Re: Implicit String-Literal Concatenation

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: Implicit String-Literal Concatenation
Date	2024-02-28 15:31 -0800
Organization	None to speak of
Message-ID	<87y1b4tbcq.fsf@nosuchdomain.example.com> (permalink)
References	(3 earlier) <urlgfn$3d1ah$3@dont-email.me> <urlmo7$3eg2j$1@dont-email.me> <urn6sv$3s62i$2@dont-email.me> <877ciouua2.fsf@nosuchdomain.example.com> <uroe02$4eoh$1@dont-email.me>

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bart <bc@freeuk.com> writes:
> On 28/02/2024 21:57, Keith Thompson wrote:
>> David Brown <david.brown@hesbynett.no> writes:
>> [...]
>>> They won't use strings, they will use data blobs - binary data.  Then
>>> there is no issue with null bytes.  And yes, implementations will skip
>>> the token generation (unless you are doing something weird, such as
>>> using #embed to read the parameters to a function call).
>>>
>>> Tests with prototype implementations gave extremely fast results.
>> I'm not sure how that would work.  #embed is a preprocessor
>> directive,
>> and at least in the abstract model it has to expand to valid C code.
>> I would have expected that it would simply generate the list of
>> comma-separated integer constants described in the standard; later
>> phases would simply parse that list and generate code as if that
>> sequence had been written in the original source file.  Do you know of
>> an implementation that does something else?
>> For example, say you have a file "foo.dat" containing 4 bytes with
>> values 0, 1, 2, and 3.  This would be perfectly valid:
>>      struct foo {
>>          unsigned char a;
>>          unsigned short b;
>>          unsigned int c;
>>          double d;
>>      };
>>      struct foo obj = {
>> #embed "foo.dat"
>>      };
>
> It would be unfortunate if your example was allowed. Clearly a binary
> representation of an instance of your struct would probably require 16 
> bytes rather than 4, of which one may be padding.

Depending on the sizes and alignments of the various types, sure.
So what?

N3096 is the latest public C23 draft.

https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf

#embed is defined in section 6.10.3.

    The expansion of a #embed directive is a token sequence
    formed from the list of integer constant expressions described
    below. The group of tokens for each integer constant expression
    in the list is separated in the token sequence from the group
    of tokens for the previous integer constant expression in the
    list by a comma. The sequence neither begins nor ends in a
    comma. If the list of integer constant expressions is empty,
    the token sequence is empty. The directive is replaced by its
    expansion and, with the presence of certain embed parameters,
    additional or replacement token sequences.

It's a preprocessor directive.  The preprocessor operates on text and
proprocessing tokens, not on raw data.  There is no way to directly
represent raw data in C source code.  (I suppose string literals do so
to some extent, but they can't represent generalized raw data.)

The usage I described above is allowed.  I see nothing unfortunate about
it.  If you only want to use #embed with arrays of unsigned char, then
do that.

Its primary intended use is to read binary file contents at compile time
and allow a program to treat those contents as a raw representation,
particularly as the initialization for an array of unsigned char.  There
was no reason to impose arbitrary restrictions to make it impossible to
use for any other purposes.

I suppose it would have been possible for #embed to expand to the raw
data itself, a binary copy of the input file.  That would require C
source code, which currently is plain text, to be able to support
delimited chunks of binary data.  It would require changes to portions
of the compiler after the preprocessor.  Presumably you'd be able to
write the same representation directly in a C source file, which means
that C source files would no longer necessarily be representable as
text.  I can see that causing all kinds of problems.

Fortunately, none of that was necessary, since the authors came up with
a way to define #embed in the preprocessor without making any other
changes to how C source code is represented.  The fact that it can be
used in other odd ways doesn't bother me.  The code I wrote above is
valid; I never said it was acceptable style.

> Certainly if you were to write it out to disk as binary, it would need
> more than 4.

Yes.  So what?

>> #embed isn't defined to translate an input file to a sequence of bytes.
>> It's defined to translate an input file to a sequence of integer
>> constant expressions.
>
> Maybe it should be defined exactly like that, because that is what
> people might expect. You example is better off using a normal text
> file which contains an actual comma-delimited list (and which can mix
> ints and floats), and a regular #include.

I certainly wouldn't advocate writing code like the above.  My point is
that, given the definition of #embed in the C23 standard, it's valid and
has well defined semantics.

If you have suggestions for alternate ways to define #embed, they might
be interesting, but it's too late to change the existing specification.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Medtronic
void Void(void) { Void(); } /* The recursive call of the void */

Thread

Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-24 23:05 +0000
  Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-25 17:38 +0100
    Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-25 20:43 +0000
      Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-25 21:20 +0000
  Re: Implicit String-Literal Concatenation Blue-Maned_Hawk <bluemanedhawk@invalid.invalid> - 2024-02-25 16:45 +0000
  Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-25 20:25 +0000
  Re: Implicit String-Literal Concatenation Łukasz 'Maly' Ostrowski <l3vi4than@gmail.com> - 2024-02-26 21:12 +0100
    Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-26 20:31 +0000
      Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-27 13:18 +0100
        Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-27 23:10 +0000
          Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-28 00:50 +0100
  Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-02-26 20:42 +0000
  Re: Implicit String-Literal Concatenation porkchop@invalid.foo (Mike Sanders) - 2024-02-26 22:03 +0000
    Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-26 23:17 +0000
      Re: Implicit String-Literal Concatenation porkchop@invalid.foo (Mike Sanders) - 2024-02-27 17:27 +0000
    Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-27 09:36 +0100
      Re: Implicit String-Literal Concatenation porkchop@invalid.foo (Mike Sanders) - 2024-02-27 17:31 +0000
      Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-27 18:56 +0000
        Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-27 23:21 +0100
          Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-27 22:52 +0000
            Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-02-28 01:09 +0000
              Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-28 12:50 +0100
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-28 20:56 +0000
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-28 21:34 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-28 23:52 +0000
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 00:15 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-29 02:53 +0000
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 09:20 +0000
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 15:48 +0000
                Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-29 17:03 +0100
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 16:17 +0000
                Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-29 18:12 +0100
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 17:30 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 13:20 -0800
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 21:44 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 14:06 -0800
                Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-03-01 18:09 +0100
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-03-01 10:49 -0800
                Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-03-01 22:06 +0100
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 09:20 -0800
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 08:58 +0100
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-29 21:05 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-03-01 09:16 +0100
                Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-03-01 16:55 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-03-01 18:28 +0100
      Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-27 20:25 +0000
        Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-27 12:35 -0800
          Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-27 23:03 +0000
        Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-27 22:12 +0000
          Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-28 12:54 +0100
            Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-28 13:13 +0000
              Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-28 15:08 +0100
              Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-28 13:36 -0800
                Re: Implicit String-Literal Concatenation Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-02-29 11:56 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 16:19 +0100
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-29 21:36 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 13:53 -0800
                Re: Implicit String-Literal Concatenation Richard Harnden <richard.nospam@gmail.invalid> - 2024-03-01 12:59 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-03-01 20:59 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 08:08 -0800
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 14:31 +0000
                Re: Implicit String-Literal Concatenation Richard Harnden <richard.nospam@gmail.invalid> - 2024-02-29 15:22 +0000
                Re: Implicit String-Literal Concatenation "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-02-29 13:10 -0800
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 13:45 -0800
                Re: Implicit String-Literal Concatenation "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-02-29 14:03 -0800
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 14:14 -0800
                Re: Implicit String-Literal Concatenation "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-03-02 13:48 -0800
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-03-05 04:48 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-03-04 20:55 -0800
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-03-07 21:08 +0000
                Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-03-07 21:44 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-03-07 14:25 -0800
                Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-03-07 23:00 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-03-07 15:46 -0800
                Re: Implicit String-Literal Concatenation "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-03-07 16:17 -0800
                Re: Implicit String-Literal Concatenation Richard Harnden <richard.nospam@gmail.invalid> - 2024-03-08 00:26 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-03-07 14:16 -0800
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 16:30 +0100
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 08:25 -0800
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 08:18 -0800
                Re: Implicit String-Literal Concatenation Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-02-29 18:17 +0100
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 09:22 -0800
                Re: Implicit String-Literal Concatenation Kaz Kylheku <433-929-6894@kylheku.com> - 2024-02-29 19:26 +0000
                Re: Implicit String-Literal Concatenation James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-02-29 14:45 -0500
              Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-28 13:41 -0800
            Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-28 13:57 -0800
              Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-28 23:01 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-28 15:31 -0800
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 00:47 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-28 17:12 -0800
                Re: Implicit String-Literal Concatenation tTh <tth@none.invalid> - 2024-02-29 16:29 +0100
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 16:15 +0000
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 15:53 +0000
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 09:06 -0800
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 17:28 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 18:58 +0100
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 18:05 +0000
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-02-29 18:09 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-02-29 21:27 +0000
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-03-01 11:52 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-03-05 04:47 +0000
                Re: Implicit String-Literal Concatenation scott@slp53.sl.home (Scott Lurndal) - 2024-03-05 15:09 +0000
                Re: Implicit String-Literal Concatenation Lawrence D'Oliveiro <ldo@nz.invalid> - 2024-03-06 01:49 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 20:51 +0100
              Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 10:10 +0100
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-02-29 10:18 +0000
                Re: Implicit String-Literal Concatenation tTh <tth@none.invalid> - 2024-02-29 16:34 +0100
                Re: Implicit String-Literal Concatenation bart <bc@freeuk.com> - 2024-03-01 11:58 +0000
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-03-01 13:17 +0100
                Re: Implicit String-Literal Concatenation Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-02-29 09:03 -0800
                Re: Implicit String-Literal Concatenation David Brown <david.brown@hesbynett.no> - 2024-02-29 19:08 +0100

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