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Re: Calling libiptc API from a separate thread in a C program throws segmentati

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On Tue, 2012-01-17, Kaz Kylheku wrote:
> On 2012-01-17, kkgarg78 <nospam_kamalkgarg@gmail.com.invalid> wrote:
>> I am working on performing iptables update through a custom c program using
>>  libiptc.
>>  
>>  The requirement is to invoke iptc APIs from a separate thread every 2 seconds.
>
> Even if there is no change to the rules? Are you monitoring for changes,
> or updating?
>
>>  The problem, I am facing is that call to both iptc_init() and iptc_free()
>> works
>>  well when called from main function. However, call to iptc_free() fails with
>>  "Segmentation Fault" when called from thread_func().
>
> I'm looing at the library sources online. It does not look thread-safe at all,
> though it's not obvious how that could cause a crash in the above program,
> since your use of the library is serialized (no concurrrency between
> your main and thread_func).
>
> You should be fine if you have a dedicated thread which calls this
> library, or put a big mutex around it.
>
>>  Compilation:
>>  
>>  # gcc -o test thread_iptc.c -lpthread -lext4 -lext6 -lip4tc -lip6tc -liptc
>>  -lxtables -ldl
>
> This is not how you compile programs for multithreading. You want:
>
>   # gcc -o test -pthread thread_iptc.c -lext4 -lext6 -lip4tc -lip6tc -liptc
>   -lxtables -ldl

If he wants to be able to debug the crashes, he should also add -g,
and also enable at least minimal compiler warnings.

> I.e. the -pthread gcc option, not -lpthread. It is not enough to link the
> threading library (and -pthread does that).
>
> One consequence of using -lpthread alone is that the "errno" variable will
> not be properly thread-specific.
>
> At one point -pthread was equivalent to -D_REENTRANT and -lpthread,
> but that may change in the future if it has not already; it's better
> to use the -pthread interface.

Either the gcc manual is incomplete, or your explanation is not
entirely correct.  For at least x86 and amd64, there is no -pthread
option.  It's there for ppc though.

/Jorgen

-- 
  // Jorgen Grahn <grahn@  Oo  o.   .     .
\X/     snipabacken.se>   O  o   .

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Calling libiptc API from a separate thread in a C program throws segmentati kkgarg78 <nospam_kamalkgarg@gmail.com.invalid> - 2012-01-17 04:40 -0600
  Re: Calling libiptc API from a separate thread in a C program throws segmentati Kaz Kylheku <kaz@kylheku.com> - 2012-01-17 18:48 +0000
    Re: Calling libiptc API from a separate thread in a C program throws segmentati Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-01-17 21:26 +0000
      Re: Calling libiptc API from a separate thread in a C program throws segmentati Kaz Kylheku <kaz@kylheku.com> - 2012-01-17 22:03 +0000
        Re: Calling libiptc API from a separate thread in a C program throws segmentati Richard Kettlewell <rjk@greenend.org.uk> - 2012-01-17 22:07 +0000
          Re: Calling libiptc API from a separate thread in a C program throws segmentati Kaz Kylheku <kaz@kylheku.com> - 2012-01-17 22:34 +0000
            Re: Calling libiptc API from a separate thread in a C program throws segmentati Richard Kettlewell <rjk@greenend.org.uk> - 2012-01-17 22:51 +0000
              Re: Calling libiptc API from a separate thread in a C program throws segmentati Kaz Kylheku <kaz@kylheku.com> - 2012-01-17 23:13 +0000
        Re: Calling libiptc API from a separate thread in a C program throws segmentati Jorgen Grahn <grahn+nntp@snipabacken.se> - 2012-01-17 22:56 +0000
  Re: Calling libiptc API from a separate thread in a C program throws segmentati Richard Kettlewell <rjk@greenend.org.uk> - 2012-01-21 11:29 +0000

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