[PATCH] manual: Document that EOPNOTSUPP and ENOTSUP are equal, not distinct

[Adhemerval Zanella Netto]

On 25/02/26 23:20, Nicolas Boulenguez wrote:
> Hello.
> The section 2.1 of the glibc manual says that EWOULDBLOCK == EAGAIN,
> but forgets to mention that ENOTSUP == EOPNOTSUPP.
> 
> https://sourceware.org/legacy-ml/libc-alpha/2019-08/msg00629.html
> https://sourceware.org/bugzilla/show_bug.cgi?id=2363
> https://bugs.debian.org/337013
> 
> Signed-off-by: Nicolas Boulenguez <nicolas@debian.org>

LGTM, it address the previous remarks from Carlos.

I will install it with just an addition on the title to reference
the BZ 2363.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> ---
>  manual/errno.texi | 40 ++++++++++++++++++++++++----------------
>  1 file changed, 24 insertions(+), 16 deletions(-)
> 
> diff --git a/manual/errno.texi b/manual/errno.texi
> index 84a142c4a5..a27223c14f 100644
> --- a/manual/errno.texi
> +++ b/manual/errno.texi
> @@ -84,11 +84,18 @@ letter or digit; you should consider names of this form to be
>  reserved names.  @xref{Reserved Names}.
>  
>  The error code values are all positive integers and are all distinct,
> -with one exception: @code{EWOULDBLOCK} and @code{EAGAIN} are the same.
> -Since the values are distinct, you can use them as labels in a
> -@code{switch} statement; just don't use both @code{EWOULDBLOCK} and
> -@code{EAGAIN}.  Your program should not make any other assumptions about
> +so they can be used as labels in a @code{switch} statement.
> +Your program should not make any other assumptions about
>  the specific values of these symbolic constants.
> +There are two exceptions to this rule:
> +@itemize @bullet
> +@item The values of @code{EAGAIN} and @code{EWOULDBLOCK} are
> +equal on every supported operating system.
> +@item The values of @code{ENOTSUP} and @code{EOPNOTSUPP} are equal
> +on some supported operating systems, for example GNU/Linux.
> +@end itemize
> +To make your program portable, you should check for both codes and
> +treat them the same.
>  
>  The value of @code{errno} doesn't necessarily have to correspond to any
>  of these macros, since some library functions might return other error
> @@ -383,8 +390,7 @@ not representable because of overflow or underflow.
>  @standards{POSIX.1, errno.h}
>  @errno{EAGAIN, 35, Resource temporarily unavailable}
>  The call might work if you try again
> -later.  The macro @code{EWOULDBLOCK} is another name for @code{EAGAIN};
> -they are always the same in @theglibc{}.
> +later.
>  
>  This error can happen in a few different situations:
>  
> @@ -395,12 +401,6 @@ non-blocking mode selected.  Trying the same operation again will block
>  until some external condition makes it possible to read, write, or
>  connect (whatever the operation).  You can use @code{select} to find out
>  when the operation will be possible; @pxref{Waiting for I/O}.
> -
> -@strong{Portability Note:} In many older Unix systems, this condition
> -was indicated by @code{EWOULDBLOCK}, which was a distinct error code
> -different from @code{EAGAIN}.  To make your program portable, you should
> -check for both codes and treat them the same.
> -
>  @item
>  A temporary resource shortage made an operation impossible.  @code{fork}
>  can return this error.  It indicates that the shortage is expected to
> @@ -411,16 +411,16 @@ Such shortages are usually fairly serious and affect the whole system,
>  so usually an interactive program should report the error to the user
>  and return to its command loop.
>  @end itemize
> +
> +@strong{Portability Note:} In @theglibc{},
> +@code{EAGAIN} and @code{EWOULDBLOCK} are equal.
> +Portable code should check for both errors and treat them the same.
>  @end deftypevr
>  
>  @deftypevr Macro int EWOULDBLOCK
>  @standards{BSD, errno.h}
>  @errno{EWOULDBLOCK, EAGAIN, Operation would block}
>  In @theglibc{}, this is another name for @code{EAGAIN} (above).
> -The values are always the same, on every operating system.
> -
> -C libraries in many older Unix systems have @code{EWOULDBLOCK} as a
> -separate error code.
>  @end deftypevr
>  
>  @deftypevr Macro int EINPROGRESS
> @@ -492,6 +492,10 @@ implemented for all communications protocols.  On @gnuhurdsystems{}, this
>  error can happen for many calls when the object does not support the
>  particular operation; it is a generic indication that the server knows
>  nothing to do for that call.
> +
> +@strong{Portability Note:} Depending on the operating system, the
> +values of @code{EOPNOTSUPP} and @code{ENOTSUP} may be equal.
> +Portable code should check for both errors and treat them the same.
>  @end deftypevr
>  
>  @deftypevr Macro int EPFNOSUPPORT
> @@ -769,6 +773,10 @@ values.
>  
>  If the entire function is not available at all in the implementation,
>  it returns @code{ENOSYS} instead.
> +
> +@strong{Portability Note:} Depending on the operating system, the
> +values of @code{EOPNOTSUPP} and @code{ENOTSUP} may be equal.
> +Portable code should check for both errors and treat them the same.
>  @end deftypevr
>  
>  @deftypevr Macro int EILSEQ