Built-in module struct

struct Cstructures

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.

The module defines the following exception and functions:

$\begin{excdesc}{error} Exception raised on various occasions; argument is a string describing what is wrong. \end{excdesc}$

$\begin{funcdesc}{pack}{fmt\, v1\, v2\, {\rm\ldots}} Return a string containing ... ... arguments must match the values required by the format exactly. \end{funcdesc}$

$\begin{funcdesc}{unpack}{fmt\, string} Unpack the string (presumably packed by ... ...\code{len(\var{string})} must equal \code{calcsize(\var{fmt})}). \end{funcdesc}$

$\begin{funcdesc}{calcsize}{fmt} Return the size of the struct (and hence of the string) corresponding to the given format. \end{funcdesc}$

Format characters have the following meaning; the conversion between C and Python values should be obvious given their types:

$\begin{tableiii}{\vert c\vert l\vert l\vert}{samp}{Format}{C}{Python} \lineiii{... ...}{integer} \lineiii{f}{float}{float} \lineiii{d}{double}{float} \end{tableiii}$

A format character may be preceded by an integral repeat count; e.g. the format string '4h' means exactly the same as '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).

Examples (all on a big-endian machine):

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

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.

(More format characters are planned, e.g. '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.)