Returning a String from C to Python

Question

I'm new to both C and Python, I'm trying to get the C function getText() into Python but all I'm getting are numbers

This is foo.c

#include <stdio.h>

const char * getText(void)
{
    return "world hello";
}
const char * getText2(void)
{
    return "hello world";
}

And this is my python code from the terminal

>>> import ctypes
>>> testlib = ctypes.CDLL('/home/user/dir/libfoo.so')
>>> testlib.getText()
743175865

I have already compiled the shared object and testing it with puts("Hello world") works as it appears in the terminal.

I'm sure I'm accessing the getText() wrongly from python but I don't know which one. Any suggestions or help would be appreciated

mhawke · Accepted Answer · 2015-12-02 06:01:38Z

7

There is an example of how to handle string return types in the ctypes documentation. Unless the foreign function returns and int, you need to set the return type for the foreign function with the .restype attribute. For your code:

import ctypes
testlib = ctypes.CDLL('/home/user/dir/libfoo.so')
testlib.getText.restype = testlib.getText2.restype = ctypes.c_char_p
print testlib.getText()
print testlib.getText2()

Output

world hello
hello world

answered Dec 2, 2015 at 6:01

mhawke

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Comments

Brij · Accepted Answer · 2015-12-02 06:02:10Z

-1

your foo.c return local array instead you have to return a pointer to dynamically declared array using malloc.

const char * getText(char* name)
{    
  char hello[] = "world hello";
  char *var1 = malloc ( sizeof(char) * ( strlen(name) + strlen(hello) + 1 ) );
  if( var1 == NULL) exit(1);
   strcpy( var1 , hello);
   strcat(var1, name);    
   return var1;
 }

answered Dec 2, 2015 at 6:02

Brij

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1 Comment

mhawke Over a year ago

No. Since the returned strings are declared as literals they will exist in the string table in global address space, and therefore they will be accessible after the function returns.

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