#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     16void 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     4096int        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_VSNPRINTFvsnprintf(buf, MAXLINE, fmt, ap);    /* safe */
#elsevsprintf(buf, fmt, ap);                /* not safe */
#endifn = 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