c array to python

Question

I need to handle a big array of data from C program to python instance (Django) using shared library (.so). Is it possible to return them through ctypes ?

For example:

import ctypes
a = ctypes.CDLL('foo.so')
b = a.collect()
latitude  = b.latitude
longitude = b.longitude

and a C:

main()
{
    /* steps to get the data */
    return <kind of struct or smth else>;
}

I am the newbie so is there a ways of delivery of such kind of data ?

Kos · Accepted Answer · 2013-01-29 18:48:11Z

1

One option is to return the values via pointer parameters:

// c 
void collect(int* outLatitude, int* outLongitude) {
    *outLatitude = 10;
    *outLongitude = 20;
}

and

# python
x = ctypes.c_int()
y = ctypes.c_int()
library.collect(ctypes.byref(x), ctypes.byref(y))
print x.value, y.value

If you need more than that, you could return a structure:

// c
typedef struct  {
    int latitude, longitude;
} Location;

Location collect();

and

# python
class Location(ctypes.Structure):
    _fields_ = [('latitude', ctypes.c_int), ('longitude', ctypes.c_int)]

library.collect.restype = Location
loc = library.collect()
print loc.latitude, loc.longitude

BTW: You have mentioned Django; I'd be careful with concurrency here. Note that your C library may be called from different threads.

answered Jan 29, 2013 at 18:48

Kos

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5 Comments

Kos Over a year ago

BTW other solutions: a) See projects like ctypesgen to read headers and write these Python definitions for you. b) Use boost::python or SWIG to compile the C/C++ code into a real Python module. You can also read my article: 5 ways to use Python with native code.

dima.h Over a year ago

there are no problems with c_int. Now i can't figure out how to use c_char_p and pass strings to python. Can you give a sample ?

Kos Over a year ago

See Fundamental data types in the docs, I can't describe that better. (For simple read-only passing, you can just pass a Python string to the function, assuming it has argtypes set correctly - ctypes will autoconvert)

dima.h Over a year ago

i meant a pointer to string from C to python and get it as x.value for example

Kos Over a year ago

Oh, in this way. Set the function's restype to c_char_p and it should return Python strings.

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