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Subject: Re: libssh2_channel_read issues

Re: libssh2_channel_read issues

From: Matthew Booth <mbooth_at_redhat.com>
Date: Thu, 12 Apr 2012 11:25:15 +0100

On 04/11/2012 05:55 PM, Zulfiqar Malik wrote:
> I am trying to write a very small shell client for some automation
> related things and am running into problems with libssh2_channel_read.
> Here are the two scenarios (application has to be single threaded.):
> 1. Blocking mode: libssh2_channel_read blocks if there's no data and
> never returns. Its impossible to use this function reliably.
> 2. Non-blocking mode: libssh2_channel_read doesn't block but keeps on
> returning -37 (LIBSSH2_ERROR_EAGAIN) and I end up with essentially the
> same problem i.e. stuck in an infinite loop. To top it off
> libssh2_channel_eof doesn't work and always returns 0.

You don't say what's happening at the other end. Can we assuming it's
something like:

libssh2_channel_exec(channel, "cat foo");

and that you want to exit when the contents of foo have been returned?
Is is possible, for example, that your channel is not returning eof
because it isn't eof?

> I don't know what's wrong or what needs to be done but I cannot seem to
> find a way through this problem. I have tried libssh2 v 1.3.1 and the
> latest one 1.4.1 with the same result. Given below is my readOutput
> method which reads the output:
> for (;;count++)
> {
> /* loop until we block */
> int rc;
> do {
> char buffer[1024];
> rc = libssh2_channel_read(channel, buffer, sizeof(buffer));
> if (rc > 0) {
> bytecount += rc;
> buffer[rc] = '\0';

Probably not related, but ^^^ corrupts the stack if libssh2_channel_read
returns a full buffer. If you're going to explicitly null terminate the
buffer, you need:

rc = libssh2_channel_read(channel, buffer, sizeof(buffer) - 1);

> fprintf(stderr, "%s", buffer);
> count = 0;
> }
> else {
> if (rc != LIBSSH2_ERROR_EAGAIN)
> fprintf(stderr, "libssh2_channel_read returned
> %d\n", rc);
> }
> if (libssh2_channel_eof(channel)) {
> rc = 0;
> break;
> }
> }
> while(rc > 0);
> //::Sleep(3);
> /* this is due to blocking that would occur otherwise so we loop on
> this condition */
> if(rc == LIBSSH2_ERROR_EAGAIN)
> {
> ::Sleep(5);
> //waitsocket(sock, sshSession);

I'm assuming this references something like the waitsocket() example at
http://www.libssh2.org/examples/ssh2_exec.html. Why the sleep instead?

> }
> else
> break;
> }

Matt

-- 
Matthew Booth, RHCA, RHCSS
Red Hat Engineering, Virtualisation Team
GPG ID:  D33C3490
GPG FPR: 3733 612D 2D05 5458 8A8A 1600 3441 EA19 D33C 3490
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Received on 2012-04-12

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