#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.
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. Remove it from your programs now; 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); }