Fred Drake | 295da24 | 1998-08-10 19:42:37 +0000 | [diff] [blame] | 1 | \section{\module{struct} --- |
| 2 | Interpret strings as packed binary data.} |
Fred Drake | b91e934 | 1998-07-23 17:59:49 +0000 | [diff] [blame] | 3 | \declaremodule{builtin}{struct} |
| 4 | |
| 5 | \modulesynopsis{Interpret strings as packed binary data.} |
| 6 | |
Fred Drake | abdea22 | 1998-03-16 05:22:08 +0000 | [diff] [blame] | 7 | \indexii{C@\C{}}{structures} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 8 | |
| 9 | This module performs conversions between Python values and C |
| 10 | structs represented as Python strings. It uses \dfn{format strings} |
| 11 | (explained below) as compact descriptions of the lay-out of the C |
| 12 | structs and the intended conversion to/from Python values. |
| 13 | |
| 14 | The module defines the following exception and functions: |
| 15 | |
Fred Drake | 7ddd043 | 1998-03-08 07:44:13 +0000 | [diff] [blame] | 16 | |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 17 | \begin{excdesc}{error} |
| 18 | Exception raised on various occasions; argument is a string |
| 19 | describing what is wrong. |
| 20 | \end{excdesc} |
| 21 | |
Fred Drake | cce1090 | 1998-03-17 06:33:25 +0000 | [diff] [blame] | 22 | \begin{funcdesc}{pack}{fmt, v1, v2, {\rm \ldots}} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 23 | Return a string containing the values |
| 24 | \code{\var{v1}, \var{v2}, {\rm \ldots}} packed according to the given |
| 25 | format. The arguments must match the values required by the format |
| 26 | exactly. |
| 27 | \end{funcdesc} |
| 28 | |
Fred Drake | cce1090 | 1998-03-17 06:33:25 +0000 | [diff] [blame] | 29 | \begin{funcdesc}{unpack}{fmt, string} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 30 | Unpack the string (presumably packed by \code{pack(\var{fmt}, {\rm \ldots})}) |
| 31 | according to the given format. The result is a tuple even if it |
| 32 | contains exactly one item. The string must contain exactly the |
| 33 | amount of data required by the format (i.e. \code{len(\var{string})} must |
| 34 | equal \code{calcsize(\var{fmt})}). |
| 35 | \end{funcdesc} |
| 36 | |
| 37 | \begin{funcdesc}{calcsize}{fmt} |
| 38 | Return the size of the struct (and hence of the string) |
| 39 | corresponding to the given format. |
| 40 | \end{funcdesc} |
| 41 | |
| 42 | Format characters have the following meaning; the conversion between C |
| 43 | and Python values should be obvious given their types: |
| 44 | |
Fred Drake | ee60191 | 1998-04-11 20:53:03 +0000 | [diff] [blame] | 45 | \begin{tableiii}{c|l|l}{samp}{Format}{C Type}{Python} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 46 | \lineiii{x}{pad byte}{no value} |
| 47 | \lineiii{c}{char}{string of length 1} |
| 48 | \lineiii{b}{signed char}{integer} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 49 | \lineiii{B}{unsigned char}{integer} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 50 | \lineiii{h}{short}{integer} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 51 | \lineiii{H}{unsigned short}{integer} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 52 | \lineiii{i}{int}{integer} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 53 | \lineiii{I}{unsigned int}{integer} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 54 | \lineiii{l}{long}{integer} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 55 | \lineiii{L}{unsigned long}{integer} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 56 | \lineiii{f}{float}{float} |
| 57 | \lineiii{d}{double}{float} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 58 | \lineiii{s}{char[]}{string} |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 59 | \lineiii{p}{char[]}{string} |
Guido van Rossum | 6ac06b3 | 1998-09-21 14:44:34 +0000 | [diff] [blame] | 60 | \lineiii{P}{void *}{integer} |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 61 | \end{tableiii} |
| 62 | |
Guido van Rossum | 6c4f003 | 1995-03-07 10:14:09 +0000 | [diff] [blame] | 63 | A format character may be preceded by an integral repeat count; e.g.\ |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 64 | the format string \code{'4h'} means exactly the same as \code{'hhhh'}. |
| 65 | |
Guido van Rossum | e20aef5 | 1997-08-26 20:39:54 +0000 | [diff] [blame] | 66 | Whitespace characters between formats are ignored; a count and its |
| 67 | format must not contain whitespace though. |
| 68 | |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 69 | For the \character{s} format character, the count is interpreted as the |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 70 | size of the string, not a repeat count like for the other format |
| 71 | characters; e.g. \code{'10s'} means a single 10-byte string, while |
| 72 | \code{'10c'} means 10 characters. For packing, the string is |
| 73 | truncated or padded with null bytes as appropriate to make it fit. |
| 74 | For unpacking, the resulting string always has exactly the specified |
| 75 | number of bytes. As a special case, \code{'0s'} means a single, empty |
| 76 | string (while \code{'0c'} means 0 characters). |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 77 | |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 78 | The \character{p} format character can be used to encode a Pascal |
| 79 | string. The first byte is the length of the stored string, with the |
| 80 | bytes of the string following. If count is given, it is used as the |
| 81 | total number of bytes used, including the length byte. If the string |
| 82 | passed in to \function{pack()} is too long, the stored representation |
| 83 | is truncated. If the string is too short, padding is used to ensure |
| 84 | that exactly enough bytes are used to satisfy the count. |
| 85 | |
| 86 | For the \character{I} and \character{L} format characters, the return |
Guido van Rossum | 6530717 | 1997-01-03 19:21:53 +0000 | [diff] [blame] | 87 | value is a Python long integer. |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 88 | |
Guido van Rossum | 6ac06b3 | 1998-09-21 14:44:34 +0000 | [diff] [blame] | 89 | For the \character{P} format character, the return value is a Python |
| 90 | integer or long integer, depending on the size needed to hold a |
| 91 | pointer when it has been cast to an integer type. A NULL pointer will |
| 92 | always be returned as the Python integer 0. When packing pointer-sized |
| 93 | values, Python integer or long integer objects may be used. For |
| 94 | example, the Alpha and Merced processors use 64-bit pointer values, |
| 95 | meaning a Python long integer will be used to hold the pointer; other |
| 96 | platforms use 32-bit pointers and will use a Python integer. |
| 97 | |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 98 | By default, C numbers are represented in the machine's native format |
| 99 | and byte order, and properly aligned by skipping pad bytes if |
| 100 | necessary (according to the rules used by the C compiler). |
| 101 | |
| 102 | Alternatively, the first character of the format string can be used to |
| 103 | indicate the byte order, size and alignment of the packed data, |
| 104 | according to the following table: |
| 105 | |
Fred Drake | ee60191 | 1998-04-11 20:53:03 +0000 | [diff] [blame] | 106 | \begin{tableiii}{c|l|l}{samp}{Character}{Byte order}{Size and alignment} |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 107 | \lineiii{@}{native}{native} |
| 108 | \lineiii{=}{native}{standard} |
| 109 | \lineiii{<}{little-endian}{standard} |
| 110 | \lineiii{>}{big-endian}{standard} |
| 111 | \lineiii{!}{network (= big-endian)}{standard} |
| 112 | \end{tableiii} |
| 113 | |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 114 | If the first character is not one of these, \character{@} is assumed. |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 115 | |
| 116 | Native byte order is big-endian or little-endian, depending on the |
| 117 | host system (e.g. Motorola and Sun are big-endian; Intel and DEC are |
| 118 | little-endian). |
| 119 | |
| 120 | Native size and alignment are determined using the C compiler's sizeof |
| 121 | expression. This is always combined with native byte order. |
| 122 | |
| 123 | Standard size and alignment are as follows: no alignment is required |
| 124 | for any type (so you have to use pad bytes); short is 2 bytes; int and |
Guido van Rossum | dbadd55 | 1997-01-03 04:20:09 +0000 | [diff] [blame] | 125 | long are 4 bytes. Float and double are 32-bit and 64-bit IEEE floating |
| 126 | point numbers, respectively. |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 127 | |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 128 | Note the difference between \character{@} and \character{=}: both use native |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 129 | byte order, but the size and alignment of the latter is standardized. |
| 130 | |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 131 | The form \character{!} is available for those poor souls who claim they |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 132 | can't remember whether network byte order is big-endian or |
| 133 | little-endian. |
| 134 | |
| 135 | There is no way to indicate non-native byte order (i.e. force |
Fred Drake | cf0fb8b | 1998-07-23 21:18:25 +0000 | [diff] [blame] | 136 | byte-swapping); use the appropriate choice of \character{<} or |
| 137 | \character{>}. |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 138 | |
Guido van Rossum | 6ac06b3 | 1998-09-21 14:44:34 +0000 | [diff] [blame] | 139 | The \character{P} format character is only available for the native |
| 140 | byte ordering (selected as the default or with the \character{@} byte |
| 141 | order character). The byte order character \character{=} chooses to |
| 142 | use little- or big-endian ordering based on the host system. The |
| 143 | struct module does not interpret this as native ordering, so the |
| 144 | \character{P} format is not available. |
| 145 | |
Guido van Rossum | 1254346 | 1996-12-31 02:22:14 +0000 | [diff] [blame] | 146 | Examples (all using native byte order, size and alignment, on a |
| 147 | big-endian machine): |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 148 | |
Fred Drake | 1947991 | 1998-02-13 06:58:54 +0000 | [diff] [blame] | 149 | \begin{verbatim} |
Guido van Rossum | dbadd55 | 1997-01-03 04:20:09 +0000 | [diff] [blame] | 150 | >>> from struct import * |
| 151 | >>> pack('hhl', 1, 2, 3) |
| 152 | '\000\001\000\002\000\000\000\003' |
| 153 | >>> unpack('hhl', '\000\001\000\002\000\000\000\003') |
| 154 | (1, 2, 3) |
| 155 | >>> calcsize('hhl') |
| 156 | 8 |
| 157 | >>> |
Fred Drake | 1947991 | 1998-02-13 06:58:54 +0000 | [diff] [blame] | 158 | \end{verbatim} |
Guido van Rossum | e47da0a | 1997-07-17 16:34:52 +0000 | [diff] [blame] | 159 | % |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 160 | Hint: to align the end of a structure to the alignment requirement of |
| 161 | a particular type, end the format with the code for that type with a |
Guido van Rossum | 6c4f003 | 1995-03-07 10:14:09 +0000 | [diff] [blame] | 162 | repeat count of zero, e.g.\ the format \code{'llh0l'} specifies two |
Guido van Rossum | 5fdeeea | 1994-01-02 01:22:07 +0000 | [diff] [blame] | 163 | pad bytes at the end, assuming longs are aligned on 4-byte boundaries. |
Fred Drake | 7ddd043 | 1998-03-08 07:44:13 +0000 | [diff] [blame] | 164 | This only works when native size and alignment are in effect; |
| 165 | standard size and alignment does not enforce any alignment. |
| 166 | |
| 167 | \begin{seealso} |
| 168 | \seemodule{array}{packed binary storage of homogeneous data} |
| 169 | \end{seealso} |