/Doc/c-api/marshal.rst

  1.. highlightlang:: c
  2
  3.. _marshalling-utils:
  4
  5Data marshalling support
  6========================
  7
  8These routines allow C code to work with serialized objects using the same
  9data format as the :mod:`marshal` module.  There are functions to write data
 10into the serialization format, and additional functions that can be used to
 11read the data back.  Files used to store marshalled data must be opened in
 12binary mode.
 13
 14Numeric values are stored with the least significant byte first.
 15
 16The module supports two versions of the data format: version 0 is the
 17historical version, version 1 (new in Python 2.4) shares interned strings in
 18the file, and upon unmarshalling.  Version 2 (new in Python 2.5) uses a binary
 19format for floating point numbers.  *Py_MARSHAL_VERSION* indicates the current
 20file format (currently 2).
 21
 22
 23.. cfunction:: void PyMarshal_WriteLongToFile(long value, FILE *file, int version)
 24
 25   Marshal a :ctype:`long` integer, *value*, to *file*.  This will only write
 26   the least-significant 32 bits of *value*; regardless of the size of the
 27   native :ctype:`long` type.
 28
 29   .. versionchanged:: 2.4
 30      *version* indicates the file format.
 31
 32
 33.. cfunction:: void PyMarshal_WriteObjectToFile(PyObject *value, FILE *file, int version)
 34
 35   Marshal a Python object, *value*, to *file*.
 36
 37   .. versionchanged:: 2.4
 38      *version* indicates the file format.
 39
 40
 41.. cfunction:: PyObject* PyMarshal_WriteObjectToString(PyObject *value, int version)
 42
 43   Return a string object containing the marshalled representation of *value*.
 44
 45   .. versionchanged:: 2.4
 46      *version* indicates the file format.
 47
 48
 49The following functions allow marshalled values to be read back in.
 50
 51XXX What about error detection?  It appears that reading past the end of the
 52file will always result in a negative numeric value (where that's relevant),
 53but it's not clear that negative values won't be handled properly when there's
 54no error.  What's the right way to tell? Should only non-negative values be
 55written using these routines?
 56
 57
 58.. cfunction:: long PyMarshal_ReadLongFromFile(FILE *file)
 59
 60   Return a C :ctype:`long` from the data stream in a :ctype:`FILE\*` opened
 61   for reading.  Only a 32-bit value can be read in using this function,
 62   regardless of the native size of :ctype:`long`.
 63
 64
 65.. cfunction:: int PyMarshal_ReadShortFromFile(FILE *file)
 66
 67   Return a C :ctype:`short` from the data stream in a :ctype:`FILE\*` opened
 68   for reading.  Only a 16-bit value can be read in using this function,
 69   regardless of the native size of :ctype:`short`.
 70
 71
 72.. cfunction:: PyObject* PyMarshal_ReadObjectFromFile(FILE *file)
 73
 74   Return a Python object from the data stream in a :ctype:`FILE\*` opened for
 75   reading.  On error, sets the appropriate exception (:exc:`EOFError` or
 76   :exc:`TypeError`) and returns *NULL*.
 77
 78
 79.. cfunction:: PyObject* PyMarshal_ReadLastObjectFromFile(FILE *file)
 80
 81   Return a Python object from the data stream in a :ctype:`FILE\*` opened for
 82   reading.  Unlike :cfunc:`PyMarshal_ReadObjectFromFile`, this function
 83   assumes that no further objects will be read from the file, allowing it to
 84   aggressively load file data into memory so that the de-serialization can
 85   operate from data in memory rather than reading a byte at a time from the
 86   file.  Only use these variant if you are certain that you won't be reading
 87   anything else from the file.  On error, sets the appropriate exception
 88   (:exc:`EOFError` or :exc:`TypeError`) and returns *NULL*.
 89
 90
 91.. cfunction:: PyObject* PyMarshal_ReadObjectFromString(char *string, Py_ssize_t len)
 92
 93   Return a Python object from the data stream in a character buffer
 94   containing *len* bytes pointed to by *string*.  On error, sets the
 95   appropriate exception (:exc:`EOFError` or :exc:`TypeError`) and returns
 96   *NULL*.
 97
 98   .. versionchanged:: 2.5
 99      This function used an :ctype:`int` type for *len*. This might require
100      changes in your code for properly supporting 64-bit systems.