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Subject:	sending an empty UDP packet.
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From:	Tanaka Akira <akr AT m17n DOT org>
Date:	Fri, 30 Jun 2006 19:18:05 +0900
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Hi.  I have a trouble to send an empty UDP packet.

$ uname -a
CYGWIN_NT-5.1 D2RP7F1X 1.5.19(0.150/4/2) 2006-01-20 13:28 i686 Cygwin

I run a UDP server (written in Ruby) on port 8888 as follows
on a terminal:

$ ~/ruby/bin/ruby -rsocket -ve 's = UDPSocket.new; s.bind("0.0.0.0", 8888); loop { p s.recv(100) }'
ruby 1.9.0 (2006-06-08) [i386-cygwin]

I compiled following UDP client on another terminal.

$ cat send.c 
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int main(int argc, char **argv)
{
  int s;
  int ret;
  char *buf;
  struct sockaddr_in addr;

  if (argc != 4) {
    puts("usage: send ipaddr port data");
    exit(1);
  }

  addr.sin_family = AF_INET;
  ret = inet_aton(argv[1], &addr.sin_addr);
  if (ret == 0) { perror("inet_aton"); exit(1); }
  addr.sin_port = htons(atoi(argv[2]));
  buf = argv[3];

  s = socket(PF_INET, SOCK_DGRAM, 0);
  if (s == -1) { perror("socket"); exit(1); }

  ret = sendto(s, buf, strlen(buf), 0, (struct sockaddr *)&addr, sizeof(addr));
  if (ret == -1) { perror("sendto"); exit(1); }

  return 0;
}

$ gcc -o send.exe send.c

I expect send.exe can send an empty packet as follows.
But the server receive no data.  The server prints nothing.

$ ./send.exe 127.0.0.1 8888 ''
$ ./send.exe 127.0.0.1 8888 ''
$ ./send.exe 127.0.0.1 8888 ''
$ ./send.exe 127.0.0.1 8888 ''

When I use send.exe to send 1, 2 and 3-bytes data, it works
fine.  The server prints "a", "ab", "abc".

$ ./send.exe 127.0.0.1 8888 a
$ ./send.exe 127.0.0.1 8888 ab
$ ./send.exe 127.0.0.1 8888 abc

I can send an empty UDP packet using a mswin32 (non-cygwin)
ruby as follows.  The server prints "".
(The server still runs by cygwin ruby.  cygwin ruby can
receive empty UDP packet.)

$ ~/19/bin/ruby -rsocket -ve 'UDPSocket.new.send("", 0, "127.0.0.1", 8888)'
ruby 1.9.0 (2005-07-22) [i386-mswin32]

But I cannot send an empty UDP packet.

$ ~/ruby/bin/ruby -rsocket -ve 'UDPSocket.new.send("", 0, "127.0.0.1", 8888)'
ruby 1.9.0 (2006-06-08) [i386-cygwin]

Is there a way to send an empty UDP packet from cygwin?
-- 
Tanaka Akira

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