--- In [email protected], "Indika Bandara" <[EMAIL PROTECTED]>
wrote:
>
> here are my requirements
>
> very very simple indeed !
>
> client:
> send filename
> send filesize
> send file
>
> server:
> read fixed lenght of filename, filesize
> allocate space for file
> store file
> send some command back
>
> these tasks work fine on my loopback interface. since TCP
> doesnt lose them in a real network, hope i can assure that
> it works
>
> i use unsgined char buffers to read.
>
> eg.
> unsigned char *buffer=new unsigned char[size];
> bzero(buffer,size);
>
> i receive the files correctly.
>
> do i have to send starting/ending sequences to mark the
> start/end of packets.
>
> for example.
> after creating socket and calling connect();
>
> i call at client
> write(10 bytes);
> write(15);
>
> then at server i call
> read(10);
> read(15);
>
> will i get what i send? will the server not read any garbage?
Indika,
don't worry, TCP/IP will handle these issues for you. But -as usual
with read() and write()- you should always check that the amount of
bytes you receive during one read() call equals the amount of bytes
you expect.
So, in order to create a fairly reliable protocol, you should always
encapsulate the data (be it a protocol command or a single block of
data or whatever) in a packet that you can check for errors yourself;
for example, use the first four bytes (in a clearly defined order!) to
transfer the actual length of a block of data; after these four bytes
only send the actual data to transfer. So you will have to implement
two read()s:
1) if (read( fd, & Length_Buffer, sizeof( Length_Buffer)) != 4)
{ perror( "reading length bytes");
exit( EXIT_FAILURE);
}
2) after you have converted the four bytes stored in Length_Buffer to
a size_t value (let's name it Length), do this:
if (read( fd, & Buffer, Length) != Length)
{ perror( "reading actual contents");
exit( EXIT_FAILURE);
}
and so on. I hope you understand the general picture.
I suggest that you always exchnge data between server and client in
two distinct packets (and every packet should have its length
prefixing it, as mentioned above):
a) Example send data from client to server: the first packet sent from
client to server is a command packet, the server responds with an
acknowledgement packet; the second packet from client to server
contains the actual data plus a checksum (just for the paranoid); the
second packet from server to client is a response indicating whether
the checksum fitted or not.
b) Example receive data from server at client: the first packet sent
by the client is a Send request; the server then sends two packets,
first an acknowledgement (or a "I refuse!" packet in case the file is
not available on the server), the second contains the actual file plus
a checksum; the second packet from the client to the server then
states whether the file was received and whether the checksum was correct.
c) Example terminate connection: the client sends a packet to the
server, the server sends a packet back indicating whether it will
terminate the connection or not (if not, maybe with an indication as
to why it refuses to close the connection).
I hope you understand what I mean. If not, please ask again.
Regards,
Nico
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