I will give an example from The GNU C Library documentation:
13.1 Opening and Closing Files
This section describes the primitives for opening and closing files
using file descriptors. The open and creat functions are declared in
the header file fcntl.h, while close is declared in unistd.h.
My question is:
Can unistd.h and fcntl.h be considered as Standard C? As far as I know, they should be part of the Posix standard?
Can we say C Standard Library = Posix functions + C API? I am confused because Wikipedia page for C Standard Library does not include unistd.h but the GNU C Library documentation includes it?
No, unistd.h, fcntl.h, etc, are not standard C.
In general, standard C doesn't include functions that deal with low level file manipulation. For example, fopen, fread, and fwrite are part of standard C library. While POSIX open, read, write functions are not standard C.
As far as I can see, in C11 standard, there is no unistd.h and fcntl.h. So, strictly speaking, they are not part of the C standard.
When it comes to the implementation part, the GNU C library (glibc) is one of them. From the wiki page
glibc provides the functionality required by the Single UNIX Specification, POSIX (1c, 1d, and 1j) and some of the functionality required by ISO C11, ISO C99, Berkeley Unix (BSD) interfaces, the System V Interface Definition (SVID) and the X/Open Portability Guide (XPG), Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems along with all X/Open UNIX extensions.
In addition, glibc also provides extensions that have been deemed useful or necessary while developing GNU.
So, as a part of the POSIX standard, they are available in glibc.
Reference: Check the C11 standard draft version here.
Related
I'm a little confused by "C standard lib" and "C POSIX lib", because I found that, many header files defined in "C POSIX lib" are also part of "C standard lib".
So, I assume that, "C standard lib" is a lib defined by ANSI C organization, and there are different implementation on different platforms (Win32/Unix-like), and "C POSIX lib" is just a implementation for "C standard lib" on Unix-like OSes, right?
But "C POSIX lib" contains some headers not specified in "C standard lib", such as <sys/types.h>, <sys/wait.h>, and <pthread.h>.
Take <pthread.h> as an example, I presume its "C standard lib" counterpart is <threads.h>, then if I want to write a multi-threaded program on Linux, which header file should I include, <pthread.h> or <threads.h>?
POSIX is a superset of the standard C library, and it's important to note that it defers to it. If C and POSIX is ever in conflict, C wins.
Sockets, file descriptors, shared memory etc. are all part of POSIX, but do not exist in the C library.
pthread.h is used for POSIX threads and threads.h is a new header for C11 and is part of the C library. Perhaps pthreads will be deprecated sometime in the future in favor of the C ones, however you probably can't count on C11 to have widespread deployment yet. Therefore if you want portability you should prefer pthreads for now. If portability is not a concern, and you have C11 threads available, you should probably use those.
The C POSIX library is a specification of a C standard library for POSIX systems. It was developed at the same time as the ANSI C standard. Some effort was made to make POSIX compatible with standard C; POSIX includes additional functions to those introduced in standard C.
POSIX 7 quote
http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap01.html#tag_14_01
1.1 Relationship to Other Formal Standards
Great care has been taken to ensure that this volume of POSIX.1-2008 is fully aligned with the following standards:
ISO C (1999)
ISO/IEC 9899:1999, Programming Languages - C, including ISO/IEC 9899:1999/Cor.1:2001(E), ISO/IEC 9899:1999/Cor.2:2004(E), and ISO/IEC 9899:1999/Cor.3.
Parts of the ISO/IEC 9899:1999 standard (hereinafter referred to as the ISO C standard) are referenced to describe requirements also mandated by this volume of POSIX.1-2008. Some functions and headers included within this volume of POSIX.1-2008 have a version in the ISO C standard; in this case CX markings are added as appropriate to show where the ISO C standard has been extended (see Codes). Any conflict between this volume of POSIX.1-2008 and the ISO C standard is unintentional.
I have listed some major API extensions at: I never really understood: what is POSIX?
ANSI C is still alive, I think: ANSI C is inherited and extended by ISO C, Cxx. POSIX have been obeying ANSI C absolutely."
We can write ANSI C on Windows, Unix-Like, embedded device easily; but Cxx, or POSIX may have issue.
When working with strdup on Windows I found out that _strdup is Windows specific, but when I ran the same code on Linux it required strdup without the underscore. Does anyone know the history behind this difference, as-well as some information on how you have dealt with this problem when writing cross-platform code?
There are several functions that are part of the POSIX specification, i.e. Linux and most other UNIX variants, that are not part of standard C. These include strdup, write, read, and others.
The reasoning for the leading underscore is as follows, taken from the MSDN docs:
The Universal C Run-Time Library (UCRT) supports most of the C
standard library required for C++ conformance. It implements the C99
(ISO/IEC 9899:1999) library, with certain exceptions: The type-generic
macros defined in , and strict type compatibility in
. The UCRT also implements a large subset of the POSIX.1
(ISO/IEC 9945-1:1996, the POSIX System Application Program Interface)
C library. However, it's not fully conformant to any specific POSIX
standard. The UCRT also implements several Microsoft-specific
functions and macros that aren't part of a standard.
Functions specific to the Microsoft implementation of Visual C++ are
found in the vcruntime library. Many of these functions are for
internal use and can't be called by user code. Some are documented for
use in debugging and implementation compatibility.
The C++ standard reserves names that begin with an underscore in the
global namespace to the implementation. Both the POSIX functions and
Microsoft-specific runtime library functions are in the global
namespace, but aren't part of the standard C runtime library. That's
why the preferred Microsoft implementations of these functions have a
leading underscore. For portability, the UCRT also supports the
default names, but the Microsoft C++ compiler issues a deprecation
warning when code that uses them is compiled. Only the default names
are deprecated, not the functions themselves. To suppress the warning,
define _CRT_NONSTDC_NO_WARNINGS before including any headers in code
that uses the original POSIX names.
I've handled that by having a #define that check if the program is being compiled for Windows, and if so create another #define to map the POSIX name to the Windows specific name. There are a few choices you can check, although probably the most reliable is _MSC_VER which is defined if MSVC is the compiler.
#ifdef _MSC_VER
#define strdup(p) _strdup(p)
#endif
I am currently learning C. I understand that many common functions, like printf and scanf are not actually part of the C language -- they are part of the "standard library" of functions.
My question is, why aren't such functions built into the language? Is it a philosophical/design consideration? A matter of efficiency when compiling programs? A necessity in order to act as a "middle layer" to ensure compatibility with different operating systems? Something else entirely?
They are part of C. A C implementation consists of a compiler and a library (and other components, like a linker).
The C core language includes facilities that make it possible to write library code that can be used by other programs. The standard library portion of the standard specifies a library that can be implemented using the facilities defined in the core language.
Some languages do have things like a print command built into the language. C's facilities for writing and invoking library code written in C are powerful enough that that's not necessary.
Furthermore, most of the library is optional for "freestanding" implementations (mostly for embedded systems). There are implementations that support the full core language but that don't provide most of the C standard library.
And a compiler and library can be provided separately. For example, gcc is a compiler; it's commonly used with different library implementations on different systems (GNU libc on Linux, "newlib" on Cygwin, the Microsoft library on Windows with MinGW, and so forth). Mixing and matching like that would be much more difficult if the library were integrated into the core language.
The C language standard (the link is to the newest freely available draft) defines C. Section 6 defines the core language; section 7 defines the standard library.
The thing is that standard allows two kinds of conforming implementation: hosted and freestading, see N1570 4/p6:
The two forms of conforming implementation are hosted and
freestanding. A conforming hosted implementation shall accept any
strictly conforming program. A conforming freestanding implementation
shall accept any strictly conforming program that does not use complex
types and in which the use of the features specified in the library
clause (clause 7) is confined to the contents of the standard headers
<float.h>, <iso646.h>, <limits.h>, <stdalign.h>, <stdarg.h>, <stdbool.h>,
<stddef.h>, <stdint.h>, and <stdnoreturn.h>. A conforming implementation may have
extensions (including additional library functions), provided they do
not alter the behavior of any strictly conforming program.
By such design, where the libraries are organized as standard headers it's convienent to simply "cut-off" some header if it is not supported.
Note that C standard defines all headers along with function for standard C library. It's included in C standard indeed, not separate thing somewhere else.
They're part of the language, they're just not part of the grammar.
Factoring I/O out into a separate function library buys you the following:
The grammar becomes easier to parse;
You can target a wider range of platforms; any I/O foibles are handled by the library functions, not by hacking the code generator;
You can implement only as much as is needed to support the platform (i.e., an embedded controller probably doesn't need to read or write formatted output);
I'm a little confused by "C standard lib" and "C POSIX lib", because I found that, many header files defined in "C POSIX lib" are also part of "C standard lib".
So, I assume that, "C standard lib" is a lib defined by ANSI C organization, and there are different implementation on different platforms (Win32/Unix-like), and "C POSIX lib" is just a implementation for "C standard lib" on Unix-like OSes, right?
But "C POSIX lib" contains some headers not specified in "C standard lib", such as <sys/types.h>, <sys/wait.h>, and <pthread.h>.
Take <pthread.h> as an example, I presume its "C standard lib" counterpart is <threads.h>, then if I want to write a multi-threaded program on Linux, which header file should I include, <pthread.h> or <threads.h>?
POSIX is a superset of the standard C library, and it's important to note that it defers to it. If C and POSIX is ever in conflict, C wins.
Sockets, file descriptors, shared memory etc. are all part of POSIX, but do not exist in the C library.
pthread.h is used for POSIX threads and threads.h is a new header for C11 and is part of the C library. Perhaps pthreads will be deprecated sometime in the future in favor of the C ones, however you probably can't count on C11 to have widespread deployment yet. Therefore if you want portability you should prefer pthreads for now. If portability is not a concern, and you have C11 threads available, you should probably use those.
The C POSIX library is a specification of a C standard library for POSIX systems. It was developed at the same time as the ANSI C standard. Some effort was made to make POSIX compatible with standard C; POSIX includes additional functions to those introduced in standard C.
POSIX 7 quote
http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap01.html#tag_14_01
1.1 Relationship to Other Formal Standards
Great care has been taken to ensure that this volume of POSIX.1-2008 is fully aligned with the following standards:
ISO C (1999)
ISO/IEC 9899:1999, Programming Languages - C, including ISO/IEC 9899:1999/Cor.1:2001(E), ISO/IEC 9899:1999/Cor.2:2004(E), and ISO/IEC 9899:1999/Cor.3.
Parts of the ISO/IEC 9899:1999 standard (hereinafter referred to as the ISO C standard) are referenced to describe requirements also mandated by this volume of POSIX.1-2008. Some functions and headers included within this volume of POSIX.1-2008 have a version in the ISO C standard; in this case CX markings are added as appropriate to show where the ISO C standard has been extended (see Codes). Any conflict between this volume of POSIX.1-2008 and the ISO C standard is unintentional.
I have listed some major API extensions at: I never really understood: what is POSIX?
ANSI C is still alive, I think: ANSI C is inherited and extended by ISO C, Cxx. POSIX have been obeying ANSI C absolutely."
We can write ANSI C on Windows, Unix-Like, embedded device easily; but Cxx, or POSIX may have issue.
I sometimes hear people using the terms "libc" and "C standard library" interchangeably. I understand that "libc" is the name (or part of the names) of many popular C standard library implementations. My guess is that because of the widespread use of these implementations, people started referring to the C standard library in general as "libc" although it is not an official name.
Is it correct to refer to the C standard library as "libc"?
I always thought that "libc" just happens to be the name (or part of the names) of many popular C standard library implementations.
This is correct. "libc" is the name of some implementations of the C Standard Library.
As an example of a C Standard Library implementation that is not named "libc," the Microsoft C Standard Library implementation is a part of the "C Run-Time Libraries," usually referred to as the "CRT."
Is it correct to refer to the C standard library as "libc"?
The C Standard Library is not named "libc," so using that term to refer to it generically (and not to a particular implementation) would be incorrect. That said, in most contexts, if you did use the term "libc" to refer to the C Standard Library, you are still likely to be understood.
"libc" is indeed the name of the implementation. It often includes functions that are not part of the C standard, and might not include functions that are part of the C standard. (A common case of the latter is the Standard C math functions being split into a separate "libm".)
'libc' refers to the C standard library. However, the libc has several implementations :
glibc : an implementation of libc written for the GNU Project
klibc : a minimalistic subset implementation of the libc
...
LibC (http://www.gnu.org/s/libc/) is one particular implementation of the C library standard (http://en.wikipedia.org/wiki/C_standard_library#ISO_Standard).