sysctl — read/write system parameters
#include <unistd.h> #include <linux/unistd.h> #include <linux/sysctl.h> #include <errno.h> _syscall1(int, _sysctl, struct __sysctl_args *, args) /* Using syscall(2) may be preferable; see intro(2) */
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.
#include <linux/unistd.h> #include <linux/types.h> #include <linux/sysctl.h> _syscall1(int, _sysctl, struct __sysctl_args *, args); int sysctl(int *name, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen) { struct __sysctl_args args = { name, nlen, oldval, oldlenp, newval, newlen }; return _sysctl(&args); } #define SIZE(x) sizeof(x)/sizeof(x[0]) #define OSNAMESZ 100 char osname[OSNAMESZ]; int osnamelth; int name[] = { CTL_KERN, KERN_OSTYPE }; int main(void) { osnamelth = sizeof(osname); if (sysctl(name, SIZE(name), osname, &osnamelth, 0, 0)) perror("sysctl"); else printf("This machine is running %*s\n", osnamelth, osname); return 0; }
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
.
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