• Uncategorized

About python : How-to-find-more-information-about-the-file-descriptor

Question Detail

I am trying to debug a process that hangs, the output of strace for the process id has last line :
recvfrom(9, <detached ...>
From this what I understand that the process is waiting on the socket.

But I don’t know which or what kind of socket is this. How can I discover more about this ? does the file descriptor 9 will give me more information ? How can I use this file discover to know more about what it is waiting for ?

its a python process, running in linux.

Question Answer

Since you are using strace I assume you are on Linux and you know the process id. In that case you can find a lot of info in /proc. Here is an example of what to do.

Do ls -l /proc/<pid>/fd. There will be an entry corresponding to the fd you are interested in and that fd should be a socket as such:

$ ls -l /proc/3311/fd
total 0
lrwx------ 1 alanau alanau 64 Sep 24 20:37 0 -> /dev/pts/0
lrwx------ 1 alanau alanau 64 Sep 24 20:37 1 -> /dev/pts/0
lrwx------ 1 alanau alanau 64 Sep 24 20:37 2 -> /dev/pts/0
lrwx------ 1 alanau alanau 64 Sep 24 20:37 3 -> socket:[23182]

In the above example, 3 is the socket of interest. The number after socket is the inode number, 23182 in this case.

Now look in the file /proc/net/tcp to find that inode:

$ cat /proc/net/tcp | grep 23182
   2: 0F02000A:C43B 8EDC3AD8:0050 01 00000000:00000000 00:00000000 00000000  1000        0 **23182** 1 0000000000000000 23 0 0 10 -1

The first line of /proc/net/tcp tells you what each field represents:

 sl  local_address rem_address   st tx_queue rx_queue tr tm->when retrnsmt   uid  timeout inode

So in this example, the remote address is 8EDC3AD8:0050. That’s the ip_addres:port_number in hex and network byte ordering. If you convert that to decimal it is 216.58.220.142:80.

Which tells you it is in fact a TCP connection to port 80 of google.com!

If you don’t find the inode number in /proc/net/tcp then try the other protocols in the directory, probably /proc/net/udp.

Look earlier in the strace output for when the file descriptor was returned from open() (or perhaps socket()), there you’ll see the additional arguments used in the call.

You may also like...

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.