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Date: Wed, 24 Apr 2013 10:09:59 -0400
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Porting to 64 bit Cygwin (was Re: Difference in 32/64-bit curl.)
Message-ID: <20130424140959.GA2170@ednor.casa.cgf.cx>
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References: <51766C22 DOT 10702 AT bahnhof DOT se> <51770372 DOT 4070408 AT t-online DOT de> <20130424125930 DOT GQ26397 AT calimero DOT vinschen DOT de> <20130424134749 DOT GR26397 AT calimero DOT vinschen DOT de>
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Maybe the below email should be a FAQ reference?

cgf

On Wed, Apr 24, 2013 at 03:47:49PM +0200, Corinna Vinschen wrote:
>On Apr 24 14:59, Corinna Vinschen wrote:
>> On Apr 23 23:56, Christian Franke wrote:
>> > Possibly a __builtin_va_list related gcc bug.
>> 
>> This is rather unlikely.  That code is shared between Cygwin and
>> Mingw, and chances are that the bug would have been found already.
>> 
>> What about a type issue?  int vs. long?
>
>For clearness I decided to add a quick lecture.  Hope that's ok.
>
>The Cygwin x86_64 toolchain is using the LP64(*) data model.  That means,
>in contrast to Windows, which uses an LLP64(*) data model, sizeof(long)
>!= sizeof(int), just as on Linux.
>
>For comparison:
>
>                 Cygwin   Windows  Cygwin
>		 Linux    x86_64   Linux
>		 Windows           x86_64
>		 i686
>
>sizeof(int)         4        4        4
>sizeof(long)        4        4        8
>sizeof(size_t)      4        8        8
>sizeof(void*)       4        8        8
>
>This difference can result in interesting problems, especially when
>using Win32 functions, especially when using pointers to Windows
>datatypes like LONG, ULONG, DWORD.  Given that Windows is LLP64, all of
>these are 4 byte in size, while `long' on Cygwin is 8 bytes.
>
>Take the example ReadFile:
>
>  ReadFile (HANDLE, LPVOID, DWORD, LPDWORD, LPOVERLAPPED);
>
>In the 32 bit Cygwin and Mingw environments, as well as in the 64 bit
>Mingw environment, it is no problem to substitute DWORD with unsigned
>long:
>
>  unsigned long number_of_bytes_read;
>  [...]
>  ReadFile (fhdl, buf, buflen, &number_of_bytes_read, NULL);
>
>However, in 64 bit Cygwin, using LP64, number_of_bytes_read is 8 bytes
>in size.  But ReadFile expects a pointer to a 4 byte type.  So the
>function will only change the lower 4 bytes of number_of_bytes_read,
>while the content of the upper 4 bytes is undefined.
>
>Here are a few donts which may help porting applications from the
>ILP32 to the new LP64 data model.  Note that this is not a Cygwin-only
>problem.  Many Linux applications suffered the same somewhat liberal
>handling of datatypes when x86_64 was new.
>
>- DON'T mix up int and long in printf/scanf.  This:
>
>    int i; long l;
>    printf ("%d %ld\n", l, i);
>
>  may not print what you think it should.
>
>- DON'T mix int and long pointers.
>
>    int *ip = (int *) &my_long; /* Uh oh! */
>
>- DON'T mix int and pointers at all!  THis will NOT work as expected
>  anymore:
>
>    void *ptr;
>    printf ("Pointer value is %x\n", (int) ptr);
>
>- DON'T use functions returning pointers without declaration.  For instance
>
>    printf ("Error message is: %s\n", strerror (errno));
>
>  This code will CRASH, unless you included string.h.  The implicit
>  rule in C is that an undeclared function is of type int.  But int
>  is 4 byte and pointers are 8 byte, so the string pointer given to
>  printf is missing the upper 4 bytes.  Hilarity ensues ;)
>
>- DON'T use C base types together with Win32 functions.  Keep in mind
>  that DWORD, LONG, ULONG are *not* the same as long and unsigned long.
>  Try to use only Win32 datatypes in conjunction with Win32 API function
>  calls to avoid type problems.
>
>- DON'T mix Windows dataypes with POSIX type-specific MIN/MAX values.
>
>    unsigned long l_max = ULONG_MAX;	/* That's right. */
>    ULONG w32_biggest = ULONG_MAX;	/* Hey, wait!  What? */
>    ULONG w32_biggest = UINT_MAX;	/* Ok. */
>
>  Always keep in mind that ULONG (or DWORD) != unsigned long but
>  rather == unsigned int now.
>
>
>HTH,
>Corinna
>
>
>(*) http://en.wikipedia.org/wiki/LLP64#64-bit_data_models
>
>-- 
>Corinna Vinschen                  Please, send mails regarding Cygwin to
>Cygwin Maintainer                 cygwin AT cygwin DOT com
>Red Hat
>
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>

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