Pthreads: Prototypes missing if -std=c11

Pthreads: Prototypes missing if -std=c11

[bz0815]

Hello,

gcc does not recognize some functions from pthread.h if option -std=c11 is used:

#include <pthread.h>
#define THREADS 5

int main (int argc, char *argv[])
{
  pthread_barrier_t barrier;
  pthread_barrier_init(&barrier, NULL, THREADS);
}

gcc -std=c11 -o test test.c
test.c: In function ‘main’:
test.c:8:3: warning: implicit declaration of function ‘pthread_barrier_init’ [-Wimplicit-function-declaration]
   pthread_barrier_init(&barrier, NULL, THREADS);
   ^

There is no warning without that option:

gcc -o test test.c

My system is:

$ uname -a
CYGWIN_NT-10.0 DESKTOP 2.8.1(0.312/5/3) 2017-07-03 14:11 x86_64 Cygwin

The compiler used is:

$ gcc --version
gcc (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

It seems to be a __POSIX_VISIBLE issue.

Thanks


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Re: Pthreads: Prototypes missing if -std=c11

[Yaakov Selkowitz]

On 2017-07-12 11:15, bz0815@tirol.com wrote:
> gcc does not recognize some functions from pthread.h if option -std=c11 is used:

Exactly, and the same happens with glibc.  When you use -std=c*, that
means you are declaring strict ISO C, and all extensions are disabled by
default unless explicitly enabled (e.g. with -D_POSIX_C_SOURCE=200112L).
 If you are not compatible with ISO C, then you should be using
-std=gnu* instead.

-- 
Yaakov

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Re: Pthreads: Prototypes missing if -std=c11

[bz0815]

> On 2017-07-12 11:15, bz0815@tirol.com wrote:
>> gcc does not recognize some functions from pthread.h if option -std=c11 is used:
>
> Exactly, and the same happens with glibc.  When you use -std=c*, that means you are declaring strict ISO C, and all extensions are disabled by > default unless explicitly enabled (e.g. with -D_POSIX_C_SOURCE=200112L).
>  If you are not compatible with ISO C, then you should be using
> -std=gnu* instead.
>
> --
> Yaakov
>
> --

Many thanks for the info, Yaakov. Accoring to the manual -std determines the language standard and c11 means ISO C11, the 2011 revision of the ISO C standard. Howevery, I find no restriction on POSIX in ISO/IEC 9899:2011. So why should -std=c11 have an influence on Pthreads? Am I missing something?


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Re: Pthreads: Prototypes missing if -std=c11

[cyg Simple]

On 7/13/2017 11:36 AM, bz0815@tirol.com wrote:
>> On 2017-07-12 11:15, bz0815@tirol.com wrote:
>>> gcc does not recognize some functions from pthread.h if option -std=c11 is used:
>>
>> Exactly, and the same happens with glibc.  When you use -std=c*, that means you are declaring strict ISO C, and all extensions are disabled by > default unless explicitly enabled (e.g. with -D_POSIX_C_SOURCE=200112L).
> 
>>  If you are not compatible with ISO C, then you should be using
>> -std=gnu* instead.
>>
>> --
>> Yaakov
>>
>> --
> 
> Many thanks for the info, Yaakov. Accoring to the manual -std determines the language standard and c11 means ISO C11, the 2011 revision of the ISO C standard. Howevery, I find no restriction on POSIX in ISO/IEC 9899:2011. So why should -std=c11 have an influence on Pthreads? Am I missing something?

The option means explicit standard C without extensions or additions.
POSIX is atop of the C standard and requires specific definition by the
user if it's required.

-- 
cyg Simple

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Re: Pthreads: Prototypes missing if -std=c11

[Brian Inglis]

On 2017-07-13 09:36, bz0815@tirol.com wrote:
>> On 2017-07-12 11:15, bz0815@tirol.com wrote:
>>> gcc does not recognize some functions from pthread.h if option -std=c11
>>> is used:
>>
>> Exactly, and the same happens with glibc. When you use -std=c*, that means 
>> you are declaring strict ISO C, and all extensions are disabled by default 
>> unless explicitly enabled (e.g. with -D_POSIX_C_SOURCE=200112L).
>> 
>>  If you are not compatible with ISO C, then you should be using
>> -std=gnu* instead.

> Many thanks for the info, Yaakov. Accoring to the manual -std determines the 
> language standard and c11 means ISO C11, the 2011 revision of the ISO C 
> standard. Howevery, I find no restriction on POSIX in ISO/IEC 9899:2011. So
> why should -std=c11 have an influence on Pthreads? Am I missing something?
ISO/IEC 9899 specifies the features and facilities that shall or may be provided
by all standard conforming freestanding or hosted implementations. It considers
proposals from stakeholders for new language and library features based on
available implementations.

Threading is optional in standards conforming implementations: conforming
programs must compile cleanly if the feature is not available, and use only the
facilities declared in threads.h.
See https://en.wikipedia.org/wiki/C11_(C_standard_revision)#Optional_features

POSIX specifies common features and facilities that shall or may be provided by
one class of hosted implementations, with additional requirements to those
specified for standard conforming implementations.

POSIX does not provide threads.h but does provide pthreads.h, so you have a
choice, depending on whether you want to be strictly conforming or support POSIX.

See or search for Linux glibc man 7 feature_test_macros for more info about
macros and values.

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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