Mailinglist Archive: opensuse-programming (51 mails)

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threads and file descriptors
  • From: Per Jessen <per@xxxxxxxxxxxx>
  • Date: Sun, 07 May 2006 11:06:10 +0200
  • Message-id: <e3kda2$k75$2@xxxxxxxxxxxxxxxx>
I'm seeing a fairly reproducable problem in which the same file
descriptor is being used concurrently in two threads, but

I understand that open file descriptors are shared by all threads, but
somehow it looks like calling socket() in one thread is able to return a
filedescriptor that's already in use by another thread.

Here's an example (a little complicated, but bear with me):

this is just stderr debugging output from my app which is running 3
threads at this time. The output is in chronological order.
The thread# is in [] to the left, my comments are prefixed with >>> :

[01] using fd 16 for /tmp/ibwd.1
[01] state-transition: 2->3
[01] writing to left ( 0 bytes left): 250 Ok: queued as 29C9E4637
[01] state 3; left event LEFT_RSET: [6 bytes] RSET
[01] all done <message-id>: 2995 bytes
[01] writing to right ( 0 bytes left): RSET
[01] state-transition: 3->1
[01] state 1; right event RIGHT_354: [37 bytes] 354
[01] using fd 14 for /tmp/ibwd.1

>>> above thread#1 has opened a file and gets fd 14 for it.

[01] state-transition: 1->2
[02] crm114[25958, 9 categories] stage done, 0.509 seconds.
[02] connected to juggernautd(14) at localhost:787

>>> above thread#2 opens a tcp connection to another app, and ALSO uses
fd 14.

[01] state 2; left event LEFT_EOM: [3 bytes] .
write_string(): unable to write to 14; error 9: Bad file descriptor

>>> above thread#1 chokes when trying to write to fd 14.

Each thread is independent of the others, except during allocation of
work. Are there any special precautions I need to take when doing IO
in a multi-threaded environment?

/Per Jessen, Z├╝rich

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