Thread-safety of iconv()

Thread-safety of iconv()

[Marko Myllynen]

Hi Michael, Haitao,

iconv(3) states:

ATTRIBUTES
   Multithreading (see pthreads(7))
       The iconv() function is thread-safe.

But iconv_open(3) says:

A conversion descriptor contains a conversion  state.   After  creation
using iconv_open(), the state is in the initial state.  Using iconv(3)
modifies the descriptor's conversion state.  (This implies that a  con-
version descriptor can not be used in multiple threads simultaneously.)
To bring the state back to the initial state, use iconv(3) with NULL as
inbuf argument.

This looks a bit unclear, do you think you could clarify iconv(3) and/or
iconv_open(3) a bit in thread-safety regard?

Thanks,

-- 
Marko Myllynen
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Re: Thread-safety of iconv()

[Peng Haitao]

On 09/15/2014 05:32 PM, Marko Myllynen wrote:
> Hi Michael, Haitao,
> 
> iconv(3) states:
> 
> ATTRIBUTES
>    Multithreading (see pthreads(7))
>        The iconv() function is thread-safe.
> 
> But iconv_open(3) says:
> 
> A conversion descriptor contains a conversion  state.   After  creation
> using iconv_open(), the state is in the initial state.  Using iconv(3)
> modifies the descriptor's conversion state.  (This implies that a  con-
> version descriptor can not be used in multiple threads simultaneously.)
> To bring the state back to the initial state, use iconv(3) with NULL as
> inbuf argument.
> 
> This looks a bit unclear, do you think you could clarify iconv(3) and/or
> iconv_open(3) a bit in thread-safety regard?
> 

Yes, I will clarify it.

When calling iconv() as iconv(cd, inbuf, ...) and iconv(cd, NULL, ...) 
in multiple threads simultaneously, Segmentation fault may occur.

So, I think the glibc's level "MT-Safe race:cd" is good.
http://www.gnu.org/software/libc/manual/html_mono/libc.html#index-iconv-1

Thanks.

-- 
Best Regards,
Peng
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Re: Thread-safety of iconv()

[Carlos O'Donell]

On 09/18/2014 02:50 AM, Peng Haitao wrote:
> On 09/15/2014 05:32 PM, Marko Myllynen wrote:
>> Hi Michael, Haitao,
>>
>> iconv(3) states:
>>
>> ATTRIBUTES
>>    Multithreading (see pthreads(7))
>>        The iconv() function is thread-safe.
>>
>> But iconv_open(3) says:
>>
>> A conversion descriptor contains a conversion  state.   After  creation
>> using iconv_open(), the state is in the initial state.  Using iconv(3)
>> modifies the descriptor's conversion state.  (This implies that a  con-
>> version descriptor can not be used in multiple threads simultaneously.)
>> To bring the state back to the initial state, use iconv(3) with NULL as
>> inbuf argument.
>>
>> This looks a bit unclear, do you think you could clarify iconv(3) and/or
>> iconv_open(3) a bit in thread-safety regard?

It is conditionally thread safe :-)

>>
> 
> Yes, I will clarify it.
> 
> When calling iconv() as iconv(cd, inbuf, ...) and iconv(cd, NULL, ...) 
> in multiple threads simultaneously, Segmentation fault may occur.
> 
> So, I think the glibc's level "MT-Safe race:cd" is good.
> http://www.gnu.org/software/libc/manual/html_mono/libc.html#index-iconv-1

The `cd' object is opaque to the user, and therefore in the glibc manual
Alex and I felt it was appropriate to call out that it *is* thread safe
to call this function but that the `iconv_t`-type object `cd` has itself
no locking and can be corrupted by multiple calls with the same object,
hence `race:cd` e.g. there is a data-race calling this function with the
same cd object.

Alex, please correct me if I'm wrong, but I think that sums it up.

Cheers,
Carlos.

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Re: Thread-safety of iconv()

[Alexandre Oliva]

On Sep 18, 2014, "Carlos O'Donell" <carlos-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:

> On 09/18/2014 02:50 AM, Peng Haitao wrote:
>> On 09/15/2014 05:32 PM, Marko Myllynen wrote:
>>> This looks a bit unclear, do you think you could clarify iconv(3) and/or
>>> iconv_open(3) a bit in thread-safety regard?

> It is conditionally thread safe :-)

It is MT-Safe, but it assumes callers arrange for mutual exclusion on
the cd object passed as argument.

It is no different from e.g. memset being MT-Safe, in spite of the race
when two threads call memset concurrently, writing to the same buffer.

It's caller's responsibility to avoid such races in both cases.

> Alex, please correct me if I'm wrong, but I think that sums it up.

Yup.  A recent blog post covers this issue (among others) in some
detail:

http://developerblog.redhat.com/2014/09/10/multi-thread-async-signal-and-async-cancel-safety-docs-in-gnu-libc/

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist|Red Hat Brasil GNU Toolchain Engineer
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