Why does a string change in Python after being used in a function with a C module?

Question

MySQL

I immediately apologize for the quality of the code, I'm just learning, if you need any more information - let me know.
The string changes mysteriously after being used in the mysql query comparison function with those already found. The function also uses a self-written C module.
Source line:

delete from blabla_1archive_visits where `date`<'2014-01-10';

Turns into:

deletefromblabla_1archive_visitswhere`date`<'2014-01-10';

Etc.

Calling a function with a gag:

diff2_s = diff2(str_query[:], BD, queryId)

The function where the plug occurs:

The plug is between these prints.

def diff2(str_query,BD, queryId):
        print 'pre diff2\n3 $$$ ' ,str_query, '\n\n\n'
        line = 0
        cursor.execute('''SELECT id,query FROM queryTable WHERE database = ?''', (BD,))
        rows = cursor.fetchall()
        for row in rows:
            if diff3_ctypes(str_query, row[1]) > 80.:
                print 'after diff2\n4 $$$ ' ,str_query, '\n\n\n'
                line = row[0]
                break
            else: continue
        if line: return line
        else: return False

C module:

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

float diff3 (char str1[],char str2[]) {
    char *token1, *last1;
    char *token2, *last2;
    char *i1[10000];
    char *i2[10000];
    float summ = 0;
    float res = 0;
    float j1 = 0, j2 = 0;
    float x1,x2;

    token1 = strtok_r(str1, " ,=().", &last1);
    while (token1 != NULL) {
        i1[(int)j1] = token1;
        j1 = j1+1;
        token1 = strtok_r(NULL, " ,=().", &last1);
    }
    token2 = strtok_r(str2, " ,=().", &last2);
    while (token2 != NULL) {
        i2[(int)j2] = token2;
        j2=j2+1;
        token2 = strtok_r(NULL, " ,=().", &last2);
    }
    x1 = j1;
    x2 = j2;
    if (x1<=x2)
    {
        for (float k = 0.0; k < x1; k = k+1)
        {
            if (!strcmp(i1[(int)k],i2[(int)k]))
            {
                summ = summ+1.0;
            }
        }
        res = summ/x1*100.0;
    }else{
        for (float k = 0.0; k < x2; k = k + 1)
        {
            if (!strcmp(i1[(int)k],i2[(int)k]))
            {
                summ = summ + 1.0;
            }
        }
        res = summ/x2*100.0;
    }
    return res;
}

Answer 1

The diff3 function doesn't work for me at all. At least it stably changes the values ​​​​that I pass to it, and even returns NAN instead of a value.
A cursory examination showed that memory is being overwritten inside the strtok_r function. This function writes values ​​to the pointer passed as the third parameter, but yours is not even initialized. I suspect that the memory somehow miraculously (and there are no others) gets into your own variable str1 or str2. I didn’t analyze the code further, but I suspect that there may still be problems with writing to variables through pointers.
Deal with this function separately, debug it first, independently of the rest of the code.