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# PERL pack Function

## Syntax

pack EXPR, LIST

## Definition and Usage

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.

## Return Value

- 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.

Character | Description
a | ASCII character string padded with null characters
A | ASCII character string padded with spaces
b | String of bits, lowest first
B | String of bits, highest first
c | A signed character (range usually -128 to 127)
C | An unsigned character (usually 8 bits)
d | A double-precision floating-point number
f | A single-precision floating-point number
h | Hexadecimal string, lowest digit first
H | Hexadecimal string, highest digit first
i | A signed integer
I | An unsigned integer
l | A signed long integer
L | An unsigned long integer
n | A short integer in network order
N | A long integer in network order
p | A pointer to a string
s | A signed short integer
S | An unsigned short integer
u | Convert to uuencode format
v | A short integer in VAX (little-endian) order
V | A long integer in VAX order
x | A null byte
X | Indicates "go back one byte"
@ | Fill with nulls (ASCII 0)

## Example

Try out following example:

    #!/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[0]\n";
    print "Array $array[1]\n";
    print "Array $array[2]\n";

It will produce following binary result:

    bits are A
    bits are
    bits are ÿT
    Array 255
    Array T
    Array 30

Check Socket Programming session to see the use of pack function.

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