\section{\module{struct} ---         Interpret strings as packed binary data}\declaremodule{builtin}{struct} \modulesynopsis{Interpret strings as packed binary data.} \indexii{C}{structures}\indexiii{packing}{binary}{data} This module performs conversions between Python values and Cstructs represented as Python strings.  It uses \dfn{format strings}(explained below) as compact descriptions of the lay-out of the Cstructs and the intended conversion to/from Python values.  This canbe used in handling binary data stored in files or from networkconnections, among other sources. 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, \textrm{\ldots}}  Return a string containing the values  \code{\var{v1}, \var{v2}, \textrm{\ldots}} packed according to the given  format.  The arguments must match the values required by the format  exactly.\end{funcdesc} \begin{funcdesc}{pack_into}{fmt, buffer, offset, v1, v2, \moreargs}  Pack the values \code{\var{v1}, \var{v2}, \textrm{\ldots}} according to the given  format, write the packed bytes into the writable \var{buffer} starting at  \var{offset}.  Note that the offset is not an optional argument.   \versionadded{2.5}\end{funcdesc} \begin{funcdesc}{unpack}{fmt, string}  Unpack the string (presumably packed by \code{pack(\var{fmt},  \textrm{\ldots})}) 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  (\code{len(\var{string})} must equal \code{calcsize(\var{fmt})}).\end{funcdesc} \begin{funcdesc}{unpack_from}{fmt, buffer\optional{,offset \code{= 0}}}  Unpack the \var{buffer} according to tthe given format.  The result is a tuple even if it contains exactly one item. The  \var{buffer} must contain at least the amount of data required by the  format (\code{len(buffer[offset:])} must be at least  \code{calcsize(\var{fmt})}).   \versionadded{2.5}\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 betweenC and Python values should be obvious given their types: \begin{tableiv}{c|l|l|c}{samp}{Format}{C Type}{Python}{Notes}  \lineiv{x}{pad byte}{no value}{}  \lineiv{c}{\ctype{char}}{string of length 1}{}  \lineiv{b}{\ctype{signed char}}{integer}{}  \lineiv{B}{\ctype{unsigned char}}{integer}{}  \lineiv{h}{\ctype{short}}{integer}{}  \lineiv{H}{\ctype{unsigned short}}{integer}{}  \lineiv{i}{\ctype{int}}{integer}{}  \lineiv{I}{\ctype{unsigned int}}{long}{}  \lineiv{l}{\ctype{long}}{integer}{}  \lineiv{L}{\ctype{unsigned long}}{long}{}  \lineiv{q}{\ctype{long long}}{long}{(1)}  \lineiv{Q}{\ctype{unsigned long long}}{long}{(1)}  \lineiv{f}{\ctype{float}}{float}{}  \lineiv{d}{\ctype{double}}{float}{}  \lineiv{s}{\ctype{char[]}}{string}{}  \lineiv{p}{\ctype{char[]}}{string}{}  \lineiv{P}{\ctype{void *}}{integer}{}\end{tableiv} \noindentNotes: \begin{description}\item[(1)]  The \character{q} and \character{Q} conversion codes are available in  native mode only if the platform C compiler supports C \ctype{long long},  or, on Windows, \ctype{__int64}.  They are always available in standard  modes.  \versionadded{2.2}\end{description}  A format character may be preceded by an integral repeat count.  Forexample, the format string \code{'4h'} means exactly the same as\code{'hhhh'}. Whitespace characters between formats are ignored; a count and itsformat must not contain whitespace though. For the \character{s} format character, the count is interpreted as thesize of the string, not a repeat count like for the other formatcharacters; for example, \code{'10s'} means a single 10-byte string, while\code{'10c'} means 10 characters.  For packing, the string istruncated or padded with null bytes as appropriate to make it fit.For unpacking, the resulting string always has exactly the specifiednumber of bytes.  As a special case, \code{'0s'} means a single, emptystring (while \code{'0c'} means 0 characters). The \character{p} format character encodes a "Pascal string", meaninga short variable-length string stored in a fixed number of bytes.The count is the total number of bytes stored.  The first byte stored isthe length of the string, or 255, whichever is smaller.  The bytesof the string follow.  If the string passed in to \function{pack()} is toolong (longer than the count minus 1), only the leading count-1 bytes of thestring are stored.  If the string is shorter than count-1, it is paddedwith null bytes so that exactly count bytes in all are used.  Note thatfor \function{unpack()}, the \character{p} format character consumes countbytes, but that the string returned can never contain more than 255characters. For the \character{I}, \character{L}, \character{q} and \character{Q}format characters, the return value is a Python long integer. For the \character{P} format character, the return value is a Pythoninteger or long integer, depending on the size needed to hold apointer when it has been cast to an integer type.  A \NULL{} pointer willalways be returned as the Python integer \code{0}. When packing pointer-sizedvalues, Python integer or long integer objects may be used.  Forexample, the Alpha and Merced processors use 64-bit pointer values,meaning a Python long integer will be used to hold the pointer; otherplatforms use 32-bit pointers and will use a Python integer. By default, C numbers are represented in the machine's native formatand byte order, and properly aligned by skipping pad bytes ifnecessary (according to the rules used by the C compiler). Alternatively, the first character of the format string can be used toindicate 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, \character{@} is assumed. Native byte order is big-endian or little-endian, depending on thehost 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\keyword{sizeof} expression.  This is always combined with native byteorder. Standard size and alignment are as follows: no alignment is requiredfor any type (so you have to use pad bytes);\ctype{short} is 2 bytes;\ctype{int} and \ctype{long} are 4 bytes;\ctype{long long} (\ctype{__int64} on Windows) is 8 bytes;\ctype{float} and \ctype{double} are 32-bit and 64-bitIEEE floating point numbers, respectively. Note the difference between \character{@} and \character{=}: both usenative byte order, but the size and alignment of the latter isstandardized. The form \character{!} is available for those poor souls who claim theycan't remember whether network byte order is big-endian orlittle-endian. There is no way to indicate non-native byte order (forcebyte-swapping); use the appropriate choice of \character{<} or\character{>}. The \character{P} format character is only available for the nativebyte ordering (selected as the default or with the \character{@} byteorder character). The byte order character \character{=} chooses touse little- or big-endian ordering based on the host system. Thestruct module does not interpret this as native ordering, so the\character{P} format is not available. Examples (all using native byte order, size and alignment, on abig-endian machine): \begin{verbatim}>>> 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\end{verbatim} Hint: to align the end of a structure to the alignment requirement ofa particular type, end the format with the code for that type with arepeat count of zero.  For example, the format \code{'llh0l'}specifies two pad bytes at the end, assuming longs are aligned on4-byte boundaries.  This only works when native size and alignment arein effect; standard size and alignment does not enforce any alignment. \begin{seealso}  \seemodule{array}{Packed binary storage of homogeneous data.}  \seemodule{xdrlib}{Packing and unpacking of XDR data.}\end{seealso} \subsection{Struct Objects \label{struct-objects}} The \module{struct} module also defines the following type: \begin{classdesc}{Struct}{format}  Return a new Struct object which writes and reads binary data according to  the format string \var{format}.  Creating a Struct object once and calling  its methods is more efficient than calling the \module{struct} functions  with the same format since the format string only needs to be compiled once.  \versionadded{2.5}\end{classdesc} Compiled Struct objects support the following methods and attributes: \begin{methoddesc}[Struct]{pack}{v1, v2, \moreargs}  Identical to the \function{pack()} function, using the compiled format.  (\code{len(result)} will equal \member{self.size}.)\end{methoddesc} \begin{methoddesc}[Struct]{pack_into}{buffer, offset, v1, v2, \moreargs}  Identical to the \function{pack_into()} function, using the compiled format.\end{methoddesc} \begin{methoddesc}[Struct]{unpack}{string}  Identical to the \function{unpack()} function, using the compiled format.  (\code{len(string)} must equal \member{self.size}).\end{methoddesc} \begin{methoddesc}[Struct]{unpack_from}{buffer\optional{,offset                                              \code{= 0}}}  Identical to the \function{unpack_from()} function, using the compiled format.  (\code{len(buffer[offset:])} must be at least \member{self.size}).\end{methoddesc} \begin{memberdesc}[Struct]{format}  The format string used to construct this Struct object.\end{memberdesc}