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NSPR Reference
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Chapter 19   BitMaps

This chapter describes the bitmap type and the functions for manipulating the bitmap type.

Bitmap Type
Bitmap Functions and Macros
Logarithmic Functions and Macros

Bitmap Type

The prbitmap_t type represents a bitmap structure.

prbitmap_t

Represents a bitmap.


syntax 
#include <prbit.h>


typedef unsigned long prbitmap_t;


description
Use prbitmap_t as an array to define a bitmap.

Bitmap Functions and Macros

The API defined for bitmaps is consistent across all platforms. The bitmap macros are:

PR_TEST_BIT
PR_SET_BIT
PR_CLEAR_BIT

Logarithmic functions and macros are also defined in the bitmap API, but are not limited to use with bitmaps. See Logarithmic Functions and Macros.

PR_TEST_BIT

Tests a bit within a bitmap array.


Syntax 
#include <prbit.h>


PRIntn PR_TEST_BIT (
   prbitmap_t *_map, 
   PRInt32 _bit);


Parameter
The macro has these parameters:

_map

Address of the bitmap containing the bit to be tested.

_bit

The relative position within the bitmap of the bit to be tested.


Returns
Non-zero if the bit is on, otherwise zero.


Description
This macro tests the bit at the specified relative position within the specified bitmap. The macro evaluates to TRUE (non-zero) if the bit is set. If the bit is clear, the macro evaluates to zero.

PR_SET_BIT

Sets a bit within a bitmap array.


Syntax 
#include <prbit.h>


void PR_SET_BIT (
   prbitmap_t *_map,
   PRInt32 _bit);


Parameter
The macro has these parameters:

_map

Address of the bitmap containing the bit to be set.

_bit

The relative position within the bitmap of the bit to be set.


Returns
Nothing.


Description
This macro sets the bit at the specified relative position in the specified bitmap to one.

PR_CLEAR_BIT

Clears a bit within a bitmap array.


Syntax 
#include <prbit.h>


void PR_CLEAR_BIT (
   prbitmap_t *_map, 
   PRInt32 _bit);


Parameter
The macro has these parameters:

_map

Address of the bitmap containing the bit to be cleared.

_bit

The relative position within the bitmap of the bit to be cleared.


Returns
Nothing.


Description
This macro sets the bit at the specified relative position in the specified bitmap to zero.

Logarithmic Functions and Macros

The bitmap files also define these logarithmic functions and macros, that need not be used specifically with bitmaps:

PR_CeilingLog2
PR_FloorLog2
PR_CEILING_LOG2
PR_FLOOR_LOG2

PR_CeilingLog2

Returns the ceiling of log2 of a number.


Syntax 
#include <prbit.h>


PRIntn PR_CeilingLog2 (PRUint32 i);


Parameter
The function has this parameter:

i

An unsigned 32-bit number.


Returns
Ceiling of log2 of i.


Description
This function takes the log2 of the specified 32-bit integer, and returns the smallest integer that is greater than or equal to it. This is the same as the C operation Ceil(Log2(i));


Example 
PRIntn j = PR_CeilingLog2( 32 ); /* After execution, j == 5 */

PR_FloorLog2

Returns the floor of log2 of a number.


Syntax 
#include <prbit.h>


PRIntn PR_FloorLog2 (PRUint32 i);


Parameter
The function has this parameter:

i

An unsigned 32-bit number.


Returns
Floor of log2 of i.


Description
This function takes the log2 of the specified 32-bit integer, and returns the largest integer that is less than or equal to it. This is the same as the C operation Floor(Log2(i));

PR_CEILING_LOG2

Returns ceiling of log2 of a number in a variable.


Syntax 
#include <prbit.h>


void PR_CEILING_LOG2 (
   PRUint32 _log,
   PRUint32 i);


Parameter
The macro has these parameters:

_log

A variable that will contain the result of the operation.

i

An unsigned 32-bit number.


Returns
Nothing.


Description
This macro takes the log2 of the specified 32-bit integer, and finds the smallest integer that is greater than or equal to it. The result is bound to the variable _log. The parameters must be variables of the proper type predefined by the caller.

PR_FLOOR_LOG2

Returns floor of log2 of a number in a variable.


Syntax 
#include <prbit.h>


void PR_FLOOR_LOG2 (
   PRUint32 _log,
   PRUint32 i);


Parameter
The macro has these parameters:

_log

A variable that will contain the result of the operation.

i

An unsigned 32-bit number.


Returns
Nothing.


Description
This macro takes the log2 of the specified 32-bit integer, and finds the largest integer that is less than or equal to it. The result is bound to the variable _log. The parameters must be variables of the proper type predefined by the caller.

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Last Updated May 18, 2001