RFC 142 (v1) Enhanced Pack/Unpack
[email protected] (Perl6 RFC Librarian)
| Newsgroups | perl.perl6.language,perl.perl6.announce |
|---|---|
| Message-ID | <[email protected]> |
This and other RFCs are available on the web at http://dev.perl.org/rfc/ =head1 TITLE Enhanced Pack/Unpack =head1 VERSION Maintainer: Edwin Wiles <[email protected]> Date: 22 Aug 2000 Version: 1 Mailing List: [email protected] Number: 142 =head1 ABSTRACT Pack and Unpack are percieved as being difficult to use, and possibly missing desirable features. =head1 DESCRIPTION The existing pack and unpack methods depend upon a simple grammar which leads to opaque format specifications, which are often difficult to get right, and which carry no information regarding variable names. A more descriptive grammar, which includes variable name associations, would make pack and unpack easier to use. =head1 IMPLEMENTATION Given the expressed desire to shrink the overall size of the perl executable, this should be implemented as a seperate module; included with the core distribution. =head2 Definition $foo = new Structure(...definition...); Define a new structure type. Can use previously defined user data types. See the section on definitions. Structure::define( $typename, \&from_sub, \&to_sub ); Define a user data type. The 'from_sub' extracts data from the packed form. The 'to_sub' puts data back into the packed form. See the section on user defined types. =head2 Input $foo->read(<INPUT>); # sysread binary data from given IO reference. $foo->set($var); # accept binary data from normal perl # variable. $foo->append($var); # append binary data to the existing data in # the structure. =head2 Output $foo->write(<OUTPUT>); # syswrite binary data to given IO reference. $var = $foo->get(); # output binary data to normal perl variable. =head2 Maniuplation $foo->{'name'} = $val; # set "name" to value $val = $foo->{'name'}; # get value of "name" [Note: There is an alternative method, using the Class::Class method of exposing the variables via their names. That is still a possibility, but this is deemed easier to implement at this time.] =head2 Data Definition DEFINITION := '[' ELEMENTS ']' ELEMENTS := ELEMENT [',' ELEMENTS] ELEMENT := NAME '=>' TYPE NAME := Text used to identify the variable for further use. You may not use 'array', it is reserved. You may not embed whitespace. TYPE := ''' BASETYPE [ '/' MODIFIERS ] ''' | '[' ARRAYDEF ']' | USERDEFINED [ '/' UDEFARGS ] BASETYPE := 'short' | 'long' | 'int' | 'double' | 'float' | 'char' | 'byte' In each of the above, unless otherwise modified, the type defaults to the signedness, endianness, and bit length that your system normally uses. The one exception to this is 'char', which Unicode may cause to be larger than a single byte, even if your system normally considers a 'char' to be a single byte. | 'chars' A null terminated string of characters. If unicode is used, this may be more than one byte per character. For use with indefinite length strings, where a "count" is not provided. If the array modifier is used, then you're expecting that many null terminated strings. | 'bytes' A null terminated string of bytes. If the array modifier is used, then you're expecting that many null terminated strings of bytes. [Note: Other basetypes desired can certainly be added. It were best if they were added at this phase. Inform me of any additional base types desired, with justifications.] UDEFARGS := UDEFARG [ ',' UDEFARGS ] UDEFARG := User defined argument, meaning dependent upon user defined code. Pretty much, any legal Perl constant. At least, by the time it hits this module, it better be constant. USERDEFINED := A user defined type name, see the section on user defined data types. ARRAYDEF := 'array' ',' COUNT ',' ELEMENTS [Note: Since an 'arraydef' already begins with an otherwise special character (eg. foo => [ 'array', 10, ...]), can we eliminate the apparently redundant 'array' keyword giving us (foo => [ 10, ...])?] COUNT := Either a constant number, or the name of an earlier element. MODIFIERS := [ ARRAYMOD ][ SIGNEDNESS ][ ENDIANNESS ] [ LENGTH ][ OFFSETDEF ] ARRAYMOD := '[' COUNT ']' This is an alternative method for declaring an array, used when the data in question is a basetype, and not an embedded repeating structure. OFFSETDEF := '@' OFFSET '/' OFFSETSTART The variable that this is applied to, starts from the indicated offset. If no OFFSETDEF is specified, the offset defaults to the end of the previous item containing no OFFSETDEF, or, if no such previous item exists, to offset zero from the beginning of the structure. OFFSET := The name of a data element defined earlier, or a numeric literal which is the offset. OFFSETSTART := 'begin' - From the beginning of the overall structure. | 'end' - From the end of the fixed portion of the structure. | 'here' - From the beginning of the offset variable. [Note: There is a proposal to allow user defined offset start types, but the necessary arguments are not well understood. Suggestions welcome.] SIGNEDNESS := u - unsigned | s - signed | If omitted, defaults to system default. ENDIANNESS := n - network - 4321 | b - big - 4321 | l - little - 1234 | p - PDP/Vax - 3412 | If omitted, defaults to system default. LENGTH := An integer expressing the length of the base type in bits. If omitted, it defaults to the normal length for your system. Some types may only support a few fixed lengths, however, integer types are expected to support any bitlength between 1 and 32, inclusive, or possibly more if big integer support is available. Nota Bene: "normal" length specifications for various sized types (like short, int, long) are useful when creating structures defined in those terms for the local platform, to be used as storage structures or API parameters. However, for cross platform work, it is critical that actual sizes be able to be specified, so that data structures can be stored and accessed in a consistent manner that need not change when the script runs on different platforms. Hence, both notations, "normal" length, and actual length, are supported by this interface. Where more than one "normal" length type is the same basic type (short, int, and long are all "integers"), any of them may be specified with the same LENGTH parameter to achieve the same results. =head3 User Defined Data Types [Note: For simplification, user defined datatypes are presumed to be fixed length. To support variable length, either the programmer must ensure that there is sufficient data, or the methods for user defined data types must be redefined to allow for reading additional data.] sub from_sub( $binaryvar, $bitoffset, $uservars, ... ) sub to_sub( $binaryvar, $bitoffset, $udvar, $uservars, ... ) $binaryvar - is the variable containing the binary data. $bitoffset - is the current offset into the binary data. $udvar - user defined variable for repacking into binary. $uservars - how ever many user variables the user needs to define the type. =head3 Examples Given the following: struct foo { int bar; int baz; int count; }; Followed by 'count' copies of: struct stroff { int length; int offset; }; Followed by 'count' variable length, not necessarily null terminated collections of bytes. Possibly strings, possibly not, but we'll consider them strings for now. The 'offset' is from the beginning of the overall structure. The corresponding definition would be: ['bar'=>'int', 'baz'=>'int', 'count'=>'int', 'stroff'=>[ 'array', 'count', 'length'=>'int', 'offset'=>'int', 'str'=>'byte[length]@offset/begin' ]] Notice that this groups the trailing string with the corresponding 'stroff' structure. Here is an example of using each of the various modifiers individually, and then several together. ['i1' => 'int/[14]'] # an array of 14 normal length int ['i2' => 'int/s'] # a signed normal length int ['i3' => 'int/l'] # a little endian normal length int ['i4' => 'int/16'] # an 16-bit int ['i5' => 'int/@0/begin'] # an int at offset 0 from begin of structure ['i6' => 'int/[14]sl16@0/begin'] # an array of 14, signed, little endian, 16-bit, integers # at offset 0 from begin of structure. ['type' => 'int/8', 'i7' => 'int/32@0/begin', 'f1' => 'float/32@0/begin' ] # i7 and f1 are two different interpretations of the same # bitstream =head1 REFERENCES Class::Class useful for automatic creation of get/set methods for variables on the basis of their names. http://search.cpan.org/doc/BINKLEY/Class-Class-0.18/lib/Class/Class.pm http://search.cpan.org/search?dist=Class-Class File::Binary possibly useful for read/write of binary information. http://search.cpan.org/doc/SIMONW/File-Binary-0.3/blib/lib/File/Binary.pm http://search.cpan.org/search?dist=File-Binary PDL::IO::FlexRaw - is almost exactly what we're looking for. While it is described as being specifically for Fortran77 binary files, we should be able to adapt it to anything. http://search.cpan.org/doc/KGB/PDL-2.005/IO/FlexRaw/FlexRaw.pm http://search.cpan.org/search?dist=PDL Combine this with Class::Class and an extended FlexRaw/pack data description format, and we've got a powerful tool for binary data manipulation. =head1 CREDITS The following individuals have contributed to this RFC. [If I've missed anyone, LET ME KNOW!] Glenn Linderman <[email protected]> (Co-Author) Dominic Dunlop <[email protected]> (Contributor)