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(python2.1-lib.info)struct
Interpret strings as packed binary data
=======================================
Interpret strings as packed binary data.
This module performs conversions between Python values and C structs
represented as Python strings. It uses "format strings" (explained
below) as compact descriptions of the lay-out of the C structs and the
intended conversion to/from Python values. This can be used in
handling binary data stored in files or from network connections, among
other sources.
The module defines the following exception and functions:
`error'
Exception raised on various occasions; argument is a string
describing what is wrong.
`pack(fmt, v1, v2, ...)'
Return a string containing the values `V1, V2, ...' packed
according to the given format. The arguments must match the
values required by the format exactly.
`unpack(fmt, string)'
Unpack the string (presumably packed by `pack(FMT, ...)')
according to the given format. The result is a tuple even if it
contains exactly one item. The string must contain exactly the
amount of data required by the format (i.e. `len(STRING)' must
equal `calcsize(FMT)').
`calcsize(fmt)'
Return the size of the struct (and hence of the string)
corresponding to the given format.
Format characters have the following meaning; the conversion between C
and Python values should be obvious given their types:
Format C Type Python Notes
------ ------ ------ ------
x pad byte no value
c `char' string of length
1
b `signed char' integer
B `unsigned char' integer
h `short' integer
H `unsigned short' integer
i `int' integer
I `unsigned int' long (1)
l `long' integer
L `unsigned long' long
f `float' float
d `double' float
s `char[]' string
p `char[]' string
P `void *' integer
Notes:
`(1)'
The `I' conversion code will convert to a Python long if the C
`int' is the same size as a C `long', which is typical on most
modern systems. If a C `int' is smaller than a C `long', an
Python integer will be created instead.
A format character may be preceded by an integral repeat count; e.g.
the format string `'4h'' means exactly the same as `'hhhh''.
Whitespace characters between formats are ignored; a count and its
format must not contain whitespace though.
For the `s' format character, the count is interpreted as the size of
the string, not a repeat count like for the other format characters;
e.g. `'10s'' means a single 10-byte string, while `'10c'' means 10
characters. For packing, the string is truncated or padded with null
bytes as appropriate to make it fit. For unpacking, the resulting
string always has exactly the specified number of bytes. As a special
case, `'0s'' means a single, empty string (while `'0c'' means 0
characters).
The `p' format character can be used to encode a Pascal string. The
first byte is the length of the stored string, with the bytes of the
string following. If count is given, it is used as the total number of
bytes used, including the length byte. If the string passed in to
`pack()' is too long, the stored representation is truncated. If the
string is too short, padding is used to ensure that exactly enough
bytes are used to satisfy the count.
For the `I' and `L' format characters, the return value is a Python
long integer.
For the `P' format character, the return value is a Python integer or
long integer, depending on the size needed to hold a pointer when it
has been cast to an integer type. A `NULL' pointer will always be
returned as the Python integer `0'. When packing pointer-sized values,
Python integer or long integer objects may be used. For example, the
Alpha and Merced processors use 64-bit pointer values, meaning a Python
long integer will be used to hold the pointer; other platforms use
32-bit pointers and will use a Python integer.
By default, C numbers are represented in the machine's native format
and byte order, and properly aligned by skipping pad bytes if necessary
(according to the rules used by the C compiler).
Alternatively, the first character of the format string can be used to
indicate the byte order, size and alignment of the packed data,
according to the following table:
Character Byte order Size and alignment
------ ----- -----
@ native native
= native standard
< little-endian standard
> big-endian standard
! network (= big-endian) standard
If the first character is not one of these, `@' is assumed.
Native byte order is big-endian or little-endian, depending on the host
system (e.g. Motorola and Sun are big-endian; Intel and DEC are
little-endian).
Native size and alignment are determined using the C compiler's
`sizeof' expression. This is always combined with native byte order.
Standard size and alignment are as follows: no alignment is required
for any type (so you have to use pad bytes); `short' is 2 bytes; `int'
and `long' are 4 bytes. `float' and `double' are 32-bit and 64-bit
IEEE floating point numbers, respectively.
Note the difference between `@' and `=': both use native byte order,
but the size and alignment of the latter is standardized.
The form `!' is available for those poor souls who claim they can't
remember whether network byte order is big-endian or little-endian.
There is no way to indicate non-native byte order (i.e. force
byte-swapping); use the appropriate choice of `<' or `>'.
The `P' format character is only available for the native byte ordering
(selected as the default or with the `@' byte order character). The
byte order character `=' chooses to use little- or big-endian ordering
based on the host system. The struct module does not interpret this as
native ordering, so the `P' format is not available.
Examples (all using native byte order, size and alignment, on a
big-endian machine):
>>> from struct import *
>>> pack('hhl', 1, 2, 3)
'\x00\x01\x00\x02\x00\x00\x00\x03'
>>> unpack('hhl', '\x00\x01\x00\x02\x00\x00\x00\x03')
(1, 2, 3)
>>> calcsize('hhl')
8
Hint: to align the end of a structure to the alignment requirement of a
particular type, end the format with the code for that type with a
repeat count of zero, e.g. the format `'llh0l'' specifies two pad bytes
at the end, assuming longs are aligned on 4-byte boundaries. This only
works when native size and alignment are in effect; standard size and
alignment does not enforce any alignment.
See also:
Note: array Packed binary storage of homogeneous data. Note:
xdrlib Packing and unpacking of XDR data.
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