Python Ctypes Null Terminated String Block

Question

I am using a ctypes implementation of CreateProcessWithLogonW, and everything works well except I cannot figure out how to handle this section:

A pointer to an environment block for the new process. If this parameter is NULL, the new process uses the environment of the calling process.

An environment block consists of a null-terminated block of null-terminated strings. Each string is in the following form:

name=value\0

To generated the raw string I execute the following:

lpEnvironment = '\0'.join(['%s=%s' % (k, os.environ[k]) for k in os.environ]) + '\0\0'
print lpEnvironment
'XAUTHORITY=/home/username/.Xauthority\x00MUTT_DIR=/home/username/.mutt\x00LASTDIRFILE=/home/username/.lastpwd-geany\x00LOGNAME=username\...\x00\x00'

However run I make a ctypes variable out of it, it truncates the information:

ctypes.c_wchar_p(lpEnvironment)
c_wchar_p(u'XAUTHORITY=/home/username/.Xauthority')

How can I pass the lpEnvironment information correctly?

ephemient · Accepted Answer · 2010-12-09 00:11:07Z

6

As far as I can tell, the whole string does get passed across the ctypes boundary correctly in one direction, but gets truncated on the way back.

>>> ctypes.create_string_buffer('abc\0def').value
'abc'
>>> ctypes.create_string_buffer('abc\0def').raw
'abc\x00def'

Unfortunately (at least for me, Python 2.6.5 on Linux) the result of create_unicode_buffer doesn't have a .raw property. However,

>>> ctypes.wstring_at(ctypes.create_unicode_buffer(u'abc\0def), 7)
u'abc\x00def'

works as expected.

answered Dec 9, 2010 at 0:11

ephemient

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2 Comments

Eric Pruitt Over a year ago

Thanks! I did not know about the raw property.

Mark Tolonen Over a year ago

''.join(ctypes.create_unicode_buffer(u'abc\0def')) works, too.

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