gethostbyname和gethostbyaddr

#include <netdb.h>

struct hostent *gethostbyname(const char *ghostname);

    返回：成功返回非空指针，出错为NULL且设置h_errno

struct hostent {
    char  *h_name;        /*  official name of host */
    char  **h_aliases;    /*  pointer to array of pointers to alias name */
    int     h_addrtype;   /*  host address type: AF_INET */
    int     h_length;     /*  length of address: 4 */
    char  **h_addr_list;  /*  ptr to array of ptrs with IPv4 addrs */              
}; 

#include    <stdio.h>
#include    <netdb.h>
#include    <stdlib.h>
#include    <sys/socket.h>
#include    <arpa/inet.h>

#define     INET_ADDRSTRLEN     16

void err_ret(const char *, ...);
void err_msg(const char *, ...);

int main(int argc, char **argv)
{
    char    *ptr, **pptr;
    char    str[INET_ADDRSTRLEN];
    struct hostent    *hptr;

    while (--argc > 0) {
        ptr = *++argv;
        if ( (hptr = gethostbyname(ptr)) == NULL) {
            err_msg("gethostbyname error for host: %s: %s",
                ptr, hstrerror(h_errno));
            continue;
        }
        printf("official hostname: %s\n", hptr->h_name);

        for (pptr = hptr->h_aliases; *pptr != NULL; pptr++) {
            printf("\talias: %s\n", *pptr);
        }

        switch (hptr->h_addrtype) {
            case AF_INET: {
                pptr = hptr->h_addr_list;
                for ( ; *pptr != NULL; pptr++) {
                    printf("\taddress: %s\n",
                        inet_ntop(hptr->h_addrtype, *pptr, str, sizeof(str)));
                }
                break;
            }
            default: {
                err_ret("unknown address type");
                break;
            }
        }
    }
    exit(0);
}

#include    <stdio.h>
#include    <errno.h>
#include    <stdlib.h>
#include    <string.h>
#include    <stdarg.h>        /* ANSI C header file */
#include    <syslog.h>        /* for syslog() */

#define     MAXLINE     4096

int        daemon_proc;        /* set nonzero by daemon_init() */

static void    err_doit(int, int, const char *, va_list);

/* Nonfatal error related to system call
 * Print message and return */

void err_ret(const char *fmt, ...) {

    va_list        ap;    

    va_start(ap, fmt);    
    err_doit(1, LOG_INFO, fmt, ap);
    va_end(ap);
    return;
}

/* Fatal error related to system call
 * Print message and terminate */

void err_sys(const char *fmt, ...) {

    va_list        ap;

    va_start(ap, fmt);
    err_doit(1, LOG_ERR, fmt, ap);
    va_end(ap);
    exit(1);
}

/* Fatal error related to system call
 * Print message, dump core, and terminate */

void err_dump(const char *fmt, ...) {
    va_list        ap;

    va_start(ap, fmt);
    err_doit(1, LOG_ERR, fmt, ap);
    va_end(ap);
    abort();        /* dump core and terminate */
    exit(1);        /* shouldn‘t get here */
}

/* Nonfatal error unrelated to system call
 * Print message and return */

void err_msg(const char *fmt, ...) {

    va_list        ap;

    va_start(ap, fmt);
    err_doit(0, LOG_INFO, fmt, ap);
    va_end(ap);
    return;
}

/* Fatal error unrelated to system call
 * Print message and terminate */

void err_quit(const char *fmt, ...) {

    va_list        ap;

    va_start(ap, fmt);
    err_doit(0, LOG_ERR, fmt, ap);
    va_end(ap);
    exit(1);
}

/* Print message and return to caller
 * Caller specifies "errnoflag" and "level" */

static void err_doit(int errnoflag, int level, const char *fmt, va_list ap) {

    int        errno_save, n;
    char    buf[MAXLINE + 1];

    errno_save = errno;        /* value caller might want printed */
#ifdef    HAVE_VSNPRINTF
    vsnprintf(buf, MAXLINE, fmt, ap);    /* safe */
#else
    vsprintf(buf, fmt, ap);                /* not safe */
#endif
    n = strlen(buf);
    if (errnoflag)
        snprintf(buf + n, MAXLINE - n, ": %s", strerror(errno_save));
    strcat(buf, "\n");

    if (daemon_proc) {
        syslog(level, buf);
    } else {
        fflush(stdout);        /* in case stdout and stderr are the same */
        fputs(buf, stderr);
        fflush(stderr);
    }
    return;
}

#include <netdb.h>

struct hostent *gethostbyaddr(const struct in_addr *, socklen_t, int family);

    返回：成功为非空指针，否则为NULL且设置h_errno
    功能：由一个二进制IP地址找到相应的主机名