iNotify Man Page: Prefix Header + Tail Array vs C Structure

iNotify Man Page: Prefix Header + Tail Array vs C Structure

[Michael Titke]

Hello!

The current version of the manual page describing /inotify/ (as well as 
the version installed with Ubuntu 15.04 frozen to Violet Indigo) 
contains a descriptive C structure describing the /inotify//messages/:

            struct inotify_event {
                int      wd;       /* Watch descriptor */
                uint32_t mask;     /* Mask describing event */
                uint32_t cookie;   /* Unique cookie associating related
                                      events (for rename(2)) */
                uint32_t len;      /* Size of name field */
                char     name[];   /* Optional null-terminated name */
            };

As part of the development of VSI I translated the above structure 
without much thinking into a corresponding byte structure description:

(define inotify-event-header
   (byte-structure-description
    (wd   int)
    (mask   4) ; [sic! that was an uint32]
    (cookie 4)
    (len    4)
    ;(name   pointer) ; That's not a pointer but an array: NAME_MAX + 1
))

Now while the prefix (or header) of the message is described adequately 
the specification of the tail array as a char name[] would in C be 
interpreted as a pointer onto a char. One might insert a length 
parameter like char name[NAME_MAX + 1] or exclude that tail array from 
the structure and describe the message in terms of /prefix/ or /header/ 
and /tail array/.

            struct inotify_event {
                int      wd;       /* Watch descriptor */
                uint32_t mask;     /* Mask describing event */
                uint32_t cookie;   /* Unique cookie associating related
                                      events (for rename(2)) */
                uint32_t len;      /* Size of name field */
                char     name[NAME_MAX + 1];   /* Optional 
null-terminated name */
            };

With that length parameter in the structure the size of the messages is 
described adequately but the actual length of the name might be confused 
with the maximum size which usually includes some (tail) padding.

I'm sorry but I don't know enough about /iNotify/ to craft a patch for 
this. Is this a datagram channel where half-read messages will vanish? 
Perhaps I should continue reading the manual page. :-)

Regards,
Michael

--
VSI: https://code.launchpad.net/viper-system-interface

  (reason: 550 5.7.1 Content-Policy reject msg: The message contains HTML subpart, therefore we consider it SPAM or Outlook Virus.  TEXT/PLAIN is accepted.! BF:<U


PS That's not really up to the standard but "... since hosts aren't 
required to relay mail at all ..." someone presumes something.
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: iNotify Man Page: Prefix Header + Tail Array vs C Structure

[walter harms]

I am not sure that i understood what you want ...

i would answer the question:

* what is the actual size of struct inotify_event ?

inotify(7) says:
the length of each inotify_event structure is thus sizeof(struct inotify_event)+len

hope that helps ..

re,
 wh

Am 13.12.2015 13:09, schrieb Michael Titke:
> Hello!
> 
> The current version of the manual page describing /inotify/ (as well as
> the version installed with Ubuntu 15.04 frozen to Violet Indigo)
> contains a descriptive C structure describing the /inotify//messages/:
> 
>            struct inotify_event {
>                int      wd;       /* Watch descriptor */
>                uint32_t mask;     /* Mask describing event */
>                uint32_t cookie;   /* Unique cookie associating related
>                                      events (for rename(2)) */
>                uint32_t len;      /* Size of name field */
>                char     name[];   /* Optional null-terminated name */
>            };
> 
> As part of the development of VSI I translated the above structure
> without much thinking into a corresponding byte structure description:
> 
> (define inotify-event-header
>   (byte-structure-description
>    (wd   int)
>    (mask   4) ; [sic! that was an uint32]
>    (cookie 4)
>    (len    4)
>    ;(name   pointer) ; That's not a pointer but an array: NAME_MAX + 1
> ))
> 
> Now while the prefix (or header) of the message is described adequately
> the specification of the tail array as a char name[] would in C be
> interpreted as a pointer onto a char. One might insert a length
> parameter like char name[NAME_MAX + 1] or exclude that tail array from
> the structure and describe the message in terms of /prefix/ or /header/
> and /tail array/.
> 
>            struct inotify_event {
>                int      wd;       /* Watch descriptor */
>                uint32_t mask;     /* Mask describing event */
>                uint32_t cookie;   /* Unique cookie associating related
>                                      events (for rename(2)) */
>                uint32_t len;      /* Size of name field */
>                char     name[NAME_MAX + 1];   /* Optional
> null-terminated name */
>            };
> 
> With that length parameter in the structure the size of the messages is
> described adequately but the actual length of the name might be confused
> with the maximum size which usually includes some (tail) padding.
> 
> I'm sorry but I don't know enough about /iNotify/ to craft a patch for
> this. Is this a datagram channel where half-read messages will vanish?
> Perhaps I should continue reading the manual page. :-)
> 
> Regards,
> Michael
> 
> -- 
> VSI: https://code.launchpad.net/viper-system-interface
> 
>  (reason: 550 5.7.1 Content-Policy reject msg: The message contains HTML
> subpart, therefore we consider it SPAM or Outlook Virus.  TEXT/PLAIN is
> accepted.! BF:<U
> 
> 
> PS That's not really up to the standard but "... since hosts aren't
> required to relay mail at all ..." someone presumes something.
> -- 
> To unsubscribe from this list: send the line "unsubscribe linux-man" in
> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: iNotify Man Page: Prefix Header + Tail Array vs C Structure

[Michael Titke]

On 13/12/2015 18:53, walter harms wrote:
> I am not sure that i understood what you want ...
>
> i would answer the question:
>
> * what is the actual size of struct inotify_event ?
>
> inotify(7) says:
> the length of each inotify_event structure is thus sizeof(struct inotify_event)+len

Please fix the manual page: the actual size is sizeof (struct 
inotify_event) where struct inotify_event is as described below with a 
size parameter for the name field of NAME_MAX+1 and the actual string 
contained in that field has a length of /len/ (and is probably NULL 
terminated at len+1).

With your answer the actual size would be the length of the fixed prefix 
fields (w/o) plus the size of a pointer (sizeof(void*) for the name 
field w/o parameter plus len): thus you might be off by the size of a 
pointer. Each inotify message has the same width. This only refers to 
the structure described in the manual page! An actual sizeof (struct 
inotify_event) might yield the correct result as the system headers 
probably contain the correct structure. But that diversion is what makes 
the manual page a little bit confusing and presenting a pointer instead 
of a fixed size array is just plainly wrong.

Please review my original message as well as reference documentation 
regarding the C programming language.

HTH,
Michael


>
> hope that helps ..
>
> re,
>   wh
>
> Am 13.12.2015 13:09, schrieb Michael Titke:
>> Hello!
>>
>> The current version of the manual page describing /inotify/ (as well as
>> the version installed with Ubuntu 15.04 frozen to Violet Indigo)
>> contains a descriptive C structure describing the /inotify//messages/:
>>
>>             struct inotify_event {
>>                 int      wd;       /* Watch descriptor */
>>                 uint32_t mask;     /* Mask describing event */
>>                 uint32_t cookie;   /* Unique cookie associating related
>>                                       events (for rename(2)) */
>>                 uint32_t len;      /* Size of name field */
>>                 char     name[];   /* Optional null-terminated name */
>>             };
>>
>> As part of the development of VSI I translated the above structure
>> without much thinking into a corresponding byte structure description:
>>
>> (define inotify-event-header
>>    (byte-structure-description
>>     (wd   int)
>>     (mask   4) ; [sic! that was an uint32]
>>     (cookie 4)
>>     (len    4)
>>     ;(name   pointer) ; That's not a pointer but an array: NAME_MAX + 1
>> ))
>>
>> Now while the prefix (or header) of the message is described adequately
>> the specification of the tail array as a char name[] would in C be
>> interpreted as a pointer onto a char. One might insert a length
>> parameter like char name[NAME_MAX + 1] or exclude that tail array from
>> the structure and describe the message in terms of /prefix/ or /header/
>> and /tail array/.
>>
>>             struct inotify_event {
>>                 int      wd;       /* Watch descriptor */
>>                 uint32_t mask;     /* Mask describing event */
>>                 uint32_t cookie;   /* Unique cookie associating related
>>                                       events (for rename(2)) */
>>                 uint32_t len;      /* Size of name field */
>>                 char     name[NAME_MAX + 1];   /* Optional
>> null-terminated name */
>>             };
>>
>> With that length parameter in the structure the size of the messages is
>> described adequately but the actual length of the name might be confused
>> with the maximum size which usually includes some (tail) padding.
>>
>> I'm sorry but I don't know enough about /iNotify/ to craft a patch for
>> this. Is this a datagram channel where half-read messages will vanish?
>> Perhaps I should continue reading the manual page. :-)
>>
>> Regards,
>> Michael
>>
>> -- 
>> VSI: https://code.launchpad.net/viper-system-interface
>>
>>   (reason: 550 5.7.1 Content-Policy reject msg: The message contains HTML
>> subpart, therefore we consider it SPAM or Outlook Virus.  TEXT/PLAIN is
>> accepted.! BF:<U
>>
>>
>> PS That's not really up to the standard but "... since hosts aren't
>> required to relay mail at all ..." someone presumes something.
>> -- 
>> To unsubscribe from this list: send the line "unsubscribe linux-man" in
>> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html

--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: iNotify Man Page: Prefix Header + Tail Array vs C Structure

[Michael Kerrisk (man-pages)]

Hello Michael,

On 12/14/2015 10:46 AM, Michael Titke wrote:
> 
> 
> On 13/12/2015 18:53, walter harms wrote:
>> I am not sure that i understood what you want ...
>>
>> i would answer the question:
>>
>> * what is the actual size of struct inotify_event ?
>>
>> inotify(7) says:
>> the length of each inotify_event structure is thus sizeof(struct inotify_event)+len
> 
> Please fix the manual page: the actual size is sizeof (struct 
> inotify_event) where struct inotify_event is as described below with a 
> size parameter for the name field of NAME_MAX+1 

If I understand you correctly, what you're saying is not correct.
The 'name' field is variable length,

> and the actual string 
> contained in that field has a length of /len/ (and is probably NULL 
> terminated at len+1).

Again, if I understand what you are trying to say, this is not
correct. The man page is I think rather clear:

       The name field is present only when an event is returned for  a
       file  inside  a watched directory; it identifies the file path‐
       name relative to the watched directory.  This pathname is null-
       terminated,  and may include further null bytes ('\0') to align
       subsequent reads to a suitable address boundary.

       The len field counts all of the bytes in  name,  including  the
       null  bytes; the length of each inotify_event structure is thus
       sizeof(struct inotify_event)+len.

> With your answer the actual size would be the length of the fixed prefix 
> fields (w/o) plus the size of a pointer (sizeof(void*) 

That's not what Walter said.

> for the name 
> field w/o parameter plus len): thus you might be off by the size of a 
> pointer. Each inotify message has the same width. This only refers to 
> the structure described in the manual page! An actual sizeof (struct 
> inotify_event) might yield the correct result as the system headers 
> probably contain the correct structure. 

If you grep through the headers in /usr/include, I think
you'll get a surprise...

> But that diversion is what makes 
> the manual page a little bit confusing and presenting a pointer instead 
> of a fixed size array is just plainly wrong.

The man page is not presenting a pointer. See below.

> Please review my original message as well as reference documentation 
> regarding the C programming language.

Sorry to say, but I think you might want to review the C standard.
The syntax shown in the man page is precisely the C99 way for
declaring a flexible length array at the end of a structure:

https://en.wikipedia.org/wiki/Flexible_array_member

IOW, I think the page is fine.

Cheers,

Michael


>>> The current version of the manual page describing /inotify/ (as well as
>>> the version installed with Ubuntu 15.04 frozen to Violet Indigo)
>>> contains a descriptive C structure describing the /inotify//messages/:
>>>
>>>             struct inotify_event {
>>>                 int      wd;       /* Watch descriptor */
>>>                 uint32_t mask;     /* Mask describing event */
>>>                 uint32_t cookie;   /* Unique cookie associating related
>>>                                       events (for rename(2)) */
>>>                 uint32_t len;      /* Size of name field */
>>>                 char     name[];   /* Optional null-terminated name */
>>>             };
>>>
>>> As part of the development of VSI I translated the above structure
>>> without much thinking into a corresponding byte structure description:
>>>
>>> (define inotify-event-header
>>>    (byte-structure-description
>>>     (wd   int)
>>>     (mask   4) ; [sic! that was an uint32]
>>>     (cookie 4)
>>>     (len    4)
>>>     ;(name   pointer) ; That's not a pointer but an array: NAME_MAX + 1
>>> ))
>>>
>>> Now while the prefix (or header) of the message is described adequately
>>> the specification of the tail array as a char name[] would in C be
>>> interpreted as a pointer onto a char. One might insert a length
>>> parameter like char name[NAME_MAX + 1] or exclude that tail array from
>>> the structure and describe the message in terms of /prefix/ or /header/
>>> and /tail array/.
>>>
>>>             struct inotify_event {
>>>                 int      wd;       /* Watch descriptor */
>>>                 uint32_t mask;     /* Mask describing event */
>>>                 uint32_t cookie;   /* Unique cookie associating related
>>>                                       events (for rename(2)) */
>>>                 uint32_t len;      /* Size of name field */
>>>                 char     name[NAME_MAX + 1];   /* Optional
>>> null-terminated name */
>>>             };
>>>
>>> With that length parameter in the structure the size of the messages is
>>> described adequately but the actual length of the name might be confused
>>> with the maximum size which usually includes some (tail) padding.
>>>
>>> I'm sorry but I don't know enough about /iNotify/ to craft a patch for
>>> this. Is this a datagram channel where half-read messages will vanish?
>>> Perhaps I should continue reading the manual page. :-)
>>>
>>> Regards,
>>> Michael
>>>
>>> -- 
>>> VSI: https://code.launchpad.net/viper-system-interface
>>>
>>>   (reason: 550 5.7.1 Content-Policy reject msg: The message contains HTML
>>> subpart, therefore we consider it SPAM or Outlook Virus.  TEXT/PLAIN is
>>> accepted.! BF:<U
>>>
>>>
>>> PS That's not really up to the standard but "... since hosts aren't
>>> required to relay mail at all ..." someone presumes something.
>>> -- 
>>> To unsubscribe from this list: send the line "unsubscribe linux-man" in
>>> the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: iNotify Man Page: Prefix Header + Tail Array vs C Structure

[Michael Titke]

There won't be any future discussions on this level. Just put a wontfix 
on it but stop burbling. There won't be any more reports regarding man 
pages. That project is to be criticized by all means.
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html