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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:
- exception error
- Exception raised on various occasions; argument is a string describing what is wrong.
-
- 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 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 (len(string)
must equal 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:
| 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 |
|
| l |
long |
integer |
|
| L |
unsigned long |
long |
|
| q |
long long |
long |
(1) |
| Q |
unsigned long long |
long |
(1) |
| f |
float |
float |
|
| d |
double |
float |
|
| s |
char[] |
string |
|
| p |
char[] |
string |
|
| P |
void * |
integer |
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Notes:
- (1)
- The "q" and "Q"
conversion codes are available in native mode only if the platform C compiler supports C long long, or, on Windows, __int64. They are
always available in standard modes. New in version 2.2.
A format character may be preceded by an integral repeat count. For example, 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; for
example, '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 encodes a "Pascal
string", meaning a short variable-length string stored in a fixed number of bytes. The
count is the total number of bytes stored. The first byte stored is the length of the string,
or 255, whichever is smaller. The bytes of the string follow. If the string passed in to pack() is too long (longer than the count minus 1), only the leading
count-1 bytes of the string are stored. If the string is shorter than count-1, it is padded
with null bytes so that exactly count bytes in all are used. Note that for unpack(), the "p" format character
consumes count bytes, but that the string returned can never contain more than 255 characters.
For the "I", "L",
"q" and "Q" 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:
| @ |
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. For
example, Motorola and Sun processors are big-endian; Intel and DEC processors 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; long long (__int64 on Windows) is 8 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 (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. For example, 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:
- Module array:
- Packed binary storage of homogeneous data.
- Module xdrlib:
- Packing and unpacking of XDR data.
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