Calling Go from Python

Question

I tried to run a simple in a go script from python and I got a segmentation fault. Here is my code:

main.go

package main

import (
    /*
typedef struct foo{
int a;
int b;
int c;
int d;
int e;
int f;
} foo;
*/
    "C"
)

func main() {}

//export Foo
func Foo(t []int) C.foo {
    return C.foo{}
}

main.py

# loading shared object
lib = cdll.LoadLibrary("main.so")

# go type
class GoSlice(Structure):
    _fields_ = [("data", POINTER(c_void_p)), ("len", c_longlong), ("cap", c_longlong)]

lib.Foo.argtypes = [GoSlice]
lib.Foo.restype = c_void_p

t = GoSlice((c_void_p * 5)(1, 2, 3, 4, 5), 5, 5)
f = lib.Foo(t)
print(f)

With that code, I got

140362617287784
[1]    23067 segmentation fault  python3 main.py

Now If I remove e and f from the main.go I got

None

and no more segmentation fault.

Why the number of members in the struct does matter here?

[EDIT] Both are running at the same place, I run one command clear && go build -o main.so -buildmode=c-shared main.go && python3 main.py

Are you sure your Python installation and Go generated library are both 32 or 64 bits? — georgeok
– georgeok, Commented Jun 13, 2019 at 21:29
@georgeok Hello, both are running on my computer at the same place with only one command clear && go build -o main.so -buildmode=c-shared main.go && python3 main.py — Vince
– Vince, Commented Jun 14, 2019 at 2:12

georgeok · Accepted Answer · 2019-06-14 10:16:52Z

14

Your GO/C code is correct. The problem is in the python script. The lib.Foo.restype = c_void_p call expects a void pointer but the library return a C struct. You need to define the return type a ctypes struct in python then it will work as you expect.

The main.go:

package main

import (
    /*
    typedef struct foo{
    int a;
    int b;
    int c;
    int d;
    int e;
    int f;
    } foo;
    */
    "C"
)

func main() {
}

//export Foo
func Foo(t []int) C.foo {
    foo := C.foo{}
    foo.a = 1 // setting some values to avoid seeing zeros
    foo.b = 2
    return foo
}

The main.py:

from ctypes import *

# loading shared object
lib = cdll.LoadLibrary("main.so")


# go type
class GoSlice(Structure):
    _fields_ = [("data", POINTER(c_void_p)), ("len", c_longlong), ("cap", c_longlong)]


class Foo(Structure):
    _fields_ = [('a', c_int),
                ('b', c_int),
                ('c', c_int),
                ('d', c_int),
                ('e', c_int),
                ('f', c_int)]


lib.Foo.argtypes = [GoSlice]
lib.Foo.restype = Foo

t = GoSlice((c_void_p * 5)(1, 2, 3, 4, 5), 5, 5)
f = lib.Foo(t)
print(f)
print(f.a)
print(f.b)

Then run go build -o main.so -buildmode=c-shared main.go && python main.py and it will print:

go build -o main.so -buildmode=c-shared main.go && python3 main.py 
<__main__.Foo object at 0x102608830>
1
2

answered Jun 14, 2019 at 10:16

georgeok

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1 Comment

Vince Over a year ago

It works! Thank you a lot. Actually, I read another topic on stackoverflow (I do not find it anymore) that explained (if I well understood) that c_void_p is enough as return type to handle struct. And since it was working for another struct of my program, I thought it was the correct way. Thank you for clarifying that.

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