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Problem with including <unistd.h>

From: Benjamin Lindner
Subject: Problem with including <unistd.h>
Date: Thu, 27 Jan 2011 13:05:50 +0100

Hello list,

building octave for MinGW platform currently encounters compilation
errors in connection with gnulib's unistd.h header file.
Currently octave includes <unistd.h> (among other headers), and then
uses e.g. gnulib::gethostname(), but g++ complains about gethostname
not beloning to namespace gnulib (i.e. having no definition for it).
Strangely, the error disappears, if <unistd.h> is included *twice* in
the corresponding source file.

Debugging the preprocessed source code I found that effectively the
whole unistd.h header was discarded on the first inclusion, and only
the second inclusion really works.

I tracked the problem down to the first #if statements in unistd.h
which on my platform read

/* Special invocation convention:
   - On mingw, several headers, including <winsock2.h>, include <unistd.h>,
     but we need to ensure that both the system <unistd.h> and <winsock2.h>
     are completely included before we replace gethostname.  */
#if 1 && 1 \
  && !defined _GL_WINSOCK2_H_WITNESS && defined _WINSOCK2_H
/* <unistd.h> is being indirectly included for the first time from
   <winsock2.h>; avoid declaring any overrides.  */
# if 1
#  include_next <unistd.h>
# else
#  error unexpected; report this to address@hidden
# endif

/* Normal invocation.  */
#elif !defined _GL_UNISTD_H

Now, if _WINSOCK2_H is defined before unistd.h is included (as this
was the case for octave, because windows.h was included before
unistd.h) this #if clause is true, so it skips all of the remaining
part of the header (after the #elif statement).
Only on the second inclusion, the first #if statement fails (because
_GL_WINSOCK2_H_WITNESS is now defined), so g++ proceeds beyond the
#elif statement and properly includes the rest of the header.

So currently, it would require unistd.h to be either included twice,
or be included before winsock2.h to work properly.


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