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Errors/crashes with FTP using multi interface?

From: Sam Deane <>
Date: Mon, 4 Feb 2013 12:28:24 +0000

Hi all,

I'm using the multi interface, and I'm seeing two problems when using FTP, which may well be related to each other.

I believe, although I'm not certain, that both problems occur more often when using CURLOPT_PREQUOTE or CURLOPT_POSTQUOTE.

What seems to be happening is that curl is getting confused with the multiple responses coming back from the server.

Sometimes this leads to us hitting this line in ftp.c:

      failf(data, "server did not report OK, got %d", ftpcode);

In this case, the code that it's complaining about is generally a legitimate code returned from some part of the conversation with the server.

That's the good scenario. In the bad scenario, we end up crashing when calling out from Curl_client_write.

The problem in this case is that the calculations at the top of Curl_pp_readresp are going wrong.

The calculation of perline is taking the difference of two buffer pointers.

perline = (ssize_t)(ptr-pp->linestart_resp);

In the case where it crashes, these two buffers appear to be completely different. Examining them, they both contain different responses from the FTP server.

However, they are far apart in memory, resulting in a large (and often negative) value for perline.

When we later call out to a user-registered function, this large/negative value is passed along, causing the called code to die horribly.

It may be a coincidence, but at some level both problems appear to be caused by the curl internals getting confused about which response it's supposed to be processing. This leads me to suspect that the underlying issue may be the same.

Does anyone have any ideas?

I believe that I have the threading set up correctly.

I'm on OS X, using GCD, with all accesses to the multi handle protected by a single serial queue, so that no two operations on the multi should ever occur simultaneously. Easy handles are potentially set up on other threads, but aren't touched once they are handed off to the multi.

This problem is a real blocker for us!

sam deane / @samdeane | / @elegantchaoscom | mac and ios software development

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Received on 2013-02-04