sysctl — read/write system parameters
#include <unistd.h> #include <linux/sysctl.h>
int
_sysctl( |
struct __sysctl_args * | args) ; |
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.
The invocation asked for the previous value by
setting oldval
non-NULL, but
allowed zero room in oldlenp
.
name
was
not found.
No search permission for one of the encountered
`directories', or no read permission where oldval
was non-zero, or
no write permission where newval
was
non-zero.
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(2) function is the
same in both.
The object names vary between kernel versions.
This makes this system call
worthless for applications. Use the /proc/sys
interface instead.
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); }
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