help with hamming code error detection/correction

Hi I am doing a project about hamming code, the input is the length of the hamming code and if the parity is even or odd. I am getting a few errors that I cannot figure out how to fix.

Code:

---

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
/*global vars */
  int length;
  int parity;
  char *hamming_string=NULL;

    void enter_params(){
  /* declare local vars */
  /* prompt for length of hamming code, parity */
      printf("Enter the length of the hamming code: ");
      scanf("%d", &length);
      printf("Enter the parity (0=even, 1-odd): ");
      scanf("%d", &parity);
  /* alocate space for hamming code */
      hamming_string=(char*)malloc(length*sizeof(char));
      return;
  }

    void check(){
  /* declare local vars */
      int i;
      int j;
      int k;
      int start;
      int step;
      int parity_check;
      int error_bit;
      int num_parity_bits;
      printf("Enter the Hamming code: ");
      scanf("%s", hamming_string);
      num_parity_bits=ceil(log(length)/log(2));
      for(i=0; i<num_parity_bits; i++){
        parity_check=parity;
        error_bit=0;
        start=pow(2,i);
        step=2*start;
        error_bit+=parity_check*start;
        for(j=start; j<length; j=j+step)
            for(k=j; k<start+step&&k<length; k++)
              parity_check=parity_check^(hamming_string[length-k]-'0');
      }
      if(error_bit != 0){
        if(hamming_string[length-error_bit]=='0'){
            printf("There is an error in bit: %d", hamming_string[length-error_bit]=='0');
            hamming_string[length-error_bit]='1';
            printf("\n");
        }
        else{
            hamming_string[length-error_bit]='0';
        }
        printf("The corrected Hamming code is: %s", hamming_string);
        printf("\n");
      }
     
      else{
        printf("There is no bit error");
        printf("\n");
      }
         
         
      return;
  }



  /* void exit(){
      if(hamming_string!=NULL){
        free(hamming_string);
      }
  }*/


    int main(){
      int choice = 0;
      while(choice!=3){
        printf("Error detection/correction:\n");
        printf("----------------------\n");
        printf("1) Enter parameters\n");
        printf("2) Check Hamming code\n");
        printf("3) Exit\n");
        printf("\n Enter Selection: ");
        scanf("%d", &choice);
        if(choice==1){
            enter_params();
            printf("\n");
            main();
        }
        else if(choice==2){
            check();
            printf("\n");
            main();
        }
        /*else if(choice==3){
            exit();
        }*/
        return 1;
      }
  }

---

when i input the length as 7 and the parity as 0 , then the hamming code as 1000110.
i am getting the output that there is no error, when i should get there is an error in bit 6 and the corrected hamming code should be 1100110.

then when i input the length as 7 and the parity as 1, and then the hamming code as 1000110. i get the correct error bit as 1 but when it corrects it i get 1001110 instead of 1000111.


any help would be much appreciated.
Thanks