This question and many more are among the numerous questions on this site that ask if a call to memcpy() is valid with a length\size specified with a zero value.
When answering, everyone quotes the current C standard (in this case C17 ISO/IEC9899:2017 page 283 ),
Where an argument declared as size_t n specifies the length of the array for a function, n can have
the value zero on a call to that function. Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call shall still have valid values, as
described in 7.1.4. On such a call, a function that locates a character finds no occurrence, a function
that compares two character sequences returns zero, and a function that copies characters copies
zero characters.
However, this is from C standard, not quoted from the C++ standard.
Where in the current C++ standard (i.e. C++17 ISO/IEC 14882) is this same definition listed? C and C++ have two different standards (and languages) and from my understanding, you cannot quote one and expect that same rule/behavior to be present in the other standard.
If this quotation from the C standard is valid in C++ without explicitly stating it in the standard, can someone then provide a source that backs up this connection between C and C++?
The C++17 standards says this about the C standard library:
1
The C++ standard library also makes available the facilities of the C standard library, suitably adjusted to ensure static type safety.
2
The descriptions of many library functions rely on the C standard library for the semantics of those functions. In some cases, the signatures specified in this International Standard may be different from the signatures in the C standard library, and additional overloads may be declared in this International Standard, but the behavior and the preconditions (including any preconditions implied by the use of an ISO C restrict qualifier) are the same unless otherwise stated.
As to your question,
If this quotation from the C standard is valid in C++ without explicitly stating it in the standard
The answer is "yes".
[library.c]/2, emphasis mine:
The descriptions of many library functions rely on the C standard library for the semantics of those functions.
In some cases, the signatures specified in this document may be different from the signatures in the C standard library, and additional overloads may be declared in this document, but the behavior and the preconditions (including any preconditions implied by the use of an ISO C restrict qualifier) are the same unless otherwise stated.
[cstring.syn]/1:
The contents and meaning of the header <cstring> are the same as the C standard library header <string.h>.
The C library is sort of baked into C++. As said in [intro.scope] from the C++17 standard (emphasis mine):
C++ is a general purpose programming language based on the C programming language as described in
ISO/IEC 9899:2011 Programming languages — C (hereinafter referred to as the C standard). In addition to
the facilities provided by C, C++ provides additional data types, classes, templates, exceptions, namespaces,
operator overloading, function name overloading, references, free store management operators, and additional
library facilities.
And again, in [intro.refs]:
The library described in Clause 7 of ISO/IEC 9899:2011 is hereinafter called the C standard library.1
1) With the qualifications noted in Clauses 21 through 33 and in C.5, the C standard library is a subset of the C++ standard
library.
So everything that is in the C standard library is in C++.
Related
NOTE: This is a c question, though I added c++ in case some C++ expert can provide a rationale or historical reason why C++ is using a different wording than C.
In the C standard library specification, we have this normative text, C17 7.1.3 Reserved identifiers (emphasis mine):
All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.
Now I keep reading answers on SO by various esteemed C experts, where they claim it is fine for a compiler or standard library to use identifiers with underscore + uppercase, or double underscore.
Doesn't "reserved for any use" mean reserved for anyone except future extensions to the C language itself? Meaning that the implementation is not allowed to use them.
While the second phrase above, regarding single leading underscore seems to be directed to the implementation?
In general, the C standard is written in a way that expects compiler vendors/library implementers to be the typical reader - not so much the application programmers.
Notably, C++ has a very different wording:
Each name that contains a double underscore (__) or begins with an underscore followed by an uppercase letter (2.11) is reserved to the implementation for any use.
(See What are the rules about using an underscore in a C++ identifier?)
Is this perhaps a mix-up between C and C++ and the languages are different here?
In the C standard, the meaning of the term "reserved" is defined by 7.1.3p2, immediately below the bullet list you are quoting:
No other identifiers are reserved. If the program declares or defines an identifier in a context in which it is reserved (other than as allowed by 7.1.4), or defines a reserved identifier as a macro name, the behavior is undefined.
Emphasis mine: reserved identifiers place a restriction on the program, not the implementation. Thus, the common interpretation – reserved identifiers may be used by the implementation to any purpose – is correct for C.
I have not kept up with the C++ standard and no longer feel qualified to interpret it.
While the Standard is primarily written to guide implementers, it is written as a description of what makes a program well-formed, and what its effect is. That's because the basic definition of a standards-conforming compiler is one that does the correct thing for any standards-conforming program:
A strictly conforming program shall use only those features of the language and library
specified in this International Standard....A conforming
hosted implementation shall accept any strictly conforming program.
Read separately, this is hugely restrictive of extensions to a compiler. For instance, based solely on that clause, a compiler shouldn't get to define any of its own reserved words. After all, any given word a particular compiler might want to reserve, could nevertheless show up in a strictly conforming program, forcing the compiler's hand.
The standard goes on, however:
A conforming implementation may have extensions (including additional
library functions), provided they do not alter the behavior of any strictly conforming
program.
That's the key piece. Compiler extensions need to be written in such a way that they affect nonconforming programs (ones which contain undefined behavior, or which shouldn't even compile at all), allowing them to compile and do fun extra things.
So the purpose of defining "reserved identifiers", when the language doesn't actually need those identifiers for anything, is to give implementations some extra wiggle room by providing them with some things which make a program nonconforming. The reason a compiler can recognize, say, __declspec as part of a declaration is because putting __declspec into a declaration is otherwise illegal, so the compiler is allowed to do whatever it wants!
The importance of "reserved for any use", therefore, is that it leaves no question about a compiler's power to treat such identifiers as having any meaning it cares to. Future compatibility is a comparatively distant concern.
The C++ standard works in a similar way, though it's a bit more explicit about the gambit:
A conforming implementation may have extensions (including additional library functions), provided they do
not alter the behavior of any well-formed program. Implementations are required to diagnose programs that
use such extensions that are ill-formed according to this International Standard. Having done so, however,
they can compile and execute such programs.
I suspect the difference in wording is down to the C++ standard just being clearer about how extensions are meant to work. Nevertheless, nothing in the C standard precludes an implementation from doing the same thing. (And we all basically ignore the requirement that the compiler warn you every time you use __declspec.)
Regarding the difference in wording in C versus C++, I'm posting my own little research here as reference:
The early K&R C 1st edition has this text:
...names which are intended for use only by functions of the library begin with an underscore so they are less likely to collide with names in a user's program.
K&R 2nd edition added an Appendix B which addresses the standard library, where we can read
External identifiers that begin with an underscore are reserved for use by the library, as are all
other identifiers that begin with an underscore and an upper-case letter or another underscore.
Early ANSI C drafts, as well as "C90" ISO 9899:1990, has the same text as in the current ISO standard.
The earliest C++ drafts however, has a different text, as noted by #hvd, possibly a clarification of the C standard. From DRAFT: 20 September 1994:
17.3.3.1.2 Global names
...
Each name that begins with an underscore and either an uppercase letter or another underscore (2.8) is
reserved to the implementation for any use
So apparently the wording "reserved for any use" was invented by the ANSI/ISO C90 committee, whereas the C++ committee some years later used a clearer wording, similar to the wording in the pre-standard K&R book.
The C99 rationale V5.10 says this below 7.1.3:
Also reserved for the implementor are all external identifiers beginning with an underscore, and
all other identifiers beginning with an underscore followed by a capital letter or an underscore.
This gives a name space for writing the numerous behind-the-scenes non-external macros and
functions a library needs to do its job properly.
This makes the committee's intention quite clear: "reserved for any use" means "reserved for the implementor".
Also of note, the current C standard has the following normative text elsewhere, in 6.2.5:
There may also be
implementation-defined extended signed integer types. 38)
where the informative foot note 38 says:
Implementation-defined keywords shall have the form of an identifier reserved for any use as
described in 7.1.3.
C has multiple contexts in which a symbol can have a definition:
The space of macro names,
The space of formal names of arguments to a macro (this space is specific to each function-like macro),
The space of ordinary identifiers,
The space of tag names,
The space of labels (this space is specific to each function), and
The space of structure/union members (this space is specific to each struct/union).
What "reserved for any use" means that the user code in a compliant program cannot use1 symbols that start with an underscore that is followed by an uppercase letter or another underscore in any of the above contexts. Compare with identifiers that start with a single underscore but are followed by a lowercase number or a digit. This falls into the second class of identifiers that start with an underscore. User code can can be use these identifiers as the names of macro arguments, as labels, or as the names of structure/union members.
"Reserved for any use" does not mean that the implementation cannot use such symbols. The intent of the reservation is to provide a name space that implementations can freely use without concern that the names defined by the implementation will conflict with the names defined by the user code in a compliant program.
1The standard does not quite mean "cannot use". The standard encourages the programmatic use of a small number of names that start with a double underscore. For example, a compliant implementation is required to define __STDC_VERSION__, __FILE__, __LINE__, and __func__. The 2011 version of the standard even gives an example of a presumably compliant program that references __func__.
The C Standard allows implementations to attach any meaning they see fit to reserved identifiers. Most implementations will treat unrecognized identifiers of reserved forms the same as any other recognized identifiers when there is no reason to do otherwise, thus allowing something like:
#ifdef __ACME_COMPILER
#define near __near
#else
#define near
#endif
int near foo;
to declare an identifier foo using a __near qualifier if the code is being processed in an Acme compiler (which would presumably support such a thing), but also be compatible with other compilers that would not require or benefit from the use of such a directive. Nothing would forbid a conforming implementation from defining __ACME_COMPILER and interpreting __near to mean "launch nuclear missiles", but a quality implementation shouldn't go out of its way to break code like the above. If an implementation doesn't know what __ACME_COMPILER is supposed to mean, treating it like any other unknown identifier would allow it to support useful constructs like the above.
It is months late but one point remains the others have not addressed.
Your question can be viewed from the opposite direction. The standard allows the implementation (as you have observed) to use a symbol like _Foo but, more importantly, thereby forbids the implementation from using foo. The latter is reserved for your use.
To understand, for discussion's sake, suppose that a future C standard introduced the new keyword _Foo. The hypothetical implementation was already using this symbol, so what happens?
Answer:
At first, the implementation will not yet have implemented the new standard. Until implemented, the new standard lacks practical effect.
Later, as part of implementing the new standard, the implementation quietly changes each _Foo to _Bar.
No problem.
In fact, if you think about it in this manner, you can say that the way the standard reserves such words is almost the only way it could reserve them.
This question doesn't directly relate to programming or a specific language concept. My question is can we use a reference to the C International Standard (for instance C11) to provide a normative reference to describe any concept from a C library in C++.
To be more specific, in a header <climits> defined in N3797::18.3.3 [c.limits] the C library header is described. But the C Standard provide more comprehensive information about <limits.h>'s content rather than N3797 working draft.
So everything about the C library defined in the C11 is true for the C++ implementation defined in C++11 of C library or we can't rely to what the C standard provides?
For the C standard library C++ falls back on the C standard and for C++11 it falls back on C99 not C11, before C++11 the C standard referenced was C90. This is covered in section 1.2 Normative references which says:
The following referenced documents are indispensable for the
application of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.
and includes:
ISO/IEC 9899:1999, Programming languages — C
ISO/IEC 9899:1999/Cor.1:2001(E), Programming languages — C, Technical Corrigendum 1
ISO/IEC 9899:1999/Cor.2:2004(E), Programming languages — C, Technical Corrigendum 2
ISO/IEC 9899:1999/Cor.3:2007(E), Programming languages — C, Technical Corrigendum 3
and also says:
The library described in Clause 7 of ISO/IEC 9899:1999 and Clause 7 of
ISO/IEC 9899:1999/Cor.1:2001 and Clause 7 of ISO/IEC
9899:1999/Cor.2:2003 is hereinafter called the C standard library.1
The C++ standard uses the term C standard library to refer back to C99 and the TCs and will explicitly state when C++ differs from C.
and section 17.2 The C standard library says:
The C++ standard library also makes available the facilities of the
C standard library, suitably adjusted to ensure static type safety.
The descriptions of many library functions rely on the C standard
library for the signatures and semantics of those functions. In all
such cases, any use of the restrict qualifier shall be omitted.
The cname header files which correspond to C Standard Library name.h files is covered in 17.6.1.2 Headers which says amongst other things:
Except as noted in Clauses 18 through 30 and Annex D, the contents of
each header cname shall be the same as that of the corresponding
header name.h, as specified in the C standard library (1.2) or the C
Unicode TR, as appropriate, as if by inclusion. In the C++ standard
library, however, the declarations (except for names which are defined
as macros in C) are within namespace scope (3.3.6) of the namespace
std. It is unspecified whether these names are first declared within
the global namespace scope and are then injected into namespace std by
explicit using-declarations (7.3.3)
The contents of climits as they relate to limits.h is covered in section 18.3.3 and says:
The contents are the same as the Standard C library header .
[ Note: The types of the constants defined by macros in are
not required to match the types to which the macros refer.—end note ]
Note, as I mentioned in the comment above, the normative references are not taken as a whole, the C++ standard must make explicit reference to a normative reference for it to apply to the C++ standard. See Can we apply content not explicitly cited from the normative references to the C++ standard? for more details.
According to C++11 standard [c.math], the <cmath> header is same as Standard C library header <math.h>.
(Of course, there are several differences, --- namespace, overloads etc. --- but these can be ignored here.)
And according to C99 standard annex F, "An implementation that defines __STDC_IEC_559__ shall conform to the specifications in" the annex F.
Ex. The atan2 may cause a domain error if both arguments are zero, but It must not if __STDC_IEC_559__ is defined.
In C99, many behavior is also dependent on whether __STDC_IEC_559__ is defined or not.
However, it seems that __STDC_IEC_559__ is not mentioned anywhere in C++11 standard.
If so, shall a C++ implementation conform to the specifications in the annex F?
I think that std::numeric_limits<T>::is_iec559() is a substitute, but it seems to mention about only type.
The C++ standard (n3797) includes the C standard library by reference, see s1.2/2.
The library described in Clause 7 of ISO/IEC 9899:1999 and Clause 7 of ISO/IEC 9899:1999/Cor.1:2001
and Clause 7 of ISO/IEC 9899:1999/Cor.2:2003 is hereinafter called the C standard library.
With the qualifications noted in Clauses 18 through 30 and in C.4, the C standard library is a subset of the C++ standard
library.
The standard contains no mention of that symbol, and I would not expect it be defined, since it appears to be specific to Standard C. By not defining that symbol, C++ is not bound by the contents of Annex F.
Instead the C++ standard contains multiple mentions of IEC 559 in a rather more C++-like form. For example,
Shall be true for all specializations in which is_iec559 != false
There is a specific mention in 18.3.2.4/56.
static constexpr bool is_iec559;
True if and only if the type adheres to IEC 559 standard.218
Meaningful for all floating point types.
I think it would be fair to say that C++ includes all the same capabilities (or lack of them), but adapted to the C++ world.
I know Macro implementation of putc() in C, but is it same in C++?
It will depend on your implementation of cstdio. In most cases this is really just a wrapper around stdio.h, with wrappers declared inside the std namespace, and the C and C++ compilers share the same standard library for C functions. For example, VS2010 uses stdio.h for C++, in which putc is implemented as both a macro and a function, depending on environment and other compile-time definitions.
Which version of C++? C++83 (1983)? C++98 (1998)? C++11 (2011)?
The C++98 and C++11 Specifications rely on the ISO C specifications for C Library functions, and do not put additional implementation constraints on them, other than trivial ones like renaming stdio.h to cstdio.h and allowing inclusion without the dot-h suffix.
See: C++98 Specification
See: C++11 Specification
Look in cstdio.h if you are interested in your particular compiler.
However, if we dig deeper and take a look at the ISO C standard: "ISO/IEC 9899:1990" (C89/C90), well, we find that it is unavailable for free viewing on the web (not even the final draft standard), so moving on to C99 (NOT ISO C), you find...
...that C99 (Not "ISO C") says putc() MAY be implemented as a macro,
See: C99 Specification
So if you are really developing in Obj-C++ (which uses C99), then C99 is the relevant specification to consider, not ISO C (C90). Also, since C99 lets the compiler writer decide whether to make putc() a macro or not, you should consider it an open possibility, and decide whether you really care to know about the C90 (ISO C) spec which is becoming obsolete (now that even C11 (2011) is out.)
Yes it is. Both C and C++ use <stdio.h> which has the same scheme in all implementations that I know of.
Are all the functions in a conformant C++98/03/0x implementation completely C99 conformant?
I thought C++0x added some C99 (language) features, but never heard or read anything definitive about the C library functions.
Just to avoid any confusion, I'm talking about a C++ program using functions declared in the <c*> header set.
Thanks.
Most of the C99 standard library has been imported in C++0X but not all. From memory, in what wasn't imported there are
<ctgmath> simply includes <ccomplex> and <cmath>,
<ccomplex> behaves as if it included <complex>
<cmath> has quite a few adjustment (providing overload and template functions completing the C99 provided one)
Some other headers (<cstdbool>, <iso646.h>, ...) have adjustments to take differences between language into account (bool is primitive in C++, a macro provided by <stdbool.h> in C for instance), but nothing of the scope of the math part.
The headers <xxx.h> whose <cxx> form doesn't behaves as the C99 version simply declares the content of <cxxx> in the global namespace, they aren't nearer of the C99 <xxx.h> content.
A related thing: C++0X provides some headers in both cxxx and xxx.h forms which aren't defined in C99 (<cstdalign> and <cuchar>, the second one is defined in a C TR)
(I remembered that a bunch of mathematical functions from C99 had been put in TR1 but not kept in C++0X, I was mistaken, that bunch of mathematical functions weren't part of C99 in the first place).
No. C++03 is aligned with ANSI C89/ISO C90, not C99.
The upcoming C++0x standard is expected to be aligned to some degree with C99. See paragraph 17.6.1.2 in the current draft which lists ccomplex, cinttypes, cstdint etc. Note that, as AProgrammer mentions, some headers aren't exactly the same; further, that the header cuchar is aligned with the C Technical Report 19769 rather than C99.