CODEWITHSUNDEEP
pythoncmultithreadingsegmentation-faultctypes
Tom Cornebize
Tom Cornebize

Reputation: 1422

Segmentation fault when using ctypes and threading

I wrote a Python wrapper for a C library using ctypes. Everything worked fine, until I tried it with the threading module.

Here is a small example to reproduce it:

foo.c

#include <stdlib.h>
#include <assert.h>

char *foo_init() {
    char *result = (char*)malloc(sizeof(char));
    assert(result);
    return result;
}

char foo_get(const char* p) {
    return *p;
}

void foo_set(char *p, char value) {
    *p = value;
}

void foo_del(char *p) {
    free(p);
}

foo.py

import threading
import ctypes
libfoo=ctypes.CDLL('foo.so')

libfoo.foo_init.restype = ctypes.c_void_p
libfoo.foo_get.restype = ctypes.c_char

class Foo:
    def __init__(self):
        self.__obj__ = libfoo.foo_init()

    def get(self):
        return libfoo.foo_get(self.__obj__)

    def set(self, value):
        libfoo.foo_set(self.__obj__, ctypes.c_char(value))

    def __del__(self):
        libfoo.foo_del(self.__obj__)

x = Foo()
x.set(b'x')
print(x.get())

def compute():
    y = Foo()
    y.set(b'y')
    print(y.get())

t = threading.Thread(target=compute)
t.start()
t.join()

Compilation:

gcc -Wall -shared -o foo.so -fPIC foo.c

Execution:

export LD_LIBRARY_PATH=.
python3 foo.py

Result:

b'x'
[1]    8627 segmentation fault  python3 foo.py

We see here that x.get() is correctly printed whereas y.get() does a segmentation fault. Thus, it seems that there is a problem linked to ctypes and threading.

Note that if I move the initialization of y outside of the compute function, then the program exits normally.

Moreover, each of the functions which manipulate the pointer produce a segmentation fault (e.g. if I just keep the __init__ and __del__ functions, there is still a crash).

Any idea on how to fix this?

Upvotes: 2

Views: 2360

Answers (1)

Tom Cornebize
Tom Cornebize

Reputation: 1422

@user2357112 gave me the answer in comments: adding the (good) return type and the arguments types is important.

Replacing the beginning of foo.py by the following fix the problem:

libfoo.foo_init.restype = ctypes.POINTER(ctypes.c_char)
libfoo.foo_init.argtypes = []
libfoo.foo_get.restype = ctypes.c_char
libfoo.foo_get.argtypes = [ctypes.POINTER(ctypes.c_char)]
libfoo.foo_set.argtypes = [ctypes.POINTER(ctypes.c_char), ctypes.c_char]
libfoo.foo_del.argtypes = [ctypes.POINTER(ctypes.c_char)]

Upvotes: 1

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