36.6. dl — Call C functions in shared objects

Platforms: Unix

Deprecated since version 2.6: The dl module has been removed in Python 3.0. Use the ctypes module instead.

The dl module defines an interface to the dlopen() function, which is the most common interface on Unix platforms for handling dynamically linked libraries. It allows the program to call arbitrary functions in such a library.

Warning

The dl module bypasses the Python type system and error handling. If used incorrectly it may cause segmentation faults, crashes or other incorrect behaviour.

Note

This module will not work unless sizeof(int) == sizeof(long) == sizeof(char *) If this is not the case, SystemError will be raised on import.

The dl module defines the following function:

dl.open(name[, mode=RTLD_LAZY])

Open a shared object file, and return a handle. Mode signifies late binding (RTLD_LAZY) or immediate binding (RTLD_NOW). Default is RTLD_LAZY. Note that some systems do not support RTLD_NOW.

Return value is a dlobject.

The dl module defines the following constants:

dl.RTLD_LAZY

Useful as an argument to open().

dl.RTLD_NOW

Useful as an argument to open(). Note that on systems which do not support immediate binding, this constant will not appear in the module. For maximum portability, use hasattr() to determine if the system supports immediate binding.

The dl module defines the following exception:

exception dl.error

Exception raised when an error has occurred inside the dynamic loading and linking routines.

Example:

>>> import dl, time
>>> a=dl.open('/lib/libc.so.6')
>>> a.call('time'), time.time()
(929723914, 929723914.498)

This example was tried on a Debian GNU/Linux system, and is a good example of the fact that using this module is usually a bad alternative.

36.6.1. Dl Objects

Dl objects, as returned by open() above, have the following methods:

dl.close()

Free all resources, except the memory.

dl.sym(name)

Return the pointer for the function named name, as a number, if it exists in the referenced shared object, otherwise None. This is useful in code like:

>>> if a.sym('time'):
...     a.call('time')
... else:
...     time.time()

(Note that this function will return a non-zero number, as zero is the NULL pointer)

dl.call(name[, arg1[, arg2...]])

Call the function named name in the referenced shared object. The arguments must be either Python integers, which will be passed as is, Python strings, to which a pointer will be passed, or None, which will be passed as NULL. Note that strings should only be passed to functions as const char*, as Python will not like its string mutated.

There must be at most 10 arguments, and arguments not given will be treated as None. The function’s return value must be a C long, which is a Python integer.