Microsoft KB Archive/60252

= INFO: Initializing Bitfields as Integers in C =

Article ID: 60252

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft C Professional Development System 6.0a
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 5.0 Professional Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q60252





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>  |?|?|?|?|?|?|?|?|?|?|?|?|?|?|?|?| 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 &quot;standard&quot; 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
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

Keywords: kbinfo kblangc kbcode KB60252

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