Solved

Problem with sockets--atomic read.

Posted on 1997-11-17
1
397 Views
Last Modified: 2013-12-26
I'm having a problem performing a read() call and retrieving all of the data.

I'm using IRIX 5.3 on an SGI Indy.  Here is the situation:

I have a socket opened for TCP/IP communication.  My client
process performs a writev() call.  The writev() is passed an
iovec with 2 buffers in it.  The total size of the two
buffers is less than PIPE_BUF (aka: PIPE_MAX).  The writev()
call returns to me with a positive value which is equal to
the number of bytes I expect to write.  I believe PIPE_BUF
is 10240 bytes, and my writev() call is writing 3875 bytes
(well below PIPE_BUF).  My server process then performs a
select() call to check for data on the socket and follows
that with a read().  The select() call is only checking for
data on a single socket and is used so that I can timeout
the recv() without using signals.  Anyway, the select()
detects the data on the socket and then I perform a recv().  
However, I am only able to read about 1500 bytes of the
message.  If I place a sleep(1) call just prior to the
recv() I get the whole message (3875 bytes).  This would
leave me to believe that recv() is returning prior to all
the data arriving on the socket.  I would think that an
atomic write on a TCP/IP socket would gurantee that the
message is received atomically as well, but this does not
seem to be the case.  I am wondering if there is some limit
imposed on TCP/IP sockets that is less than PIPE_BUF than I
should be using or do I just need to put that recv() call in
a loop with an accumulator and just keep trying to read
until all the data arrives?

Thanks,
Barry M. Caceres
barryc@alumni.caltech.edu
0
Comment
Question by:barryc
1 Comment
 
LVL 4

Accepted Solution

by:
jos010697 earned 200 total points
Comment Utility
Yep, you ansered your own question in your last sentence (see
above). It's quite peculiar indeed -- the writev() call can
send data atimically, while there's no guarantee at all that
recv() _receives_ the data in one sweep. The atomiicity simply
means that no other interleaved writes are done to a socket;
it doesn't mean that the data is not chopped up in smaller
parts though. A simple loop could do the job for you:

for (; c= recv(sock, buf, size, 0); c >= 0; size-= c)
   if (!size)
      break;

if (c < 0)
   /* check errno here */

kind regards,

Jos aka jos@and.nl
0

Featured Post

What Security Threats Are You Missing?

Enhance your security with threat intelligence from the web. Get trending threat insights on hackers, exploits, and suspicious IP addresses delivered to your inbox with our free Cyber Daily.

Join & Write a Comment

Suggested Solutions

Title # Comments Views Activity
Expand to include initial dialog with two choices. 9 59
List out all word 7 218
modThree challenge 4 64
zeroFront challenge 7 70
This is to be the first in a series of articles demonstrating the development of a complete windows based application using the MFC classes.  I’ll try to keep each article focused on one (or a couple) of the tasks that one may meet.   Introductio…
Introduction: Finishing the grid – keyboard support for arrow keys to manoeuvre, entering the numbers.  The PreTranslateMessage function is to be used to intercept and respond to keyboard events. Continuing from the fourth article about sudoku. …
This video will show you how to get GIT to work in Eclipse.   It will walk you through how to install the EGit plugin in eclipse and how to checkout an existing repository.
In this tutorial you'll learn about bandwidth monitoring with flows and packet sniffing with our network monitoring solution PRTG Network Monitor (https://www.paessler.com/prtg). If you're interested in additional methods for monitoring bandwidt…

728 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

10 Experts available now in Live!

Get 1:1 Help Now