DRz DRz - 4 months ago 14
Python Question

Create a CData-typed buffer in Python with CFFI

I am trying to create a buffer with a special type in Python, to send it to a C function wrapped with CFFI.

In C, I have something like:

typedef unsigned char UINT8;
typedef UINT8* PUINT8;


Then, in Python, the above lines are in the
ffi.cdef()
and my buffer instantiation code looks like:

nb_buffer = 8
buffer_size = 42

buffers = ffi.new( "PUINT8[]", nb_buffer )

for i in range( nb_buffer ):
tmp_buffer = ffi.buffer( ffi.new( "UINT8[]", 1 ), buffer_size )
buffers[ i ] = ffi.cast( "PUINT8", ffi.from_buffer( tmp_buffer ) )

c.function( buffers )


The C function receives a UINT8**.

And... that ends with a segmentation fault a little bit further, in the C function.

So I
print buffers[ i ]
after using
ffi.cast
:

<cdata 'unsigned char *' 0x2cbaca0>
<cdata 'unsigned char *' 0x2cbacd8>
<cdata 'unsigned char *' 0x2cbaca0>
<cdata 'unsigned char *' 0x2cbacd8>
<cdata 'unsigned char *' 0x2cbaca0>
<cdata 'unsigned char *' 0x2cbacd8>
<cdata 'unsigned char *' 0x2cbaca0>
<cdata 'unsigned char *' 0x2cbacd8>


What am I missing? Is the buffer being garbage collected as soon as it overwrites
tmp_buffer
?

Answer

How about:

buffers = ffi.new( "PUINT8[]", nb_buffer )
keepalive = []

for i in range( nb_buffer ):
    p = ffi.new("UINT8[]", buffer_size)
    keepalive.append(p)
    buffers[i] = p

c.function( buffers )

# keepalive stays alive at least until here