[Python-Dev] _PyString_Resize (original) (raw)

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).