Q58533: Offsetof() Macro Gives Incorrect Results or C1045

Article: Q58533
Product(s): See article
Version(s): 1.00 1.01 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickc s_quickasm | mspl13_c
Last Modified: 13-FEB-1990

The offsetof() macro, defined in stddef.h, does not work correctly
with doubles or floats. It either gives incorrect results or it gives
a C1045, floating-point overflow error. It is possible to work around
this problem by defining your own macro to get the offset of a field
in a structure.

Microsoft is researching this problem and will post new information
here as it becomes available.

Typically, you get the offset of a structure by issuing the following
statement

   off = offsetof (struct foo, bar) ;

where off is an unsigned integer (size_t), foo is a structure, and bar
is a field in the structure foo. The difficulties occur when the field
to which the offsetof() macro refers is a double or a float.

The compiler generates the C1045 floating-point overflow error in some
cases. It is possible to work around this by rearranging the contents
of the structure to which offsetof() is referring.

If the program compiles without giving fatal error C1045
(floating-point overflow), then it will return zeros for all
offsetof() references to the doubles or floats.

Currently, the only way to work around the problem is to redefine the
macro in the following way:

extern int __zero ;

#define NEWOFFSETOF(str,field)  (size_t)&(((str *)__zero)->field)

The variable __zero should be set to 0 (zero) before any NEWOFFSETOF
code is executed.

As you can see, this method does work for statically initialized data.
Because the new macro references a variable, the compiler cannot know
what to initialize the data to. It is impossible to replace the
variable __zero with an actual 0 (zero) because that is what the
original macro is defined as.

Defining the new macro to reference NULL allows you to statically
initialize data, but it also gives the "floating-point overflow"
error.

Following is a sample program, which causes the error to occur:

#include <stdio.h>
#include <stddef.h>

struct s
{
    short   a ;
    double  b ;
} ;

void main (void)
{
    printf ("The offsets are %u and %u\n",
            offsetof (struct s, a),
            offsetof (struct s, b)) ;
}

The results of the program should be as follows:

   The offsets are 0 and 2

In fact, the output is (if it does compile) as follows:

   The offsets are 0 and 0

The program can be rewritten in the following manner:

#include <stdio.h>
#include <stddef.h>

struct s
{
    short   a ;
    double  b ;
} ;

#define NEWOFFSETOF(str,field)  (size_t)&(((str *)__zero)->field)

int __zero = 0 ;

void main (void)
{
    printf ("The offsets are %u and %u\n",
            NEWOFFSETOF (struct s, a),
            NEWOFFSETOF (struct s, b)) ;
}

The results are as follows:

   The offsets are 0 and 2

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.