The decNumber Library, version 3.68
Copyright (c) IBM Corporation, 2010. All rights reserved. ©
 23 Jan 2010 
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The decimal32, decimal64, and decimal128 modules define the data structures
and conversion functions for the IEEE 754 decimalencoded compressed
decimal formats which are 32, 64, or 128 bits (4, 8, or 16 bytes)
long, respectively. These provide up to 7, 16, or 34 digits of
decimal precision in a compact and machineindependent form.
Details of the formats are available at:
http://speleotrove.com/decimal/decbits.html
These modules provide the interface between the compressed numbers
and the decNumber internal format (and also provide string conversions).
The decFloats modules provide arithmetic
and other functions which work on data in the same formats directly.
Example 7 and Example 8 in the User’s Guide show
how to work with data in the formats with or without using the
decNumber module.
Apart from the different lengths and ranges of the numbers, these
three modules are identical, so this section just describes the
decimal64 module.
The definitions and functions for the other two formats are the same,
except for the obvious name and value changes.
In this implementation each format is represented as an array of
unsigned bytes.
There is therefore just one field in the decimal64 structure:
 bytes

The bytes field represents the eight bytes of a decimal64
number, using Densely Packed Decimal encoding for the coefficient.^{[1]}
The storage of a number in the bytes array is assumed to follow
the byte ordering (‘endianness’) of the computing platform
(if bigendian, then bytes[0] contains the sign bit of the
format). The code in these modules requires that the
DECLITEND tuning parameter be set to
match the endianness of the platform.
Note that the equivalent structures in the decFloats modules are
identical except for their names. It is therefore safe to cast
pointers between them if they are the same size (for example between
decimal64 and decDouble). This means that these modules can be used
as proxies between the decNumber module and the decFloats
modules.
The decimal64 module includes private functions for coding and decoding
Densely Packed Decimal data; these functions are shared by the other
compressed format modules. Hence, when using any of these three
then decimal64.c must be compiled too.
Definitions
The decimal64.h header file defines the decimal64 data structure
described above.
It includes the decNumber.h header file, to simplify use, and (if
not already defined) it sets the DECNUMDIGITS constant to 16, so
that any declared decNumber will be the right size to contain any
decimal64 number.
If more than one of the three decimal format header files are used in
a program, they must be included in decreasing order of size so that
the largest value of DECNUMDIGITS will be used.
The decimal64.h header file also contains:
 Constants defining aspects of decimal64 numbers, including the
maximum precision, the minimum and maximum (adjusted) exponent
supported, the bias applied to the exponent, the length of the number in
bytes, and the maximum number of characters in the string form of the
number (including terminator)

Definitions of the public functions in the decimal64 module.
The decimal64 module also contains the shared routines for compressing
and expanding Densely Packed Decimal data, and uses the decDPD.h
header file. The latter contains lookup tables which are used for
encoding and decoding Densely Packed Decimal data (only three of of
the tables in the header file are used). These tables are
automatically generated and should not need altering.
Functions
The decimal64.c source file contains the public functions
defined in the header file. These comprise conversions to and from
strings and decNumbers, and some utilities.
When a decContext structure is used to report errors, the same rules are
followed as for other modules. That is, a trap may be raised, etc.
This function is used to convert a character string to decimal64 format.
It implements the tonumber conversion in the arithmetic
specification (that is, it accepts subnormal numbers, NaNs, and
infinities, and it preserves the sign and exponent of 0). If
necessary, the value will be rounded to fit.
The arguments are:
 decimal64
 (decimal64 *) Pointer to the structure to be set from the
character string.
 string
 (char *) Pointer to the input character string. This must
be a valid numeric string, as defined in the specification.
The string will not be altered.
 context
 (decContext *) Pointer to the context structure which
controls the conversion, as for the decNumberFromString
function except that the precision and exponent range are fixed
for each format (the values of emax, emin, and
digits are ignored).
Returns decimal64.
Possible errors are DEC_Conversion_syntax
(the string does not have the syntax of a
number), DEC_Overflow
(the adjusted exponent of the number is positive and is greater
than emax for the format),
or DEC_Underflow
(the adjusted exponent of the number is negative and is less
than emin for the format and the conversion is not exact).
If one of these conditions is set, the decimal64 structure will
have the value NaN, ±Infinity or the largest
possible finite number, or a finite (possibly subnormal) number
respectively, with the same sign as the converted number after
overflow or underflow.
decimal64ToString(decimal64, string)
This function is used to convert a decimal64 number to a character string,
using scientific notation if an exponent is needed (that is, there will
be just one digit before any decimal point).
It implements the toscientificstring conversion in the
arithmetic specification.
The arguments are:
 decimal64
 (decimal64 *) Pointer to the structure to be converted to a
string.
 string
 (char *) Pointer to the character string buffer which will
receive the converted number.
It must be at least DECIMAL64_String (24) characters
long.
Returns string; no error is possible from this function.
decimal64ToEngString(decimal64, string)
This function is used to convert a decimal64 number to a character string,
using engineering notation (where the exponent will be a multiple of
three, and there may be up to three digits before any decimal point) if
an exponent is needed. It implements the toengineeringstring
conversion in the arithmetic specification.
The arguments and result are the same as for the decimal64ToString
function, and similarly no error is possible from this function.
decimal64FromNumber(decimal64, number, context)
This function is used to convert a decNumber to decimal64 format.
The arguments are:
 decimal64
 (decimal64 *) Pointer to the structure to be set from the
decNumber. This may receive a numeric value (including subnormal
values and –0) or a special value.
 number
 (decNumber *) Pointer to the input structure.
The decNumber structure will not be altered.
 context
 (decContext *) Pointer to a context structure whose
status field is used to report any error and whose
other fields are used to control rounding, etc., as required.
Returns decimal64.
The possible errors are as for the decimal64FromString
function, except that DEC_Conversion_syntax
is not possible.
decimal64ToNumber(decimal64, number)
This function is used to convert a decimal64 number to decNumber form
in preparation for arithmetic or other operations.
The arguments are:
 decimal64
 (decimal64 *) Pointer to the structure to be converted to a
decNumber.
The decimal64 structure will not be altered.
 number
 (decNumber *) Pointer to the result structure.
It must have space for 16 digits of precision.
Returns number; no error is possible from this function.
decimal64Canonical(decimal64, source)
This function is used to ensure that a decimal64 number is encoded
with the canonical form. That is, all declets use the preferred 1000
encodings and an infinity has a coefficient of zero.
The arguments are:
 decimal64
 (decimal64 *) Pointer to the structure to receive a copy
of source, with canonical encoding.
 source
 (decimal64 *) Pointer to the structure to be converted to
a canonical encoding.
Returns decimal64; no error is possible from this function.
decimal64IsCanonical(decimal64)
This function is used to test whether a decimal64 number is encoded
with the canonical form. That is, that all declets use the preferred
1000 encodings and an infinity has a coefficient of zero.
The argument is:
 decimal64
 (decimal64 *) Pointer to the structure to be tested.
Returns an unsigned integer (uint32_t *) which is
1 if decimal64 has canonical encoding, or 0 otherwise.
No error is possible from this function.
Footnotes:
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