_ISOCxx_SOURCE

_ISOCxx_SOURCE

[Alejandro Colomar]

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Hi!

I was reviewing the feature_test_macros(7) page, and found the
documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
And thinking about fixing that led me to think that the design of those
macros is less than ideal.

Here's for example some of the problematic text:

        Defining _ISOC23_SOURCE also enables C11, C99, and C95 features.

The above is not true.  See for example some contradicting (and also
incorrect/incomplete) text in the HISTORY section of gets(3):

	glibc header files don’t expose the function declaration if the
	_ISOC11_SOURCE feature test macro is defined.

gets(3) is a C89 and C99 feature, which is disabled with C11 and later
standard versions.  The text saying that _ISOC23_SOURCE enables older
features is only true as much as those features remain in the C23
standard.  And in gets(3), we should say that the function is not
exposed on C11 or later (not just C11).

A better design of this macro would be to have a single identifier, and
use a value to specify the version.  This is how POSIX does it.  We
could call it _ISO_C_SOURCE, and have a value such as 202311L for
requesting C23.  That would make it easier to describe: features are
available within a range of versions; often a half-bounded interval,
starting at a version, and not ending; but other times a bounded
interval, starting at a version, and ending at another version.

What do you think?


Have a lovely day!
Alex

-- 
<https://www.alejandro-colomar.es>

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Re: _ISOCxx_SOURCE

[Joseph Myers]

On Sat, 13 Jun 2026, Alejandro Colomar via Gcc wrote:

> I was reviewing the feature_test_macros(7) page, and found the
> documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
> And thinking about fixing that led me to think that the design of those
> macros is less than ideal.

Users should normally use -std= options with the compiler to get matching 
language and library features.  These feature test macros are mainly for 
niche cases with old compilers.  So I don't think any new user-facing 
design for them should be added (the C23 and later ones already use the 
newer __GLIBC_USE style of conditionals in the header implementation).

-- 
Joseph S. Myers
josmyers@redhat.com

Re: _ISOCxx_SOURCE

[Alejandro Colomar]

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Hi Joseph,

On 2026-06-15T17:20:17+0000, Joseph Myers wrote:
> On Sat, 13 Jun 2026, Alejandro Colomar via Gcc wrote:
> 
> > I was reviewing the feature_test_macros(7) page, and found the
> > documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
> > And thinking about fixing that led me to think that the design of those
> > macros is less than ideal.
> 
> Users should normally use -std= options with the compiler to get matching 
> language and library features.  These feature test macros are mainly for 
> niche cases with old compilers.  So I don't think any new user-facing 
> design for them should be added (the C23 and later ones already use the 
> newer __GLIBC_USE style of conditionals in the header implementation).

Makes sense.  Should I document that users should avoid setting them
directly (and that they should use the corresponding compiler flag)?


Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es>

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Re: _ISOCxx_SOURCE

[Joseph Myers]

On Mon, 15 Jun 2026, Alejandro Colomar wrote:

> Hi Joseph,
> 
> On 2026-06-15T17:20:17+0000, Joseph Myers wrote:
> > On Sat, 13 Jun 2026, Alejandro Colomar via Gcc wrote:
> > 
> > > I was reviewing the feature_test_macros(7) page, and found the
> > > documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
> > > And thinking about fixing that led me to think that the design of those
> > > macros is less than ideal.
> > 
> > Users should normally use -std= options with the compiler to get matching 
> > language and library features.  These feature test macros are mainly for 
> > niche cases with old compilers.  So I don't think any new user-facing 
> > design for them should be added (the C23 and later ones already use the 
> > newer __GLIBC_USE style of conditionals in the header implementation).
> 
> Makes sense.  Should I document that users should avoid setting them
> directly (and that they should use the corresponding compiler flag)?

Indeed.  Also, users are probably using a compiler defaulting to 
-std=gnu23 or -std=gnu11 so the older _ISOC99_SOURCE and _ISOC11_SOURCE 
are doubly irrelevant for that reason.

-- 
Joseph S. Myers
josmyers@redhat.com

Re: _ISOCxx_SOURCE

[Alejandro Colomar]

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Hi Joseph,

On 2026-06-16T16:02:44+0000, Joseph Myers wrote:
> On Mon, 15 Jun 2026, Alejandro Colomar wrote:
> 
> > Hi Joseph,
> > 
> > On 2026-06-15T17:20:17+0000, Joseph Myers wrote:
> > > On Sat, 13 Jun 2026, Alejandro Colomar via Gcc wrote:
> > > 
> > > > I was reviewing the feature_test_macros(7) page, and found the
> > > > documentation of the _ISOC{99,11,23,...}_SOURCE macros to be incorrect.
> > > > And thinking about fixing that led me to think that the design of those
> > > > macros is less than ideal.
> > > 
> > > Users should normally use -std= options with the compiler to get matching 
> > > language and library features.  These feature test macros are mainly for 
> > > niche cases with old compilers.  So I don't think any new user-facing 
> > > design for them should be added (the C23 and later ones already use the 
> > > newer __GLIBC_USE style of conditionals in the header implementation).
> > 
> > Makes sense.  Should I document that users should avoid setting them
> > directly (and that they should use the corresponding compiler flag)?
> 
> Indeed.

Thanks!  Will do.

>  Also, users are probably using a compiler defaulting to 
> -std=gnu23 or -std=gnu11 so the older _ISOC99_SOURCE and _ISOC11_SOURCE 
> are doubly irrelevant for that reason.

I was also wondering...

If one uses a modern GCC with an old glibc (which doesn't know C23),
then, using -std=c23 will be problematic: the compiler will enable C23
language mode, but the library will entirely and silently ignore
_ISOC23_SOURCE, because it doesn't know about it, thus falling back to
C89 mode (I guess), and thus will for example enable things like
gets(3).

If the way to pass the language version from the compiler to the libc
were through a single macro, then the library would be able to at least
diagnose, and/or turn on the latest dialect it knows about.

Is there any limitation of which libc versions are supported by modern
GCC?  Is this a bug?


Have a lovely night!
Alex

> 
> -- 
> Joseph S. Myers
> josmyers@redhat.com
> 

-- 
<https://www.alejandro-colomar.es>

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Re: _ISOCxx_SOURCE

[Joseph Myers]

On Tue, 16 Jun 2026, Alejandro Colomar via Gcc wrote:

> If one uses a modern GCC with an old glibc (which doesn't know C23),
> then, using -std=c23 will be problematic: the compiler will enable C23
> language mode, but the library will entirely and silently ignore
> _ISOC23_SOURCE, because it doesn't know about it, thus falling back to
> C89 mode (I guess), and thus will for example enable things like
> gets(3).

Since the relevant checks in features.h are based on __STDC_VERSION__ > 
something or __STDC_VERSION__ >= something, it will fall back to the most 
recent C standard version known by that glibc version, which is probably 
what you want.

-- 
Joseph S. Myers
josmyers@redhat.com

Re: _ISOCxx_SOURCE

[Alejandro Colomar]

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Hi Joseph,

On 2026-06-16T21:52:57+0000, Joseph Myers wrote:
> On Tue, 16 Jun 2026, Alejandro Colomar via Gcc wrote:
> 
> > If one uses a modern GCC with an old glibc (which doesn't know C23),
> > then, using -std=c23 will be problematic: the compiler will enable C23
> > language mode, but the library will entirely and silently ignore
> > _ISOC23_SOURCE, because it doesn't know about it, thus falling back to
> > C89 mode (I guess), and thus will for example enable things like
> > gets(3).
> 
> Since the relevant checks in features.h are based on __STDC_VERSION__ > 
> something or __STDC_VERSION__ >= something,

Ahh, yup, it has || for either _ISOCxx_SOURCE or __STDC_VERSION__.  That
makes sense.

> it will fall back to the most 
> recent C standard version known by that glibc version, which is probably 
> what you want.

Hmmm, it is.  Then it's all fine.  Thanks!


Cheers,
Alex

-- 
<https://www.alejandro-colomar.es>

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