python - ctypes return a string from c function

Question

I'm a Python veteran, but haven't dabbled much in C. After half a day of not finding anything on the internet that works for me, I thought I would ask here and get the help I need.

What I want to do is write a simple C function that accepts a string and returns a different string. I plan to bind this function in several languages (Java, Obj-C, Python, etc.) so I think it has to be pure C?

Here's what I have so far. Notice I get a segfault when trying to retrieve the value in Python.

hello.c

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

const char* hello(char* name) {
    static char greeting[100] = "Hello, ";
    strcat(greeting, name);
    strcat(greeting, "!
");
    printf("%s
", greeting);
    return greeting;
}

main.py

import ctypes
hello = ctypes.cdll.LoadLibrary('./hello.so')
name = "Frank"
c_name = ctypes.c_char_p(name)
foo = hello.hello(c_name)
print c_name.value # this comes back fine
print ctypes.c_char_p(foo).value # segfault

I've read that the segfault is caused by C releasing the memory that was initially allocated for the returned string. Maybe I'm just barking up the wrong tree?

What's the proper way to accomplish what I want?

Answer 1

Your problem is that greeting was allocated on the stack, but the stack is destroyed when the function returns. You could allocate the memory dynamically:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

const char* hello(char* name) {
    char* greeting = malloc(100);
    snprintf("Hello, %s!
", 100, name)
    printf("%s
", greeting);
    return greeting;
}

But that's only part of the battle because now you have a memory leak. You could plug that with another ctypes call to free().

...or a much better approach is to read up on the official C binding to python (python 2.x at http://docs.python.org/2/c-api/ and python 3.x at http://docs.python.org/3/c-api/). Have your C function create a python string object and hand that back. It will be garbage collected by python automatically. Since you are writing the C side, you don't have to play the ctypes game.

...edit..

I didn't compile and test, but I think this .py would work:

import ctypes

# define the interface
hello = ctypes.cdll.LoadLibrary('./hello.so')
# find lib on linux or windows
libc = ctypes.CDLL(ctypes.util.find_library('c'))
# declare the functions we use
hello.hello.argtypes = (ctypes.c_char_p,)
hello.hello.restype = ctypes.c_char_p
libc.free.argtypes = (ctypes.c_void_p,)

# wrap hello to make sure the free is done
def hello(name):
    _result = hello.hello(name)
    result = _result.value
    libc.free(_result)
    return result

# do the deed
print hello("Frank")