Exchanging strings between C and Python

I wanted to exchange strings between a C and a Python program. I followed the template of Python using ctypes to pass a char * array and populate results (which is slightly different as it only communicates an array of strings from C to Python). The C program given below:

//simplegmp.c
#include<gmp.h>
#include <stdio.h>
#include <ctype.h>
#include<string.h>
void Cnumfunc(char** sent, char** recd)
{

    printf("This is the string %s\n",*sent);
    mpz_t p1;
    mpz_init(p1);
    mpz_set_str(p1,*sent,10);
    gmp_printf("Value of the gmp number is : %Zd \n", p1);
    mpz_add_ui(p1,p1,1);// add 1 and send back
    mpz_get_str(*recd,10,p1);


}

Compilation command for C program:

gcc -shared -o libsimple.so -fPIC simplegmp.c -lgmp

Now the python program:

#simplegm.py
import ctypes
libsimplegmp=ctypes.CDLL("./libsimple.so")
string_sent=ctypes.create_string_buffer(8)
string_recd=ctypes.create_string_buffer(8)
ptr_to_sent=ctypes.c_char_p(ctypes.addressof(string_sent))
ptr_to_recd=ctypes.c_char_p(ctypes.addressof(string_recd))
#libsimplegmp.connect()
string_sent="8374890"
libsimplegmp.Cnumfunc(ptr_to_sent,ptr_to_recd)
returnednum=ptr_to_recd.value
print("this is",returnednum)

This gives me the following:

This is the string (null)
Segmentation fault (core dumped)

I am new to this kind of programming and any help is much appreciated. Thanks.

(Question edited to latest details)