Perl pack Function
This function evaluates the expressions in LIST and packs them into a binary structure specified by EXPR. The format is specified using the characters shown in Table below −
Each character may be optionally followed by a number, which specifies a repeat count for the type of value being packed.that is nibbles, chars, or even bits, according to the format. A value of * repeats for as many values remain in LIST. Values can be unpacked with the unpack function.
For example, a5 indicates that five letters are expected. b32 indicates that 32 bits are expected. h8 indicates that 8 nybbles ( or 4 bytes) are expected. P10 indicates that the structure is 10 bytes long.
Following is the simple syntax for this function −
pack EXPR, LIST
This function returns a packed version of the data in LIST using TEMPLATE to determine how it is coded.
Here is the table which gives values to be used in TEMPLATE.
|S.No.||Character & Description|
ASCII character string padded with null characters
ASCII character string padded with spaces
String of bits, lowest first
String of bits, highest first
A signed character (range usually -128 to 127)
An unsigned character (usually 8 bits)
A double-precision floating-point number
A single-precision floating-point number
Hexadecimal string, lowest digit first
Hexadecimal string, highest digit first
A signed integer
An unsigned integer
A signed long integer
An unsigned long integer
A short integer in network order
A long integer in network order
A pointer to a string
A signed short integer
An unsigned short integer
Convert to uuencode format
A short integer in VAX (little-endian) order
A long integer in VAX order
A null byte
Indicates "go back one byte"
Fill with nulls (ASCII 0)
Following is the example code showing its basic usage −
#!/usr/bin/perl -w $bits = pack("c", 65); # prints A, which is ASCII 65. print "bits are $bits\n"; $bits = pack( "x" ); # $bits is now a null chracter. print "bits are $bits\n"; $bits = pack( "sai", 255, "T", 30 ); # creates a seven charcter string on most computers' print "bits are $bits\n"; @array = unpack( "sai", "$bits" ); #Array now contains three elements: 255, T and 30. print "Array $array\n"; print "Array $array\n"; print "Array $array\n";
When above code is executed, it produces the following result −
bits are A bits are bits are �T Array 255 Array T Array 30