Python, get String from C library

Question

I haven't worked with C for many many years now so I'm stuck on how I can get a string back from a simple library and into Python.

test.c

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

char * get_string() {

        char theString[]= "This is my test string!";

        char *myString = malloc ( sizeof(char) * (strlen(theString)) +1 );

        strcpy(myString, theString);

        return myString;
}

I compile the code using:

gcc -Wall -O3 -shared test.c -o test.so

python.py

#!/usr/bin/env python

import ctypes

def main():
    string_lib = ctypes.cdll.LoadLibrary('test.so')
    myString = string_lib.get_string()
    print ctypes.c_char_p(myString).value

if __name__ == '__main__':
    main()

When I run the Python script I get:

Segmentation fault: 11

if I just print the result I get a numeric value which I assume is the pointer... How can I make it work?

Thanks!

Do you need the -fpic flag when you compile the c code?

Paul Rooney
– Paul Rooney

2015-05-17 13:05:00 +00:00
Commented May 17, 2015 at 13:05 — Paul Rooney
– Paul Rooney, Commented May 17, 2015 at 13:05
@PaulRooney - Tried it now, same result.

Kostas
– Kostas

2015-05-17 13:08:54 +00:00
Commented May 17, 2015 at 13:08 — Kostas
– Kostas, Commented May 17, 2015 at 13:08

deets · Accepted Answer · 2015-05-17 13:16:40Z

2

The problem is that you are using 64 bit Python. This will truncate the returned pointer.

Declare the restype first:

 import ctypes

 def main():
     string_lib = ctypes.cdll.LoadLibrary('test.so')
     string_lib.get_string.restype = ctypes.c_char_p
     myString = string_lib.get_string()
     print myString

 if __name__ == '__main__':
     main()

answered May 17, 2015 at 13:16

deets

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