rtime - get time from a remote machine

       For an explanation of the terms used in this section, see attributes(7).
       ┌─────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │ Interface                                                                   │ Attribute     │ Value   │
       ├─────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │ rtime()                                                                     │ Thread safety │ MT-Safe │
       └─────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

rtime() in glibc 2.2.5 and earlier does not work properly on 64-bit machines.

       This  function  uses  the  Time  Server Protocol as described in RFC 868 to obtain the time from a remote
       machine.

       The Time Server Protocol gives the time in seconds since 00:00:00 UTC, 1  Jan  1900,  and  this  function
       subtracts  the appropriate constant in order to convert the result to seconds since the Epoch, 1970-01-01
       00:00:00 +0000 (UTC).

       When timeout is non-NULL, the udp/time socket (port 37) is used.  Otherwise, the  tcp/time  socket  (port
       37) is used.

       All errors for underlying functions (sendto(2), poll(2), recvfrom(2),  connect(2),  read(2))  can  occur.
       Moreover:

       EIO    The number of returned bytes is not 4.

       ETIMEDOUT
              The waiting time as defined in timeout has expired.

       This  example  requires  that  port  37  is  up  and  open.   You  may  check  that the time entry within
       /etc/inetd.conf is not commented out.

       The program connects to a computer called "linux".  Using "localhost" does not work.  The result  is  the
       localtime of the computer "linux".

       #include <errno.h>
       #include <netdb.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <string.h>
       #include <time.h>

       #include <rpc/auth_des.h>

       static int use_tcp = 0;
       static const char servername[] = "linux";

       int
       main(void)
       {
           int                 ret;
           time_t              t;
           struct hostent      *hent;
           struct rpc_timeval  time1 = {0, 0};
           struct rpc_timeval  timeout = {1, 0};
           struct sockaddr_in  name;

           memset(&name, 0, sizeof(name));
           sethostent(1);
           hent = gethostbyname(servername);
           memcpy(&name.sin_addr, hent->h_addr, hent->h_length);

           ret = rtime(&name, &time1, use_tcp ? NULL : &timeout);
           if (ret < 0)
               perror("rtime error");
           else {
               t = time1.tv_sec;
               printf("%s\n", ctime(&t));
           }

           exit(EXIT_SUCCESS);
       }

       Standard C library (libc, -lc)

       rtime - get time from a remote machine

       Only IPv4 is supported.

       Some in.timed versions support only TCP.  Try the example program with use_tcp set to 1.

       On  success,  0  is  returned, and the obtained 32-bit time value is stored in timep->tv_sec.  In case of
       error -1 is returned, and errno is set to indicate the error.

ntpdate(1), inetd(8)

Linux man-pages 6.9.1                              2024-06-15                                           rtime(3)

#include<rpc/auth_des.h>intrtime(structsockaddr_in*addrp,structrpc_timeval*timep,structrpc_timeval*timeout);