Re: sending images over a socket
From: Joaquin López (lsanchez_at_lsi.uji.es)
Date: 12/09/03
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Date: Tue, 09 Dec 2003 18:17:35 +0100
Lew Pitcher wrote:
> On Mon, 8 Dec 2003 17:14:30 +0100, "Joaquin López" <lsanchez@lsi.uji.es>
> wrote:
>
>>Hello, I'm writing a program for sending images over a socket.
>>
>>The size of the image is 640x480 pixels in pgm format.
>>
>>The problem is that when the client sends the image to the server (307200
>>bytes) the server only receives 65533.
>>
>>The thing is, is there any limits on the information sent from one point
>>(send) to another (recv)?.
>>
>>In such case, how is the way I have to send one image of 640x480 pixels?
>>Perhaps in two times?
>
> It is likely that you don't understand TCP communications and/or stream
> sockets.
>
> A recv() call only returns the data received up to that point. There is no
> synchronization between the data sent and the data received. That is to
> say that although you may /send/ massive amounts of data in one call, the
> corresponding recv() call does not have to (and usually won't) receive the
> data in one recv() call. This is the nature of TCP/IP, and is not a flaw
> in the sockets implementation, or anything else.
>
> It appears that your /first/ recv() gets 64k of the data sent. You need to
> continue issueing recv() calls (and appending the results together) until
> you receive all the data you sent. How you determine that you have
> received /all/ the data depends on your sending program:
> - It may close it's sending socket when it has sent all the data; the
> receiver
> will detect this as an end-of-file on the receiving socket, or
> - It may send the length of the data as a data item prior to the data
> itself;
> the receiver must fetch this length first, then recv() until the length
> has been exhausted, or
> - It may send a sentinal in the data stream to mark the end of the data;
> the
> receiver must be sensitive to this sentinal and trim it's recv()d data
> according to the placement of the sentinal, or
> - The sender and receiver always know exactly how much data is transferred
> as
> the sender always sends a fixed amount of data, and the receiver knows
> what that fixed amount is; both sender and receiver must agree on what
> this amount is prior to the transmission of the data.
>
> In all cases, both sender application and receiver application must agree
> upon the strategy to be used to detect end of data, and both applications
> must be coded to enact that strategy.
>
>
Thank you to all of us for helping me. The problem was exactly this, I
expected recv() will get all the data in only one call. It was my fault.
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