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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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sysctl(2) System Calls Manual sysctl(2)
sysctl - read/write system parameters
#include <unistd.h>
#include <linux/sysctl.h>
[[deprecated]] int _sysctl(struct __sysctl_args *args);
This system call no longer exists on current kernels! See NOTES.
The _sysctl() call reads and/or writes kernel parameters. For
example, the hostname, or the maximum number of open files. The
argument has the form
struct __sysctl_args {
int *name; /* integer vector describing variable */
int nlen; /* number of elements of this vector */
void *oldval; /* 0 or address where to store old value */
size_t *oldlenp; /* available room for old value,
overwritten by actual size of old value */
void *newval; /* 0 or address of new value */
size_t newlen; /* size of new value */
};
This call does a search in a tree structure, possibly resembling a
directory tree under /proc/sys, and if the requested item is found
calls some appropriate routine to read or modify the value.
Upon successful completion, _sysctl() returns 0. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
EACCES
EPERM No search permission for one of the encountered
"directories", or no read permission where oldval was
nonzero, or no write permission where newval was nonzero.
EFAULT The invocation asked for the previous value by setting
oldval non-NULL, but allowed zero room in oldlenp.
ENOTDIR
name was not found.
Linux.
Linux 1.3.57. Removed in Linux 5.5, glibc 2.32.
It originated in 4.4BSD. Only Linux has the /proc/sys mirror, and
the object naming schemes differ between Linux and 4.4BSD, but the
declaration of the sysctl() function is the same in both.
Use of this system call was long discouraged: since Linux 2.6.24,
uses of this system call result in warnings in the kernel log, and
in Linux 5.5, the system call was finally removed. Use the
/proc/sys interface instead.
Note that on older kernels where this system call still exists, it
is available only if the kernel was configured with the
CONFIG_SYSCTL_SYSCALL option. Furthermore, glibc does not provide
a wrapper for this system call, necessitating the use of
syscall(2).
The object names vary between kernel versions, making this system
call worthless for applications.
Not all available objects are properly documented.
It is not yet possible to change operating system by writing to
/proc/sys/kernel/ostype.
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <linux/sysctl.h>
#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
int _sysctl(struct __sysctl_args *args);
#define OSNAMESZ 100
int
main(void)
{
int name[] = { CTL_KERN, KERN_OSTYPE };
char osname[OSNAMESZ];
size_t osnamelth;
struct __sysctl_args args;
memset(&args, 0, sizeof(args));
args.name = name;
args.nlen = ARRAY_SIZE(name);
args.oldval = osname;
args.oldlenp = &osnamelth;
osnamelth = sizeof(osname);
if (syscall(SYS__sysctl, &args) == -1) {
perror("_sysctl");
exit(EXIT_FAILURE);
}
printf("This machine is running %*s\n", (int) osnamelth, osname);
exit(EXIT_SUCCESS);
}
proc(5)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sysctl(2)
Pages that refer to this page: syscalls(2), proc_sys(5), sctp(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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