Discussion:
Double-underline-itis
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BartC
2017-02-28 23:44:37 UTC
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I came across this declaration in some Linux header:

extern int mbtowc (wchar_t *__restrict __pwc,
const char *__restrict __s, size_t __n) __THROW;

There are thousands just the same. I can understand the double-underline
prefix used for implementation names that may clash with those in the
user namespace.

But what's with the double-underline on the parameter names? They
shouldn't clash with anything. (And here, I assumed __pwc was yet
another attribute that the implementation thinks is needed.)

Sometimes, as well as the double-underline at the start, they need to
have one at the end too, such as '__restrict__' (what's wrong with just
'restrict', if you need to have it all?).

Underlines are hard to see and tend to run together so you can't tell
easily if there are two or three.

Since $ isn't used much, what would have been wrong with that? It's not
pretty but at least you can see it!
--
bartc
s***@casperkitty.com
2017-02-28 23:54:32 UTC
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Post by BartC
But what's with the double-underline on the parameter names? They
shouldn't clash with anything. (And here, I assumed __pwc was yet
another attribute that the implementation thinks is needed.)
User code is allowed to declare macros with almost any names the user sees
fit, and might do so prior to including standard headers. If user code
started with

#define fmt "(Quack! My favorite number is %d)"
#include <stdio.h>

and the definition for printf were

int printf(char const *restrict fmt, ...);

then compilation of the header would fail.

Such constructs don't generally cause trouble, but I don't think they're
conforming when they use non-reserved identifiers.
Ben Bacarisse
2017-03-01 01:19:04 UTC
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Post by BartC
extern int mbtowc (wchar_t *__restrict __pwc,
const char *__restrict __s, size_t __n) __THROW;
There are thousands just the same. I can understand the
double-underline prefix used for implementation names that may clash
with those in the user namespace.
But what's with the double-underline on the parameter names? They
shouldn't clash with anything.
That's why the underscores are there. Without them, the names could
clash with user-defined macros.
Post by BartC
(And here, I assumed __pwc was yet
another attribute that the implementation thinks is needed.)
Why? I would assume it's just the parameter name unless I had strong
evidence that it was not.
Post by BartC
Sometimes, as well as the double-underline at the start, they need to
have one at the end too, such as '__restrict__' (what's wrong with
just 'restrict', if you need to have it all?).
__restrict__ can be #defined away when processing the header with a
pre-1999 version of C. You can't do that with restrict because it's not
a name reserved to the implementation. Obviously one could use
different headers for different C versions, but this method is simple
and effective.
Post by BartC
Underlines are hard to see and tend to run together so you can't tell
easily if there are two or three.
I don't have that problem. _, __ and ___ all look very different to
me. Maybe it's the font I use -- there is a slight gap between
consecutive underscores.
Post by BartC
Since $ isn't used much, what would have been wrong with that? It's
not pretty but at least you can see it!
--
Ben.
David Brown
2017-03-01 08:11:51 UTC
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Post by Ben Bacarisse
Post by BartC
extern int mbtowc (wchar_t *__restrict __pwc,
const char *__restrict __s, size_t __n) __THROW;
There are thousands just the same. I can understand the
double-underline prefix used for implementation names that may clash
with those in the user namespace.
But what's with the double-underline on the parameter names? They
shouldn't clash with anything.
That's why the underscores are there. Without them, the names could
clash with user-defined macros.
Post by BartC
(And here, I assumed __pwc was yet
another attribute that the implementation thinks is needed.)
Why? I would assume it's just the parameter name unless I had strong
evidence that it was not.
Post by BartC
Sometimes, as well as the double-underline at the start, they need to
have one at the end too, such as '__restrict__' (what's wrong with
just 'restrict', if you need to have it all?).
__restrict__ can be #defined away when processing the header with a
pre-1999 version of C. You can't do that with restrict because it's not
a name reserved to the implementation. Obviously one could use
different headers for different C versions, but this method is simple
and effective.
There is also the opposite effect. "restrict" can be #defined to
anything you want in pre-C99 or C++, so if the header used "restrict" it
would not work in pre-C99 modes, or C++ modes. But gcc (and probably
clang, and maybe other compilers) allows restricted pointers in C++ mode
and other C modes, using "__restrict__" instead of "restrict". So by
using "__restrict__" rather than "restrict", the header works for a
wider variety of compiler modes.

(Not that "restrict" in a function declaration does much other than
document the restriction, as has been discussed on another thread.)
Post by Ben Bacarisse
Post by BartC
Underlines are hard to see and tend to run together so you can't tell
easily if there are two or three.
I don't have that problem. _, __ and ___ all look very different to
me. Maybe it's the font I use -- there is a slight gap between
consecutive underscores.
Post by BartC
Since $ isn't used much, what would have been wrong with that? It's
not pretty but at least you can see it!
BartC
2017-03-01 11:45:41 UTC
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Post by Ben Bacarisse
Post by BartC
extern int mbtowc (wchar_t *__restrict __pwc,
const char *__restrict __s, size_t __n) __THROW;
There are thousands just the same. I can understand the
double-underline prefix used for implementation names that may clash
with those in the user namespace.
But what's with the double-underline on the parameter names? They
shouldn't clash with anything.
That's why the underscores are there. Without them, the names could
clash with user-defined macros.
Yes, that's what supercat said. But then, macros can cause all sorts of
mischief. Anyone producing a header file for a library, and wishing to
use parameter names in function declarations, would also be at risk of
having those names clashing with macros declared before the header.

What can do they do? Use __ prefixes too? That would partly defeat the
purpose of using parameter names to document their uses, and would
clutter up the code. And __ is probably reserved for the implementation,
so they could then clash with macros in the standard headers.
Post by Ben Bacarisse
Post by BartC
(And here, I assumed __pwc was yet
another attribute that the implementation thinks is needed.)
Why? I would assume it's just the parameter name unless I had strong
evidence that it was not.
(I was looking for what was causing my parsing of the declaration to
fail, I saw __pwc and declared that as another blank macro. As it turned
out, wchar_t wasn't defined some reason. Eventually I'll have my own
headers without any of these problems.

As it is, I have to painstakingly scan along looking for commas before I
can even see how many parameters there are. Written in a saner manner:

extern int mbtowc (wchar_t dest, char* source, size_t n);

Ah! Now it's obvious, even with those ugly "_t" suffixes; it presumably
copies source to dest converting UTF8 or whatever to a wide-character
array. Only the exactly meaning of n is unclear, but I would assume it's
the maximum capacity of dest.
Post by Ben Bacarisse
Post by BartC
Sometimes, as well as the double-underline at the start, they need to
have one at the end too, such as '__restrict__' (what's wrong with
just 'restrict', if you need to have it all?).
__restrict__ can be #defined away when processing the header with a
pre-1999 version of C. You can't do that with restrict because it's not
a name reserved to the implementation. Obviously one could use
different headers for different C versions, but this method is simple
and effective.
You mean that

#define restrict

won't work because someone could have....

I don't even understand the problem. These are system headers, designed
to work with a particular implementation, so will know whether
'restrict' is supported or not. What's the point of such a keyword being
introduced if it then goes onto use something else?

Or do the headers know the implementation doesn't recognise restrict?
That's hard to believe because my set of gcc headers use __restrict too.

(And they seem to favour a single underline for parameter names. So a
sort of primitive unary counting system:

abc 0, user identifier
_abc 1, implementation, grade 1?
__abc 2, implementation, grade 2?
___abc 3, reserved, or system identifier _abc with __ prefix?
__abc__ ? I give up!)
Post by Ben Bacarisse
Post by BartC
Underlines are hard to see and tend to run together so you can't tell
easily if there are two or three.
I don't have that problem. _, __ and ___ all look very different to
me. Maybe it's the font I use -- there is a slight gap between
consecutive underscores.
In my own editor, in Notepad, in SciTe, in the C highlighting of both
Pastebin and Github, and in my Thunderbird newsreader, underlines run
together. Only with a couple of things I tried in Ubuntu was there a
miniscule gap of what looked to be one pixel.

(Actually, that might also be the case in Windows; if I magnify the
screen pixels, then I can see a slight greyish discontinuity between the
_ characters. Probably an effect of character smoothing. But they still
look like they run together.)

--
___bart_c
Ben Bacarisse
2017-03-01 12:30:41 UTC
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Post by BartC
Post by Ben Bacarisse
Post by BartC
extern int mbtowc (wchar_t *__restrict __pwc,
const char *__restrict __s, size_t __n) __THROW;
There are thousands just the same. I can understand the
double-underline prefix used for implementation names that may clash
with those in the user namespace.
But what's with the double-underline on the parameter names? They
shouldn't clash with anything.
That's why the underscores are there. Without them, the names could
clash with user-defined macros.
Yes, that's what supercat said. But then, macros can cause all sorts
of mischief. Anyone producing a header file for a library, and wishing
to use parameter names in function declarations, would also be at risk
of having those names clashing with macros declared before the header.
I'm not sure what your point is. You asked a question and I tried to
answer it (in fact I'd say I did answer it). Are you saying that system
headers should not take this into account because some other header
might not bother?
Post by BartC
What can do they do?
They should either not use names in the header or document the names
used so annt one #defining one of them prior to including the header
will know what's gone wrong.

<snip>
Post by BartC
Post by Ben Bacarisse
Post by BartC
Sometimes, as well as the double-underline at the start, they need to
have one at the end too, such as '__restrict__' (what's wrong with
just 'restrict', if you need to have it all?).
__restrict__ can be #defined away when processing the header with a
pre-1999 version of C. You can't do that with restrict because it's not
a name reserved to the implementation. Obviously one could use
different headers for different C versions, but this method is simple
and effective.
You mean that
#define restrict
won't work because someone could have....
I don't even understand the problem. These are system headers,
designed to work with a particular implementation, so will know
whether 'restrict' is supported or not. What's the point of such a
keyword being introduced if it then goes onto use something else?
Many implementations support various C standards. As I said, you could
include a different header for C90, C99 and so on but it's easier to
#define __restrict__ to either nothing or restrict depending on the
language version.

<snip>
--
Ben.
BartC
2017-03-01 13:04:28 UTC
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Post by Ben Bacarisse
Post by BartC
You mean that
#define restrict
won't work because someone could have....
I don't even understand the problem. These are system headers,
designed to work with a particular implementation, so will know
whether 'restrict' is supported or not. What's the point of such a
keyword being introduced if it then goes onto use something else?
Many implementations support various C standards. As I said, you could
include a different header for C90, C99 and so on but it's easier to
#define __restrict__ to either nothing or restrict depending on the
language version.
I've just come across __signed__! How many C systems don't support 'signed'?

typedef __signed__ char __s8;
typedef unsigned char __u8;

Is this so __signed__ could be defined as 'unsigned' according to some
compiler option? That seems unlikely when you declaring distinct s8 and
u8 types.

(In a file from a Linux set of includes, one of various assorted headers
that get loaded even when just using a handful of standard headers in
the user-code. include\asm-generic\int-ll64.h

This was just after I had to 'remove' one time.h because it didn't
define clock(). But there were 4 other time.h files in sub-directories,
so no worries!

What a mess these system headers are. I hadn't realised they were so
Heath Robinson, looking like a bunch of hacks put together to meet some
deadline, instead of something more permanent.

Anyway these headers do seem to rely heavily on the use of "__" prefixes
absolutely everywhere. And, for some reason that I'm not sure has been
explained yet, on "__" suffixes.)
--
Bartc
Ben Bacarisse
2017-03-01 15:29:30 UTC
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Post by BartC
Post by Ben Bacarisse
Post by BartC
You mean that
#define restrict
won't work because someone could have....
I don't even understand the problem. These are system headers,
designed to work with a particular implementation, so will know
whether 'restrict' is supported or not. What's the point of such a
keyword being introduced if it then goes onto use something else?
Many implementations support various C standards. As I said, you could
include a different header for C90, C99 and so on but it's easier to
#define __restrict__ to either nothing or restrict depending on the
language version.
I've just come across __signed__! How many C systems don't support 'signed'?
Why do you assume is about support? My first reaction when I see
something like this that I don't understand is that it's probably a my
fault. Yes, there is often junk in these files but there is often stuff
I can't be sure about because it's solving a problem I've not yet
grasped.

<snip>
--
Ben.
Kenny McCormack
2017-03-01 15:47:27 UTC
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In article <***@bsb.me.uk>,
Ben Bacarisse <***@bsb.me.uk> wrote:
...
Post by Ben Bacarisse
Why do you assume is about support? My first reaction when I see
something like this that I don't understand is that it's probably a my
fault. Yes, there is often junk in these files but there is often stuff
I can't be sure about because it's solving a problem I've not yet
grasped.
Given your situation in life, I think that's a very reasonable position for
you to take. Man's got to know his limitations...!
--
"The party of Lincoln has become the party of John Wilkes Booth."

- Carlos Alazraqui -
Keith Thompson
2017-03-01 17:30:23 UTC
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BartC <***@freeuk.com> writes:
[...]
Post by BartC
I've just come across __signed__! How many C systems don't support 'signed'?
typedef __signed__ char __s8;
typedef unsigned char __u8;
[...]

The "signed" keyword and the signed char type were, IIRC, introduced
by ANSI C in 1989. A header designed to work with both pre-ANSI
and ANSI C would have to have some way of dealing with that. And
since the header still works even with C11 (as long as __signed__
is defined properly), there's not much motivation to fix it --
unless you wrongly assume that headers are intended as end user
documentation.

The use of __s8 and __u8, rather than the standard int8_t and
uint8_t, already implies that this is from an old legacy header.
--
Keith Thompson (The_Other_Keith) kst-***@mib.org <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
s***@casperkitty.com
2017-03-01 18:14:52 UTC
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Post by Keith Thompson
[...]
Post by BartC
I've just come across __signed__! How many C systems don't support 'signed'?
typedef __signed__ char __s8;
typedef unsigned char __u8;
[...]
The "signed" keyword and the signed char type were, IIRC, introduced
by ANSI C in 1989. A header designed to work with both pre-ANSI
and ANSI C would have to have some way of dealing with that. And
since the header still works even with C11 (as long as __signed__
is defined properly), there's not much motivation to fix it --
unless you wrongly assume that headers are intended as end user
documentation.
The use of __s8 and __u8, rather than the standard int8_t and
uint8_t, already implies that this is from an old legacy header.
Client code written for systems where "char" is signed might expect
that a function expecting a const pointer to an array of 8-bit signed
values would accept a pointer to a string literal. Even systems where
"char" is signed, however, will not allow implicit conversions from
string literals to "signed char const *".

I think the Standard should have allowed implementations to, at their
option, treat as identical, types which have precisely-matching
representations and semantics. Treating "char" and "signed char" identically
in such cases would make some implementations simpler, and would not affect
the meaning of any program upon which the Standard would impose any
requirements. While some kinds of type incompatibility would be indicative
of possible logic problems (e.g. between an unsigned "char" type and a
"signed char" type), I see no reason to expect that would be true in cases
where the involved types have identical representations and are treated
identically in every way (including aliasing).
Tim Rentsch
2017-04-17 01:45:31 UTC
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Post by Keith Thompson
[...]
I've just come across __signed__! How many C systems don't
support 'signed'?
typedef __signed__ char __s8;
typedef unsigned char __u8;
[...]
The "signed" keyword and the signed char type were, IIRC, introduced
by ANSI C in 1989. A header designed to work with both pre-ANSI
and ANSI C would have to have some way of dealing with that. And
since the header still works even with C11 (as long as __signed__
is defined properly), there's not much motivation to fix it --
unless you wrongly assume that headers are intended as end user
documentation.
The use of __s8 and __u8, rather than the standard int8_t and
uint8_t, already implies that this is from an old legacy header.
It probably is from an old header, but using __s8 and __u8 (as
opposed to int8_t and uint8_t) doesn't necessarily imply that.
The __u8 and __s8 types, as typedef'ed above, are more widely
applicable than the [u]int8_t types, for a variety of reasons.
Presumably someone concerned with maximizing portability
would be aware of such concerns.

TickleMe Elmo
2017-03-01 07:30:39 UTC
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