Do I Need an Extern "C" Block to Include Standard Posix C Headers

Do I need an extern C block to include standard POSIX C headers?

The behavior of <fcntl.h> and <unistd.h> in C++ is not specified by the standard (because they are also not part of the C89 standard). That said, I have never seen a platform where they (a) exist and (b) actually need to be wrapped in an extern "C" block.

The behavior of <stdio.h>, <math.h>, and the other standard C headers is specified by section D.5 of the C++03 standard. They do not require an extern "C" wrapper block, and they dump their symbols into the global namespace. However, everything in Annex D is "deprecated".

The canonical C++ form of those headers is <cstdio>, <cmath>, etc., and they are specified by section 17.4.1.2 (3) of the C++ standard, which says:

<cassert> <ciso646> <csetjmp> <cstdio> <ctime> <cctype> <climits>
<csignal> <cstdlib> <cwchar> <cerrno> <clocale> <cstdarg> <cstring>
<cwctype>

Except as noted in clauses 18 through 27, the contents of each header
cname shall be the same as that of the corresponding header name.h, as
specified in ISO/IEC 9899:1990 Programming Languages C (Clause 7), or
ISO/IEC:1990 Programming Languages—C AMENDMENT 1: C Integrity, (Clause
7), as appropriate, as if by inclusion. In the C++ Standard Library,
however, the declarations and definitions (except for names which are
defined as macros in C) are within namespace scope (3.3.5) of the
namespace std.

So the standard, non-deprecated, canonical way to use (e.g.) printf in C++ is to #include <cstdio> and then invoke std::printf.

Why do we need extern C{ #include foo.h } in C++?

C and C++ are superficially similar, but each compiles into a very different set of code. When you include a header file with a C++ compiler, the compiler is expecting C++ code. If, however, it is a C header, then the compiler expects the data contained in the header file to be compiled to a certain format—the C++ 'ABI', or 'Application Binary Interface', so the linker chokes up. This is preferable to passing C++ data to a function expecting C data.

(To get into the really nitty-gritty, C++'s ABI generally 'mangles' the names of their functions/methods, so calling printf() without flagging the prototype as a C function, the C++ will actually generate code calling _Zprintf, plus extra crap at the end.)

So: use extern "C" {...} when including a c header—it's that simple. Otherwise, you'll have a mismatch in compiled code, and the linker will choke. For most headers, however, you won't even need the extern because most system C headers will already account for the fact that they might be included by C++ code and already extern "C" their code.

What is the effect of extern C in C++?

extern "C" makes a function-name in C++ have C linkage (compiler does not mangle the name) so that client C code can link to (use) your function using a C compatible header file that contains just the declaration of your function. Your function definition is contained in a binary format (that was compiled by your C++ compiler) that the client C linker will then link to using the C name.

Since C++ has overloading of function names and C does not, the C++ compiler cannot just use the function name as a unique id to link to, so it mangles the name by adding information about the arguments. A C compiler does not need to mangle the name since you can not overload function names in C. When you state that a function has extern "C" linkage in C++, the C++ compiler does not add argument/parameter type information to the name used for linkage.

Just so you know, you can specify extern "C" linkage to each individual declaration/definition explicitly or use a block to group a sequence of declarations/definitions to have a certain linkage:

extern "C" void foo(int);
extern "C"
{
   void g(char);
   int i;
}

If you care about the technicalities, they are listed in section 7.5 of the C++03 standard, here is a brief summary (with emphasis on extern "C"):

extern "C" is a linkage-specification
Every compiler is required to provide "C" linkage
A linkage specification shall occur only in namespace scope
~~All function types, function names and variable names have a language linkage~~ See Richard's Comment: Only function names and variable names with external linkage have a language linkage
Two function types with distinct language linkages are distinct types even if otherwise identical
Linkage specs nest, inner one determines the final linkage
extern "C" is ignored for class members
At most one function with a particular name can have "C" linkage (regardless of namespace)
~~extern "C" forces a function to have external linkage (cannot make it static)~~ See Richard's comment: static inside extern "C" is valid; an entity so declared has internal linkage, and so does not have a language linkage
Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved

C: What is the use of 'extern' in header files?

A header file is used so that you won't repeat yourself. In your example, you didn't need to write

extern int one;

in headertest2.c, because it would already get included in that file via the header file.

Not repeating yourself is not a small thing. Imagine you have a hundred files that use this global variable (one). Do you want to write extern int one in all of them? What if there were 20 of these variables and 50 more functions? What if you wanted to change int to uint32_t?

Surely, duplicating definitions is tedious and error-prone.

Let's take a look at stdio.h for example. Without headers, you would have to copy-paste the following code in every file that wanted to do I/O:

/* Define ISO C stdio on top of C++ iostreams.
   Copyright (C) 1991, 1994-2008, 2009, 2010 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

/*
 *  ISO C99 Standard: 7.19 Input/output <stdio.h>
 */

#ifndef _STDIO_H

#if !defined __need_FILE && !defined __need___FILE
# define _STDIO_H   1
# include <features.h>

__BEGIN_DECLS

# define __need_size_t
# define __need_NULL
# include <stddef.h>

# include <bits/types.h>
# define __need_FILE
# define __need___FILE
#endif /* Don't need FILE.  */

#if !defined __FILE_defined && defined __need_FILE

/* Define outside of namespace so the C++ is happy.  */
struct _IO_FILE;

__BEGIN_NAMESPACE_STD
/* The opaque type of streams.  This is the definition used elsewhere.  */
typedef struct _IO_FILE FILE;
__END_NAMESPACE_STD
#if defined __USE_LARGEFILE64 || defined __USE_SVID || defined __USE_POSIX \
    || defined __USE_BSD || defined __USE_ISOC99 || defined __USE_XOPEN \
    || defined __USE_POSIX2
__USING_NAMESPACE_STD(FILE)
#endif

# define __FILE_defined 1
#endif /* FILE not defined.  */
#undef  __need_FILE

#if !defined ____FILE_defined && defined __need___FILE

/* The opaque type of streams.  This is the definition used elsewhere.  */
typedef struct _IO_FILE __FILE;

# define ____FILE_defined   1
#endif /* __FILE not defined.  */
#undef  __need___FILE

#ifdef  _STDIO_H
#define _STDIO_USES_IOSTREAM

#include <libio.h>

#if defined __USE_XOPEN || defined __USE_XOPEN2K8
# ifdef __GNUC__
#  ifndef _VA_LIST_DEFINED
typedef _G_va_list va_list;
#   define _VA_LIST_DEFINED
#  endif
# else
#  include <stdarg.h>
# endif
#endif

#ifdef __USE_XOPEN2K8
# ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
# endif
# if defined __USE_LARGEFILE64 && !defined __off64_t_defined
typedef __off64_t off64_t;
# define __off64_t_defined
# endif

# ifndef __ssize_t_defined
typedef __ssize_t ssize_t;
# define __ssize_t_defined
# endif
#endif

/* The type of the second argument to `fgetpos' and `fsetpos'.  */
__BEGIN_NAMESPACE_STD
#ifndef __USE_FILE_OFFSET64
typedef _G_fpos_t fpos_t;
#else
typedef _G_fpos64_t fpos_t;
#endif
__END_NAMESPACE_STD
#ifdef __USE_LARGEFILE64
typedef _G_fpos64_t fpos64_t;
#endif

/* The possibilities for the third argument to `setvbuf'.  */
#define _IOFBF 0        /* Fully buffered.  */
#define _IOLBF 1        /* Line buffered.  */
#define _IONBF 2        /* No buffering.  */

/* Default buffer size.  */
#ifndef BUFSIZ
# define BUFSIZ _IO_BUFSIZ
#endif

/* End of file character.
   Some things throughout the library rely on this being -1.  */
#ifndef EOF
# define EOF (-1)
#endif

/* The possibilities for the third argument to `fseek'.
   These values should not be changed.  */
#define SEEK_SET    0   /* Seek from beginning of file.  */
#define SEEK_CUR    1   /* Seek from current position.  */
#define SEEK_END    2   /* Seek from end of file.  */

#if defined __USE_SVID || defined __USE_XOPEN
/* Default path prefix for `tempnam' and `tmpnam'.  */
# define P_tmpdir   "/tmp"
#endif

/* Get the values:
   L_tmpnam How long an array of chars must be to be passed to `tmpnam'.
   TMP_MAX  The minimum number of unique filenames generated by tmpnam
        (and tempnam when it uses tmpnam's name space),
        or tempnam (the two are separate).
   L_ctermid    How long an array to pass to `ctermid'.
   L_cuserid    How long an array to pass to `cuserid'.
   FOPEN_MAX    Minimum number of files that can be open at once.
   FILENAME_MAX Maximum length of a filename.  */
#include <bits/stdio_lim.h>

/* Standard streams.  */
extern struct _IO_FILE *stdin;      /* Standard input stream.  */
extern struct _IO_FILE *stdout;     /* Standard output stream.  */
extern struct _IO_FILE *stderr;     /* Standard error output stream.  */
/* C89/C99 say they're macros.  Make them happy.  */
#define stdin stdin
#define stdout stdout
#define stderr stderr

__BEGIN_NAMESPACE_STD
/* Remove file FILENAME.  */
extern int remove (__const char *__filename) __THROW;
/* Rename file OLD to NEW.  */
extern int rename (__const char *__old, __const char *__new) __THROW;
__END_NAMESPACE_STD

#ifdef __USE_ATFILE
/* Rename file OLD relative to OLDFD to NEW relative to NEWFD.  */
extern int renameat (int __oldfd, __const char *__old, int __newfd,
             __const char *__new) __THROW;
#endif

__BEGIN_NAMESPACE_STD
/* Create a temporary file and open it read/write.

   This function is a possible cancellation points and therefore not
   marked with __THROW.  */
#ifndef __USE_FILE_OFFSET64
extern FILE *tmpfile (void) __wur;
#else
# ifdef __REDIRECT
extern FILE *__REDIRECT (tmpfile, (void), tmpfile64) __wur;
# else
#  define tmpfile tmpfile64
# endif
#endif

#ifdef __USE_LARGEFILE64
extern FILE *tmpfile64 (void) __wur;
#endif

/* Generate a temporary filename.  */
extern char *tmpnam (char *__s) __THROW __wur;
__END_NAMESPACE_STD

#ifdef __USE_MISC
/* This is the reentrant variant of `tmpnam'.  The only difference is
   that it does not allow S to be NULL.  */
extern char *tmpnam_r (char *__s) __THROW __wur;
#endif

#error Omitted due to post length limit

__BEGIN_NAMESPACE_STD
#ifndef __USE_FILE_OFFSET64
/* Get STREAM's position.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);
/* Set STREAM's position.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fsetpos (FILE *__stream, __const fpos_t *__pos);
#else
# ifdef __REDIRECT
extern int __REDIRECT (fgetpos, (FILE *__restrict __stream,
                 fpos_t *__restrict __pos), fgetpos64);
extern int __REDIRECT (fsetpos,
               (FILE *__stream, __const fpos_t *__pos), fsetpos64);
# else
#  define fgetpos fgetpos64
#  define fsetpos fsetpos64
# endif
#endif
__END_NAMESPACE_STD

#ifdef __USE_LARGEFILE64
extern int fseeko64 (FILE *__stream, __off64_t __off, int __whence);
extern __off64_t ftello64 (FILE *__stream) __wur;
extern int fgetpos64 (FILE *__restrict __stream, fpos64_t *__restrict __pos);
extern int fsetpos64 (FILE *__stream, __const fpos64_t *__pos);
#endif

__BEGIN_NAMESPACE_STD
/* Clear the error and EOF indicators for STREAM.  */
extern void clearerr (FILE *__stream) __THROW;
/* Return the EOF indicator for STREAM.  */
extern int feof (FILE *__stream) __THROW __wur;
/* Return the error indicator for STREAM.  */
extern int ferror (FILE *__stream) __THROW __wur;
__END_NAMESPACE_STD

#ifdef __USE_MISC
/* Faster versions when locking is not required.  */
extern void clearerr_unlocked (FILE *__stream) __THROW;
extern int feof_unlocked (FILE *__stream) __THROW __wur;
extern int ferror_unlocked (FILE *__stream) __THROW __wur;
#endif

__BEGIN_NAMESPACE_STD
/* Print a message describing the meaning of the value of errno.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern void perror (__const char *__s);
__END_NAMESPACE_STD

/* Provide the declarations for `sys_errlist' and `sys_nerr' if they
   are available on this system.  Even if available, these variables
   should not be used directly.  The `strerror' function provides
   all the necessary functionality.  */
#include <bits/sys_errlist.h>

#ifdef  __USE_POSIX
/* Return the system file descriptor for STREAM.  */
extern int fileno (FILE *__stream) __THROW __wur;
#endif /* Use POSIX.  */

#ifdef __USE_MISC
/* Faster version when locking is not required.  */
extern int fileno_unlocked (FILE *__stream) __THROW __wur;
#endif

#if (defined __USE_POSIX2 || defined __USE_SVID  || defined __USE_BSD || \
     defined __USE_MISC)
/* Create a new stream connected to a pipe running the given command.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern FILE *popen (__const char *__command, __const char *__modes) __wur;

/* Close a stream opened by popen and return the status of its child.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int pclose (FILE *__stream);
#endif

#ifdef  __USE_POSIX
/* Return the name of the controlling terminal.  */
extern char *ctermid (char *__s) __THROW;
#endif /* Use POSIX.  */

#ifdef __USE_XOPEN
/* Return the name of the current user.  */
extern char *cuserid (char *__s);
#endif /* Use X/Open, but not issue 6.  */

#ifdef  __USE_GNU
struct obstack;         /* See <obstack.h>.  */

/* Write formatted output to an obstack.  */
extern int obstack_printf (struct obstack *__restrict __obstack,
               __const char *__restrict __format, ...)
     __THROW __attribute__ ((__format__ (__printf__, 2, 3)));
extern int obstack_vprintf (struct obstack *__restrict __obstack,
                __const char *__restrict __format,
                _G_va_list __args)
     __THROW __attribute__ ((__format__ (__printf__, 2, 0)));
#endif /* Use GNU.  */

#if defined __USE_POSIX || defined __USE_MISC
/* These are defined in POSIX.1:1996.  */

/* Acquire ownership of STREAM.  */
extern void flockfile (FILE *__stream) __THROW;

/* Try to acquire ownership of STREAM but do not block if it is not
   possible.  */
extern int ftrylockfile (FILE *__stream) __THROW __wur;

/* Relinquish the ownership granted for STREAM.  */
extern void funlockfile (FILE *__stream) __THROW;
#endif /* POSIX || misc */

#if defined __USE_XOPEN && !defined __USE_XOPEN2K && !defined __USE_GNU
/* The X/Open standard requires some functions and variables to be
   declared here which do not belong into this header.  But we have to
   follow.  In GNU mode we don't do this nonsense.  */
# define __need_getopt
# include <getopt.h>
#endif  /* X/Open, but not issue 6 and not for GNU.  */

/* If we are compiling with optimizing read this file.  It contains
   several optimizing inline functions and macros.  */
#ifdef __USE_EXTERN_INLINES
# include <bits/stdio.h>
#endif
#if __USE_FORTIFY_LEVEL > 0 && defined __extern_always_inline
# include <bits/stdio2.h>
#endif
#ifdef __LDBL_COMPAT
# include <bits/stdio-ldbl.h>
#endif

__END_DECLS

#endif /* <stdio.h> included.  */

#endif /* !_STDIO_H */

That, is why we have header files.

C Compiler error from standard headers - undefined C++ definitions

I found the problem:

These constants were supposed to be defined in sys/cdefs.h.

For some reason this file was in /usr/include/bsd/sys/cdefs.h.

The bsd version of cdefs.h did not have these constants defined.

I removed the bsd directory and reinstalled libc6-dev.

Everything appears to be working now.

Difference between C standard library and C POSIX library

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.

Does the ISO 9899 standard has reserved any use of the _t suffix for identifiers?

Standard C allows you to use the _t suffix so long as you don't end up with a token that starts with a double underscore. (Note that C++ restricts this further in that a double underscore is not allowed anywhere in the token; worth adhering to should you anticipate your code reaching C++.)

It's POSIX that reserves _t.

Do I Need an Extern "C" Block to Include Standard Posix C Headers