Article: Q60252
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s):
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 29-NOV-2001
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The information in this article applies to:
- Microsoft C for MS-DOS
- Microsoft C/C++ for MS-DOS
- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 4.0
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0
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SUMMARY
=======
In Microsoft C, you can initialize the values of an entire bitfield structure as
an integer in several ways:
- Declare the bitfield structure as part of a union with an integer. (This is
the preferred method).
- Use an integer pointer by setting the pointer to the address of the structure
and then changing what the pointer points to.
- Enforce the bitfield type constraints to get a copy of the bitfield into an
integer variable.
For examples of these three methods, see below.
MORE INFORMATION
================
In Microsoft C, bitfields are stored in word-sized blocks with the least
significant bit representing the first bit of the bitfield. For example, the
bitfields in bitstruct, defined below in the example, are stored as follows:
< p4 > < p3 > < p2> <p1>
|?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|
Assigning the integer 0x4c to this structure results in the following bit
pattern:
|0|0|0|0|0|0|0|0|0|1|0|0|1|1|0|0|
The bitfields are given the following respective values:
p1=0 p2=3 p3=2 p4=0
If the number of bits needed for a bitfield structure exceeds 16 in code compiled
for MS-DOS or Windows, words will be added as needed to provide room for the
structure with no single bitfield crossing a word boundary. Microsoft C/C++,
32-bit Edition, stores bitfield structures in double word values. If the field
requires more than 32 bits, the compiler uses additional double words as
needed.
NOTE: There is no "standard" for storing bitfields in memory; therefore, any
program that depends on a particular storage method is not portable to systems
that use a different method.
Sample Code
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struct strtype
{
unsigned p1:2;
unsigned p2:3;
unsigned p3:5;
unsigned p4:5;
} bitstruct;
union untype
{
struct strtype un_bitstruct;
unsigned bit_integer;
} bitunion;
unsigned *intptr;
unsigned intgr;
void main(void)
{
/* Using the bitfield structure only */
/* Set the pointer to address of bitfield */
intptr = (unsigned *)&bitstruct;
/* Change the bitfield */
*intptr = 0x4c;
/* Get the new value */
intgr = *(unsigned *)&bitstruct;
/* Using an union makes this much easier (syntactically) */
/* Set the pointer */
intptr = &bitunion.bit_integer;
/* Change the bitfield */
bitunion.bit_integer = 0x4c;
/* Get the new value */
intgr = bitunion.bit_integer;
}
Additional query words: 8.00 8.00c 9.00
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Keywords : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Technology : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC500Search
Version : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type : kbinfo
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