将字符串从m个字符开始由另一个字符串替代

#include<stdio.h>int main(){    char str[100],m,n,i,substr[50];    printf("input str:\n");    scanf("%s",str);    printf("substr:\n");    scanf("%s",substr);    printf("place:\n");    scanf("%d",&m);    for(i=0;substr[i]!='\0';i++)        str[m+i-1]=substr[i];    str[m+i-1]='\0';    printf("str:%s\n",str);    return 0;}