[Python-Dev] _PyString_Resize

Tim Peters tim.one@comcast.net
Sat, 27 Apr 2002 14:56:42 -0400


Word to the wise:  learn how _PyString_Resize works.  I just fixed dozens of
misues in the code base, and beefed up the docs.  Here's the checkin
comment:

"""
Repair widespread misuse of _PyString_Resize.  Since it's clear people
don't understand how this function works, also beefed up the docs.  The
most common usage error is of this form (often spread out across gotos):

	if (_PyString_Resize(&s, n) < 0) {
		Py_DECREF(s);
		s = NULL;
		goto outtahere;
	}

The error is that if _PyString_Resize runs out of memory, it automatically
decrefs the input string object s (which also deallocates it, since its
refcount must be 1 upon entry), and sets s to NULL.  So if the "if"
branch ever triggers, it's an error to call Py_DECREF(s):  s is already
NULL!  A correct way to write the above is the simpler:

	if (_PyString_Resize(&s, n) < 0)
		goto outtahere;
"""

Even better, *most* of the time you can just do

	_PyString_Resize(&s, n);
	return s;

at the end of a function.  This works correctly in all respects regardless
of whether _PyString_Resize() succeeds or fails.

Note that there's also no need to store a trailing \0 byte yourself before
calling _PyString_Resize:  it does that automatically (for all the PyString_
alloc and realloc functions, the size you pass in is one less than the
number of bytes actually allocated for string space, and a trailing \0 byte
is always appended by magic in successful cases).