Using strlen() seems to be as old as ctypes itself.

--

I don't know which behavior is correct. It's a little bit strange that ctypes truncate the string at "\0". A "char" buffer can be an arbitrary array of bytes. It may be a binary format which doesn't use null byte as string terminator, but just to encode a 16-bit integer as two bytes.

My attempt to understand the current behavior:

https://github.com/python/cpython/pull/18419#pullrequestreview-355606890

"size = strlen(data);" instruction was added when the ctypes was added by this commit:

commit d4c9320412177895f598a93d73a0e654db27c351
Author: Thomas Heller <theller@ctypes.org>
Date:   Wed Mar 8 19:35:11 2006 +0000

    Copy ctypes-0.9.9.4 sources from external into the trunk.

Sadly, Thomas Heller no longer contributes to Python since 2011:
https://blog.python.org/2011/04/thomas-heller-steps-down-as-ctypes.html

The original project is hosted at:
https://sourceforge.net/p/ctypes/code/

It seems like s_set() function was added between these source/cfield.c two versions:

revision 1.116
date: 2006/03/15 20:35:55;  author: theller;  state: Exp;  lines: +14 -4
PyString_FromFormat()b understands the C99 "z" qualifier on all
platforms, in Python 2.5.  Patch from Tim Peters, adapted to work with
older Python versions.

revision 1.115
date: 2006/03/03 20:17:15;  author: theller;  state: Exp;  lines: +636 -279
Moving files from branch_1_0 to HEAD.

Sadly, the commit message doesn't say much about the rationale.