JCL Help:InitCrc32@Cardinal@Cardinal
JCL Help: InitCrc32 Function
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Summary
Defines a new polynomial and starting value for the 32 bit CRC functions.
Pascal
procedure InitCrc32(Polynom: Cardinal; Start: Cardinal); overload;
procedure InitCrc32(Polynom: Cardinal; Start: Cardinal; out Crc32Table: TCrc32Table); overload;
Parameters
| Parameters | Description |
| Start: Cardinal | 32 bit starting value to be used with subsequent calls to 32 bit CRC functions |
| polynomial | Generator polynomial to be used with subsequent calls to 32 bit CRC functions. The polynomial is given without the highest bit (bit x^32) |
Description
Defines a new "generator polynomial" and starting value for the 32 bit CRC functions. Beware that the check and the correction only works if the polynomial is "appropriate". Two of these polynomials are predefined: Crc32PolynomCCITT = $04C11DB7 represents the polynomial x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1 that is suggested by the CCITT and used by Ethernet and ZIP. Crc32PolynomXXXXX = $9DB11213 represents the polynomial x^32 + x^31 + x^28 + x^27 + x^26 + x^24 + x^23 + x^21 + x^20 + x^16 + x^12 + x^9 + x^4 + x + 1 which is another "appropriate" polynomial (as far as I know). Beware that not appropriate polynomials result in highly undesirable behaviour of the CRC. The starting value can by anything. $FFFFFFFF and 0 are usual definitions.
See Also
About
Unit
Donator
Michael Schnell
Notes
As a standard in the JEDI Code Library the compiler option {$J-} is set to prevent modifying typed constants. So the function CRC32Init only exists if JCLMath is recompiled with the compiler environment variable "CRCINIT" defined, which temporarily sets {$J+} for the CRC functions.
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