byte swapping 32-bit float and weird od results Post: 302317361

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LEARN ABOUT PHP

pack

PACK(3) 								 1								   PACK(3)

pack - Pack data into binary string

SYNOPSIS

       string pack (string  $format, [mixed  $args], [mixed  $...])

DESCRIPTION

	Pack given arguments into a binary string according to $format.

	The idea for this function was taken from Perl and all formatting codes work the same as in Perl. However, there are some formatting codes
       that are missing such as Perl's "u" format code.

	Note that the distinction between signed and unsigned values only affects the function unpack(3), where as function pack(3) gives the same
       result for signed and unsigned format codes.

PARAMETERS

	      o $format
		-  The	$format  string consists of format codes followed by an optional repeater argument. The repeater argument can be either an
		integer value or * for repeating to the end of the input data. For a, A, h, H the repeat count specifies how  many  characters	of
		one  data  argument  are taken, for @ it is the absolute position where to put the next data, for everything else the repeat count
		specifies how many data arguments are consumed and packed into the resulting binary string.  Currently implemented formats are:

	      pack(3) format characters

	      +-----+---------------------------------------------------+
	      |Code |							|
	      |     |							|
	      |     |			 Description			|
	      |     |							|
	      +-----+---------------------------------------------------+
	      | a   |							|
	      |     |							|
	      |     |		      NUL-padded string 		|
	      |     |							|
	      | A   |							|
	      |     |							|
	      |     |		     SPACE-padded string		|
	      |     |							|
	      | h   |							|
	      |     |							|
	      |     |		Hex string, low nibble first		|
	      |     |							|
	      | H   |							|
	      |     |							|
	      |     |		Hex string, high nibble first		|
	      |     |							|
	      | c   |							|
	      |     |							|
	      |     |			 signed char			|
	      |     |							|
	      | C   |							|
	      |     |							|
	      |     |			unsigned char			|
	      |     |							|
	      | s   |							|
	      |     |							|
	      |     | signed short (always 16 bit, machine byte order)	|
	      |     |							|
	      | S   |							|
	      |     |							|
	      |     | unsigned	short  (always	16  bit,  machine  byte |
	      |     | order)						|
	      |     |							|
	      | n   |							|
	      |     |							|
	      |     | unsigned	short  (always	16 bit, big endian byte |
	      |     | order)						|
	      |     |							|
	      | v   |							|
	      |     |							|
	      |     | unsigned short (always 16 bit, little endian byte |
	      |     | order)						|
	      |     |							|
	      | i   |							|
	      |     |							|
	      |     | signed  integer  (machine dependent size and byte |
	      |     | order)						|
	      |     |							|
	      | I   |							|
	      |     |							|
	      |     | unsigned integer (machine dependent size and byte |
	      |     | order)						|
	      |     |							|
	      | l   |							|
	      |     |							|
	      |     |  signed long (always 32 bit, machine byte order)	|
	      |     |							|
	      | L   |							|
	      |     |							|
	      |     | unsigned long (always 32 bit, machine byte order) |
	      |     |							|
	      | N   |							|
	      |     |							|
	      |     | unsigned	long  (always  32  bit, big endian byte |
	      |     | order)						|
	      |     |							|
	      | V   |							|
	      |     |							|
	      |     | unsigned long (always 32 bit, little endian  byte |
	      |     | order)						|
	      |     |							|
	      | q   |							|
	      |     |							|
	      |     | signed  long  long  (always  64 bit, machine byte |
	      |     | order)						|
	      |     |							|
	      | Q   |							|
	      |     |							|
	      |     | unsigned long long (always 64 bit,  machine  byte |
	      |     | order)						|
	      |     |							|
	      | J   |							|
	      |     |							|
	      |     | unsigned	long  long  (always  64 bit, big endian |
	      |     | byte order)					|
	      |     |							|
	      | P   |							|
	      |     |							|
	      |     | unsigned long long (always 64 bit, little  endian |
	      |     | byte order)					|
	      |     |							|
	      | f   |							|
	      |     |							|
	      |     | float (machine dependent size and representation) |
	      |     |							|
	      | d   |							|
	      |     |							|
	      |     | double  (machine	dependent  size and representa- |
	      |     | tion)						|
	      |     |							|
	      | x   |							|
	      |     |							|
	      |     |			  NUL byte			|
	      |     |							|
	      | X   |							|
	      |     |							|
	      |     |		      Back up one byte			|
	      |     |							|
	      | Z   |							|
	      |     |							|
	      |     |	     NUL-padded string (new in PHP 5.5) 	|
	      |     |							|
	      | @   |							|
	      |     |							|
	      |     |		NUL-fill to absolute position		|
	      |     |							|
	      +-----+---------------------------------------------------+
	      o $args
		-

RETURN VALUES

	Returns a binary string containing data.

CHANGELOG

       +--------+---------------------------------------------------+
       |Version |						    |
       |	|						    |
       |	|		     Description		    |
       |	|						    |
       +--------+---------------------------------------------------+
       | 5.6.3	|						    |
       |	|						    |
       |	|  The "q", "Q", "J" and "P" codes  were  added  to |
       |	| enable working with 64-bit numbers.		    |
       |	|						    |
       | 5.5.0	|						    |
       |	|						    |
       |	|  The "Z" code was added with equivalent function- |
       |	| ality to "a" for Perl compatibility.		    |
       |	|						    |
       +--------+---------------------------------------------------+
EXAMPLES

       Example #1

	      pack(3) example

	      <?php
	      $binarydata = pack("nvc*", 0x1234, 0x5678, 65, 66);
	      ?>

	       The resulting binary string will be 6 bytes long and contain the byte sequence 0x12, 0x34, 0x78, 0x56, 0x41, 0x42.

NOTES

       Caution

	      Note that PHP internally stores integer values as signed values of a machine-dependent size (C  type  long).  Integer  literals  and
	      operations that yield numbers outside the bounds of the integer type will be stored as float. When packing these floats as integers,
	      they are first cast into the integer type. This may or may not result in the desired byte pattern.

	      The most relevant case is when packing unsigned numbers that would be representable with the integer type if it  were  unsigned.	In
	      systems  where the integer type has a 32-bit size, the cast usually results in the same byte pattern as if the integer were unsigned
	      (although this relies on implementation-defined unsigned to signed conversions, as per the C standard). In systems where the integer
	      type  has  64-bit  size, the float most likely does not have a mantissa large enough to hold the value without loss of precision. If
	      those systems also have a native 64-bit C int type (most UNIX-like systems don't), the only way to use the  I  pack  format  in  the
	      upper range is to create integer negative values with the same byte representation as the desired unsigned value.

SEE ALSO

       unpack(3).

PHP Documentation Group 														   PACK(3)