the classical examination test

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


//copy the ”n-m“ size string from  point m
void StrMid(char* str1,int m,int n,char* str2)
{
    if( !str1 && !str2){
        return ;
    }

    if( n <= m || m<0 || n>strlen(str1) ){
        return ;
    }else{
        int i=0;
        for( m; m<=n ; ++m,++i ){
            str2[i] = str1[m];
        }
        str2[i]=0;
    }
}

int main(int argc, char *argv[])
{
    char str2[100]={0};
    char str1[]={"goodmorning"};
    StrMid(str1,1,3,str2);
    printf("str2 = %s",str2);
}

//the memory test, int to char, 
//but if n is 5,6, what is the result, it due to the big-endian and little-endian
int m(char* s, int n)
{
    if(n>1)
        return s[0] + m(&s[1] , n-1);
    else
        return s[0];
}

int main()
{
    int a[]={1,2,3};
    int value = m((char*)a, 4);
    printf("value= %d\n",value);
}


//which type is the IP ?
typedef enum{
    IP_A = 0,
    IP_B,
    IP_C,
    IP_D,
    IP_E,
    IP_NO 
}w;


int ipType(const char *ipStr)
{
    if(!ipStr){
        return 0;
    }
    const char *p_str= ipStr;
    int i=0; 
    for(i=0 ;*p_str!=0 && i<3; ++i){
        int num = atoi(p_str);
        if(num <0 || num>254){
            return IP_NO;
        }
        for( ;*p_str!=0 && isdigit(*p_str); ++p_str)
            ;
        if( '.' != *p_str){
            return IP_NO;
        }       
    }

    int num = atoi(p_str);
    if(num <0 || num>254){
        return IP_NO;
    }   

    int ip_head = atoi(ipStr);
    if( ip_head >= 0xF0){
        return IP_E;
    }
    if( ip_head >= 0xE0){
        return IP_D;
    }
    if( ip_head >= 0xC0){
        return IP_C;
    }
    if( ip_head >= 0x80){
        return IP_B;
    }
    if( ip_head >= 0x00){
        return IP_A;
    }           
}
int main()
{
    setbuf(stdout,NULL);
    while(1)
    {
        char ip[100]={0};
        printf("please input a IP, I will analyse it:\n");
        fgets(ip, 99, stdin);

        printf("IP type is : ");
        switch( ipType(ip) )
        {
            case IP_A: printf("IP_A\n"); break;
            case IP_B: printf("IP_B\n"); break;
            case IP_C: printf("IP_C\n"); break;
            case IP_D: printf("IP_D\n"); break;
            case IP_E: printf("IP_E\n"); break;
            case IP_NO: printf("IP_NO\n"); break;
            default: printf("IP_NO\n"); break;
        }   
    }   
    return 0;
}

//not use judgement statement , to judge two interger number, which is the bigger
void max(int a, int b)
{
    int c = a - b;  
    const char *strs[2] = {"a large","b large"};  
    //Don't expand the highest bit
    c = unsigned(c) >> (sizeof(int)*8 - 1);  
    //equal to  c = c >> (sizeof(int)*8 - 1) &0x1;  
    printf(" %s \n",strs[c] );  
}

int main(int argc, char*argv[])
{
    setbuf(stdout, NULL);
    while(1)
    {
        char buf[100]={0};
        printf("please input two integer:\n");
        fgets( buf ,100, stdin );
        int a= atoi(buf);
        char *pStr = strchr( buf,' ');
        int b= atoi(pStr);
        printf("a=%d, b=%d\n", a,b);
        max(a,b);
    }
}