|
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
|
|
|
SYSCTL(2) Linux Programmer's Manual SYSCTL(2)
sysctl - read/write system parameters
#include <unistd.h>
#include <linux/sysctl.h>
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; /* length 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.
This system call first appeared in Linux 1.3.57. It was removed
in Linux 5.5; glibc support was removed in version 2.32.
This call is Linux-specific, and should not be used in programs
intended to be portable. 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 <unistd.h>
#include <sys/syscall.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <linux/sysctl.h>
int _sysctl(struct __sysctl_args *args );
#define OSNAMESZ 100
int
main(void)
{
struct __sysctl_args args;
char osname[OSNAMESZ];
size_t osnamelth;
int name[] = { CTL_KERN, KERN_OSTYPE };
memset(&args, 0, sizeof(args));
args.name = name;
args.nlen = sizeof(name)/sizeof(name[0]);
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", osnamelth, osname);
exit(EXIT_SUCCESS);
}
proc(5)
This page is part of release 5.13 of the Linux man-pages project.
A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2021-03-22 SYSCTL(2)
Pages that refer to this page: syscalls(2), proc(5), sctp(7)
Copyright and license for this manual page
|
HTML rendering created 2022-12-18 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
|