The c function I am trying to call:
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]){
long long int n = atoll(argv[1]);
printf("main(%lld)n", n);
int m = 0;
while (n > 1) {
m++;
if (n % 2 == 0) { // even
printf("%lld = %lld/2 n", n/2, n);
n = n/2;
}
else {
printf("%lld = 3*%lld+1 n", 3*n+1, n);
n = 3*n + 1;
}
}
printf("Collatz(%lld) is '%d'n", n,m);
return m;
}
My python code that attempts to call it with parameter 4
from ctypes import *
import os, sys
print("you typed:", sys.argv[1])
filename = sys.argv[1].split(".")[0]
os.system(f"gcc -c -fPIC {filename}.c -o {filename}.o && gcc {filename}.o "
f"-shared -o {filename}.so")
soFile = CDLL(os.getcwd() + "/" + filename + ".so")
soFile.main(4) # <----------
In vim I run the following:
:!python test.py "collatz-loop" you typed: collatz-loop main(0) Collatz(0) is '0'
Notice how the output is 0. Even if I change the parameter to something other than 4, the output is always 0.
If I try to change soFile.main(4) to something with, say, two parameters, like soFile.main(2, 4), I get
:!python test.py "collatz-loop" shell returned 139
Why is this happening, and how do I fix it?
- I am new to c by the way, though very familiar with Java. They seem very alike syntactically.
- Yes, I did remember to save/write the python and c file before running the .py file.
Answer
TLDR for comment thread:
You should split it into 2 funcs, main and collatz (int collatz(long long n)) and make main just pass atoll(argv[1]) to collatz, so you can run it normally as an executable and also from your python script file as a shared library. You also need to set the argtype in ctypes to long long(I think it defaults to int).
C:
#include <stdio.h>
#include <stdlib.h>
int collatz(long long n){
int m = 0;
while (n > 1) {
m++;
if (n % 2 == 0) { // even
printf("%lld = %lld/2 n", n/2, n);
n = n/2;
}
else {
printf("%lld = 3*%lld+1 n", 3*n+1, n);
n = 3*n + 1;
}
}
printf("Collatz(%lld) is '%d'n", n,m);
return m;
}
int main(int argc, char *argv[]){
long long int n = atoll(argv[1]);
printf("main(%lld)n", n);
collatz(n);
}
Python:
from ctypes import *
import os, sys
print("you typed:", sys.argv[1])
filename = sys.argv[1].split(".")[0]
os.system(f"gcc -c -fPIC {filename}.c -o {filename}.o && gcc {filename}.o "
f"-shared -o {filename}.so")
soFile = CDLL(os.getcwd() + "/" + filename + ".so")
soFile.collatz.argtypes = [c_longlong]
soFile.collatz(4)
Note: Not directly related to this issue, but you should check to make sure argc > 2 in your C code to avoid an out of bounds read if no args are passed.