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Document the new extensions.

This commit is contained in:
Guido van Rossum 1996-12-31 02:22:14 +00:00
parent 3aa27fd315
commit 125434665b
2 changed files with 128 additions and 18 deletions
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73
Doc/lib/libstruct.tex
View File

@ -45,23 +45,81 @@ and Python values should be obvious given their types:
\lineiii{x}{pad byte}{no value}
\lineiii{c}{char}{string of length 1}
\lineiii{b}{signed char}{integer}
\lineiii{B}{unsigned char}{integer}
\lineiii{h}{short}{integer}
\lineiii{H}{unsigned short}{integer}
\lineiii{i}{int}{integer}
\lineiii{I}{unsigned int}{integer}
\lineiii{l}{long}{integer}
\lineiii{L}{unsigned long}{integer}
\lineiii{f}{float}{float}
\lineiii{d}{double}{float}
\lineiii{s}{char[]}{string}
\end{tableiii}
A format character may be preceded by an integral repeat count; e.g.\
the format string \code{'4h'} means exactly the same as \code{'hhhh'}.
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).
For the \code{'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. \code{'10s'} means a single 10-byte string, while
\code{'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, \code{'0s'} means a single, empty
string (while \code{'0c'} means 0 characters).
Examples (all on a big-endian machine):
For the \code{'I'} and \code{'L'} format characters, the return
value is a Python long integer if a Python plain integer can't
represent the required range (note: this is dependent on the size of
the relevant C types only, not of the sign of the actual value).
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:
\begin{tableiii}{|c|l|l|}{samp}{Character}{Byte order}{Size and alignment}
\lineiii{@}{native}{native}
\lineiii{=}{native}{standard}
\lineiii{<}{little-endian}{standard}
\lineiii{>}{big-endian}{standard}
\lineiii{!}{network (= big-endian)}{standard}
\end{tableiii}
If the first character is not one of these, \code{'@'} 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. In this mode, there is no support for float and
double (\code{'f'} and \code{'d'}).
Note the difference between \code{'@'} and \code{'='}: both use native
byte order, but the size and alignment of the latter is standardized.
The form \code{'!'} 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 \code{'<'} or
\code{'>'}.
Examples (all using native byte order, size and alignment, on a
big-endian machine):
\bcode\begin{verbatim}
from struct import *
pack('hhl', 1, 2, 3) == '\000\001\000\002\000\000\000\003'
unpack('hhl', '\000\001\000\002\000\000\000\003') == (1, 2, 3)
calcsize('hhl') == 8
@ -71,8 +129,5 @@ 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 \code{'llh0l'} specifies two
pad bytes at the end, assuming longs are aligned on 4-byte boundaries.
(More format characters are planned, e.g.\ \code{'s'} for character
arrays, upper case for unsigned variants, and a way to specify the
byte order, which is useful for [de]constructing network packets and
reading/writing portable binary file formats like TIFF and AIFF.)
(This only works when native size and alignment are in effect;
standard size and alignment does not enforce any alignment.)

73
Doc/libstruct.tex
View File

@ -45,23 +45,81 @@ and Python values should be obvious given their types:
\lineiii{x}{pad byte}{no value}
\lineiii{c}{char}{string of length 1}
\lineiii{b}{signed char}{integer}
\lineiii{B}{unsigned char}{integer}
\lineiii{h}{short}{integer}
\lineiii{H}{unsigned short}{integer}
\lineiii{i}{int}{integer}
\lineiii{I}{unsigned int}{integer}
\lineiii{l}{long}{integer}
\lineiii{L}{unsigned long}{integer}
\lineiii{f}{float}{float}
\lineiii{d}{double}{float}
\lineiii{s}{char[]}{string}
\end{tableiii}
A format character may be preceded by an integral repeat count; e.g.\
the format string \code{'4h'} means exactly the same as \code{'hhhh'}.
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).
For the \code{'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. \code{'10s'} means a single 10-byte string, while
\code{'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, \code{'0s'} means a single, empty
string (while \code{'0c'} means 0 characters).
Examples (all on a big-endian machine):
For the \code{'I'} and \code{'L'} format characters, the return
value is a Python long integer if a Python plain integer can't
represent the required range (note: this is dependent on the size of
the relevant C types only, not of the sign of the actual value).
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:
\begin{tableiii}{|c|l|l|}{samp}{Character}{Byte order}{Size and alignment}
\lineiii{@}{native}{native}
\lineiii{=}{native}{standard}
\lineiii{<}{little-endian}{standard}
\lineiii{>}{big-endian}{standard}
\lineiii{!}{network (= big-endian)}{standard}
\end{tableiii}
If the first character is not one of these, \code{'@'} 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. In this mode, there is no support for float and
double (\code{'f'} and \code{'d'}).
Note the difference between \code{'@'} and \code{'='}: both use native
byte order, but the size and alignment of the latter is standardized.
The form \code{'!'} 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 \code{'<'} or
\code{'>'}.
Examples (all using native byte order, size and alignment, on a
big-endian machine):
\bcode\begin{verbatim}
from struct import *
pack('hhl', 1, 2, 3) == '\000\001\000\002\000\000\000\003'
unpack('hhl', '\000\001\000\002\000\000\000\003') == (1, 2, 3)
calcsize('hhl') == 8
@ -71,8 +129,5 @@ 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 \code{'llh0l'} specifies two
pad bytes at the end, assuming longs are aligned on 4-byte boundaries.
(More format characters are planned, e.g.\ \code{'s'} for character
arrays, upper case for unsigned variants, and a way to specify the
byte order, which is useful for [de]constructing network packets and
reading/writing portable binary file formats like TIFF and AIFF.)
(This only works when native size and alignment are in effect;
standard size and alignment does not enforce any alignment.)