Data marshalling support — Python 2.7.18 documentation

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# Data marshalling support

These routines allow C code to work with serialized objects using the same data format as the `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` (new in Python 2.4) shares interned strings in the file, and upon unmarshalling. Version 2 (new in Python 2.5) uses a binary format for floating point numbers. Py_MARSHAL_VERSION indicates the current file format (currently 2).

void `PyMarshal_WriteLongToFile`(long value, FILE *file, int version)

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

  Changed in version 2.4: version indicates the file format.

void `PyMarshal_WriteObjectToFile`(PyObject *value, FILE *file, int version)

  Marshal a Python object, value, to file.

  Changed in version 2.4: version indicates the file format.

PyObject* `PyMarshal_WriteObjectToString`(PyObject *value, int version)
  Return value: New reference.

  Return a string object containing the marshalled representation of value.

  Changed in version 2.4: 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?

long `PyMarshal_ReadLongFromFile`(FILE *file)

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

int `PyMarshal_ReadShortFromFile`(FILE *file)

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

PyObject* `PyMarshal_ReadObjectFromFile`(FILE *file)
  Return value: New reference.

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

PyObject* `PyMarshal_ReadLastObjectFromFile`(FILE *file)
  Return value: New reference.

  Return a Python object from the data stream in a `FILE*` opened for reading. Unlike `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 (`EOFError` or `TypeError`) and returns NULL.

PyObject* `PyMarshal_ReadObjectFromString`(char *string, Py_ssize_t len)
  Return value: New reference.

  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 (`EOFError` or `TypeError`) and returns NULL.

  Changed in version 2.5: This function used an `int` type for len. This might require changes in your code for properly supporting 64-bit systems.