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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | 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);
Note: There is no glibc wrapper for this system call; see NOTES.
Do not use this system call! 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 call is Linux-specific, and should not be used in programs
intended to be portable. A sysctl() call has been present in Linux
since version 1.3.57. 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.
Glibc does not provide a wrapper for this system call; call it using
syscall(2). Or rather... don't call it: use of this system call has
long been discouraged, and it is so unloved that it is likely to
disappear in a future kernel version. Since Linux 2.6.24, uses of
this system call result in warnings in the kernel log. Remove it
from your programs now; use the /proc/sys interface instead.
This system call is available only if the kernel was configured with
the CONFIG_SYSCTL_SYSCALL option.
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(struct __sysctl_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 4.12 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 2016-10-08 SYSCTL(2)
Pages that refer to this page: syscalls(2), proc(5), sctp(7)