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From: Raphael Kubo da Costa <rakuco@FreeBSD.org>
Subject: Re: -lpthread vs -pthread: does -D_REENTRANT matter?
Date: Sun, 14 Oct 2012 19:52:59 +0300
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Jilles Tjoelker <jilles@stack.nl> writes:

> On Mon, Oct 08, 2012 at 12:17:08PM -0400, Eitan Adler wrote:
>> The only difference between -lpthread and -pthread that I could see is
>> that the latter also sets -D_REENTRANT.
>> However, I can't find any uses of _REENTRANT anywhere outside of a few
>> utilities that seem to define it manually.
>
>> Testing with various manually written pthread programs resulted in
>> identical binaries, let alone identical results.
>
>> Is there an actual difference between -pthread and -lpthread or is
>> this just a historical artifact?
>
> In some cases, -pthread also affects the compiler's code generation. On
> some RISC architectures, compilers may try to avoid loads and stores of
> less than 32 bits.

[...]

> Because C99 does not specify threading, it allows these transformations.
> In C11, they are forbidden. Passing -pthread disables them as well.

And does this not happen at all if one uses -lpthread instead?