atoi的实现

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

typedef enum rtValue{pNULL,txtInvalid,OverFlow,Valid}rtValue;
rtValue m_error = Valid;

int atoi_fxp(const char* nptr){
    int i,len,negtive;
    long long result;
    if(nptr == NULL){
        m_error = pNULL;
        return -1;
    }
    i = result = 0;
    negtive = 1;
    len = strlen(nptr);
    if(*nptr == '-'){
        nptr++;
        negtive=-1;
        len--;
    }
    while(i < len){
        if((nptr[i] < '0')||(nptr[i] > '9')){
            m_error = txtInvalid;
            return -1;
        }
        result = result*10 + negtive*(nptr[i] - '0');
        if((abs(result) > 0x0FFFFFFF)||(abs(result) & (1 << 31))){
            m_error = OverFlow;
            return -1;
        }
        i++;
    }
    return result;
}

int main(int argc,char** argv){
    if(argc != 2){
        perror("usage wrong\n");
        return -1;
    }
    printf("the source str is %s\n",argv[1]);
    printf("the converted data is : %d\n",atoi_fxp(argv[1]));
    switch(m_error){
        case pNULL: printf("the pointer is NULL  \n");break;
        case txtInvalid: printf("the source string is wrong\n");break;
        case OverFlow: printf("the result is overflow\n");break;
        default: break;
    }
}