How to copy bytes from a ctypes Structure to a buffer created from create_string_buffer

Question

I have a ctypes structure (for example):

from ctypes import *
class Foo(Structure):
   _fields_ = [('f1',c_uint),
               ('f2',c_uint)]

I would like to copy an instance of that structure in to a buffer that was created from create_string_buffer (that is larger in size than needed for a single instance of Foo).

Example:

f = Foo()
f.f1=1; f.f2=2;
buf = create_string_buffer(100)
buf[0:sizeof(f)] = f

I know you can read and write data to/from structures on file objects (i.e. open('bar','rb').readinto(f) ), so it seems that there should be some way to do this easily too...

Eryk Sun · Accepted Answer · 2015-04-17 21:05:45Z

11

Avoid using an operation that makes a temporary copy, or that creates temporary objects while iterating. This should be a direct copy from the source buffer to the target buffer. It's simplest to use ctypes.memmove:

f = Foo()
f.f1=1
f.f2=2

buf = (c_char * 100)()
memmove(buf, byref(f), sizeof(f))

Or use a temporary array created with from_buffer (not a copy):

memoryview(buf)[:sizeof(f)] = (c_char * sizeof(f)).from_buffer(f)

The latter doesn't work in Python 3, however. It should work, but the buffer format codes that ctypes uses aren't supported by memoryview.

If you wanted a new array that was the same size or smaller, you could use the array type's from_buffer_copy method (2.6+):

buf = (c_char * sizeof(f)).from_buffer_copy(f)

edited Apr 17, 2015 at 21:05

answered Jul 31, 2013 at 17:37

Eryk Sun

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3 Comments

marsh Over a year ago

What if I want to do it the other way around? A buffer into a ctype structure with a size and data.

Eryk Sun Over a year ago

@marsh, use memmove to copy data into an existing struct, but if you're creating a new instance, try the from_buffer_copy constructor instead.

Eric Over a year ago

You can use memoryview(buf).cast('B') to discard the unwanted format code.

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