From: "Y.K. Goh" <s6606555 AT np DOT edu DOT sg>
Sender: "Y.K. Goh" <s6606555 AT npnsnews>
Subject: Problem with socket programming
Newsgroups: comp.os.msdos.djgpp
X-User-Info: 153.20.61.145    s6606555 npnsnews 
NNTP-Posting-Host: 153.20.15.92
Message-ID: <3626a63b.0@nsuxnews>
Date: 16 Oct 1998 09:49:47 +0800
X-Trace: 16 Oct 1998 09:49:47 +0800, 153.20.15.92
Lines: 38
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp
Reply-To: djgpp AT delorie DOT com


I use the following code segment for my project which is a simple file transfer program and I experience some weird problem -- the last 2 bytes of the packet are always corrupted. It goes away when I add 2 to the total packet size. The problem is why is the packet corrupted and although the problem goes away when I add 2 to packet size, I don't understand why there is a need to add 2.

Does anyone know the problem?

--------------------------------------------
dp.packet.opcode = DATA;
dp.packet.blockno = blockno;
dp.packet.comp = ~blockno;
dp.packet.checksum = checksum(dp.packet.data, bytes_read);

dgram_send(ss, dp.buf, bytes_read + (2 * sizeof(unsigned short)) + (2 * sizeof(unsigned int)) + 2); <== added 2 to packet size and the problem goes away. But why??

printf("Sent data packet %5u (%3d bytes, %5u).\n", blockno, bytes_read, dp.packet.checksum);

--------------------------------------------------------
dgram_send function

void dgram_send(int ss, char * sbuf, int length)
{
  sendto(ss, sbuf, length, 0, (struct sockaddr*)&from, sizeof(from));
}
-----------------------------------------------
The Data Packet definition

union DATAPacket
{
  struct
  {
    unsigned short opcode;
    unsigned int blockno;
    unsigned int comp;
    unsigned short checksum;
    char data[BUFSIZE];
  } packet;
  char buf[524];
};
----------------------------------------