnamelth, osname);
118           exit(EXIT_SUCCESS);
119       }
120