sysctl方式获得unix系统的信息

sysctl方式获得unix系统的release版本号。因为sysctl的release版本号是不能被更改的，所以通过这种方式获得的信息具有确定性。
#include <stdio.h>
#include <sys/types.h>
#include <sys/sysctl.h>
#include <errno.h>
#include <stdlib.h>
int main()
{
        char *releasename = NULL;
        size_t len;
        int mib[2];
        mib[0] = CTL_KERN;
        mib[1] = KERN_OSRELEASE;

        if (0 != sysctl(mib, 2, NULL, &len, NULL, 0)){
                printf("errno is %d/n",errno);
        }
        releasename = (char *)malloc(len);
        if (NULL == releasename){
                printf("errno is %d/n",errno);
        }
        if (0 != sysctl(mib,2,releasename,&len,NULL,0)){
                printf("error is %d/n",errno);
        }
        printf("hw_model:%s/n",releasename);
        if (NULL != releasename){
                free(releasename);
                releasename = NULL;
        }
}