ereOn ereOn - 4 months ago 11
Python Question

How to support 64 bits pointers in cffi?

I'm using cffi to interface my Python module with a C-library.

I got everything working fine on Linux but I'm having a hard time with Mac OS X (Yosemite - 64 bits). Here is a minimal sample I wrote that shows the problem:

foo.h



#ifndef FOO_H
#define FOO_H
#include <stddef.h>

#ifdef __cplusplus
extern "C" {
#endif

void* mymalloc(size_t);

#ifdef __cplusplus
}
#endif
#endif


foo.cpp



#include "foo.h"

#include <cstdlib>
#include <cstdio>

void* mymalloc(size_t size) {
void* const result = ::malloc(size);
printf("%p\n", result);
return result;
}


Makefile



run: foo.py libfoo.dylib
DYLD_LIBRARY_PATH=. LIBRARY_PATH=. python foo.py

libfoo.dylib: foo.cpp foo.h
g++ -dynamiclib -o $@ $<


foo.py



import cffi

ffi = cffi.FFI()
ffi.cdef("""
void* malloc(size_t);
void* mymalloc(size_t);
""")
api = ffi.verify("", libraries=['foo'])
result = api.malloc(4)
print "malloc", result
result = api.mymalloc(4)
print "mymalloc", result


So nothing too fancy here: a simple
mymalloc(size_t)
functions that acts as a wrapper for the real
malloc(size_t)
one and shows the generated pointer before returning.

When executing
foo.py
(with
make run
), I however see the following output:

g++ -dynamiclib -o libfoo.dylib foo.cpp
DYLD_LIBRARY_PATH=. LIBRARY_PATH=. python foo.py
malloc <cdata 'void *' 0x7fdba0e01670>
0x7fdba0e00280
mymalloc <cdata 'void *' 0xffffffffa0e00280>


In the case of
mymalloc(size_t)
, it seems that cffi somehow truncated/modified the pointer integral value: I get
0xffffffffa0e00280
instead of the expected
0x7fdba0e00280
. This is basically the value of the pointer but stored only on 32 bits.
malloc(size_t)
which has the exact same prototype seems however to be correctly handled by cffi and returns a 64 bits address.

I'm at loss trying to find what I do wrong here. Would anyone have a clue ?

Answer

According to an official answer, my way of calling verify() is bogus.

The correct code should read:

import cffi

ffi = cffi.FFI()
code = """
    void* malloc(size_t);
    void* mymalloc(size_t);
"""
ffi.cdef(code)
api = ffi.verify(code, libraries=['foo'])
result = api.malloc(4)
print "malloc", result
result = api.mymalloc(4)
print "mymalloc", result

Which indeeds yields the expected result !

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