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From: Kaz Kylheku <kaz@kylheku.com>
Newsgroups: comp.os.linux.development.system
Subject: Re: Calling libiptc API from a separate thread in a C program throws segmentati
Date: Tue, 17 Jan 2012 22:34:48 +0000 (UTC)
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On 2012-01-17, Richard Kettlewell <rjk@greenend.org.uk> wrote:
> Kaz Kylheku <kaz@kylheku.com> writes:
>> Jorgen Grahn <grahn+nntp@snipabacken.se> wrote:
>>>> 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
>>
>> Whom are you going to believe, me or the GCC manual? :)
>
> errno ends up indirected either way though (at least here).

"Either way" between which alternatives?

The -pthread option will have the effect of defining _REENTRANT so that
__errno_location ends up used. But -lpthread alone will not.

In /usr/include/bits/errno.h you have this:

# ifndef __ASSEMBLER__
/* Function to get address of global `errno' variable.  */
extern int *__errno_location (void) __THROW __attribute__ ((__const__));

#  if !defined _LIBC || defined _LIBC_REENTRANT
/* When using threads, errno is a per-thread value.  */
#   define errno (*__errno_location ())
#  endif
# endif /* !__ASSEMBLER__ */

This part is included in /usr/include/errno.h, which then has this:

#ifndef errno
extern int errno;
#endif