Doc/c-api/marshal.rst

.. highlight:: 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 several versions of the data format; see
the :py:mod:`Python module documentation <marshal>` for details.

The following exceptions can be raised by these functions:
:exc:`ValueError` if the value cannot be marshalled,
:exc:`ValueError` or :exc:`TypeError` if the data is malformed,
:exc:`EOFError` if the end of the data is reached before the value is complete,
:exc:`OSError` if reading from or writing to a :c:expr:`FILE*` fails,
:exc:`KeyboardInterrupt` if reading or writing is interrupted by a signal,
and :exc:`MemoryError` if memory allocation fails.

.. versionchanged:: 3.16
   Previously, in functions taking a :c:expr:`FILE*`,
   the reading functions raised :exc:`EOFError`
   instead of :exc:`OSError` and :exc:`KeyboardInterrupt`,
   and the writing functions ignored I/O errors and interruptions.

.. c:macro:: Py_MARSHAL_VERSION

   The current format version. See :py:data:`marshal.version`.

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

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

   This function can fail, in which case it sets the error indicator.
   Use :c:func:`PyErr_Occurred` to check for that.

   .. note::

      In Python 3.15 and earlier, this function does not set the error
      indicator if writing to *file* fails.
      The same caveat applies as for :c:func:`PyMarshal_WriteObjectToFile`.

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

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

   The marshalled representation of *value* is built in memory
   and then written to *file*.
   Nothing is written if *value* cannot be marshalled.

   This function can fail, in which case it sets the error indicator.
   Use :c:func:`PyErr_Occurred` to check for that.

   .. note::

      In Python 3.15 and earlier, this function can write only part of the
      data to *file* and return without setting the error indicator, for
      example if *value* contains an object which cannot be marshalled or
      if writing to *file* fails (see :gh:`155907`).
      Code which has to support these versions cannot rely on
      :c:func:`PyErr_Occurred` alone to know that all the data was written.
      It can use :c:func:`PyMarshal_WriteObjectToString` and write the
      result itself, or validate what was written.

   .. versionchanged:: 3.16
      Previously, the data was written to *file* as it was produced,
      using a buffer of constant size,
      and incomplete data was written if *value* could not be marshalled.

.. c:function:: PyObject* PyMarshal_WriteObjectToString(PyObject *value, int version)

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

   On error, raises an exception and returns ``NULL``.


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


.. c:function:: long PyMarshal_ReadLongFromFile(FILE *file)

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

   On error, raises an exception and returns ``-1``.


.. c:function:: int PyMarshal_ReadShortFromFile(FILE *file)

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

   On error, raises an exception and returns ``-1``.


.. c:function:: PyObject* PyMarshal_ReadObjectFromFile(FILE *file)

   Return a Python object from the data stream in a :c:expr:`FILE*` opened for
   reading.

   On error, raises an exception and returns ``NULL``.


.. c:function:: PyObject* PyMarshal_ReadLastObjectFromFile(FILE *file)

   Return a Python object from the data stream in a :c:expr:`FILE*` opened for
   reading.  Unlike :c:func:`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 this variant if you are certain that you won't be reading
   anything else from the file.

   On error, raises an exception and returns ``NULL``.


.. c:function:: PyObject* PyMarshal_ReadObjectFromString(const char *data, Py_ssize_t len)

   Return a Python object from the data stream in a byte buffer
   containing *len* bytes pointed to by *data*.

   On error, raises an exception and returns ``NULL``.