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;
}