cpython/Doc/c-api/marshal.rst

.. highlightlang:: c

.. _marshalling-utils:

Data marshalling support
========================

These routines allow C code to work with serialized objects using the same data
format as the :mod:`marshal` module.  There are functions to write data into the
serialization format, and additional functions that can be used to read the data
back.  Files used to store marshalled data must be opened in binary mode.

Numeric values are stored with the least significant byte first.

The module supports two versions of the data format: version 0 is the historical
version, version 1 shares interned strings in the file, and upon unmarshalling.
Version 2 uses a binary format for floating point numbers.
*Py_MARSHAL_VERSION* indicates the current file format (currently 2).


.. cfunction:: void PyMarshal_WriteLongToFile(long value, FILE *file, int version)

   Marshal a :ctype:`long` integer, *value*, to *file*.  This will only write the
   least-significant 32 bits of *value*; regardless of the size of the native
   :ctype:`long` type.  *version* indicates the file format.


.. cfunction:: void PyMarshal_WriteObjectToFile(PyObject *value, FILE *file, int version)

   Marshal a Python object, *value*, to *file*.
   *version* indicates the file format.


.. cfunction:: PyObject* PyMarshal_WriteObjectToString(PyObject *value, int version)

   Return a string object containing the marshalled representation of *value*.
   *version* indicates the file format.


The following functions allow marshalled values to be read back in.

XXX What about error detection?  It appears that reading past the end of the
file will always result in a negative numeric value (where that's relevant), but
it's not clear that negative values won't be handled properly when there's no
error.  What's the right way to tell? Should only non-negative values be written
using these routines?


.. cfunction:: long PyMarshal_ReadLongFromFile(FILE *file)

   Return a C :ctype:`long` from the data stream in a :ctype:`FILE\*` opened for
   reading.  Only a 32-bit value can be read in using this function, regardless of
   the native size of :ctype:`long`.


.. cfunction:: int PyMarshal_ReadShortFromFile(FILE *file)

   Return a C :ctype:`short` from the data stream in a :ctype:`FILE\*` opened for
   reading.  Only a 16-bit value can be read in using this function, regardless of
   the native size of :ctype:`short`.


.. cfunction:: PyObject* PyMarshal_ReadObjectFromFile(FILE *file)

   Return a Python object from the data stream in a :ctype:`FILE\*` opened for
   reading.  On error, sets the appropriate exception (:exc:`EOFError` or
   :exc:`TypeError`) and returns *NULL*.


.. cfunction:: PyObject* PyMarshal_ReadLastObjectFromFile(FILE *file)

   Return a Python object from the data stream in a :ctype:`FILE\*` opened for
   reading.  Unlike :cfunc:`PyMarshal_ReadObjectFromFile`, this function assumes
   that no further objects will be read from the file, allowing it to aggressively
   load file data into memory so that the de-serialization can operate from data in
   memory rather than reading a byte at a time from the file.  Only use these
   variant if you are certain that you won't be reading anything else from the
   file.  On error, sets the appropriate exception (:exc:`EOFError` or
   :exc:`TypeError`) and returns *NULL*.


.. cfunction:: PyObject* PyMarshal_ReadObjectFromString(char *string, Py_ssize_t len)

   Return a Python object from the data stream in a character buffer containing
   *len* bytes pointed to by *string*.  On error, sets the appropriate exception
   (:exc:`EOFError` or :exc:`TypeError`) and returns *NULL*.