atoi API

#include<stdio.h>

#define bool int
#define false 0
#define true 1

int _strlen(char* pszName)
{
    int nLen = 0;
    while(*pszName)
    {
        nLen++;
        pszName++;
    }    
    return nLen;
}

bool _isdigit(char szName)
{
    bool bResult = false;
    if (szName >= '0' && szName <= '9')
    {
        bResult = true;
    }
    return bResult;
}

int atoi(char *pszName)
{
    if (NULL == pszName)
    {
        return 0;
    }
    int nLen = _strlen(pszName);
    if (nLen <= 0)
    {
        return 0;
    }
    
    int nIndex = 0;
    bool bFlag = true;
    if (!_isdigit(pszName[0]))
    {
        if ('-' == pszName[0])
        {
            bFlag = false;
        }
        else if ('+' == pszName[0])
        {
            bFlag = true;
        }
        else
            return 0;

        nIndex++;
    }
    
    int nResult = 0;
    for (; nIndex < nLen; nIndex++)
    {
        if (!_isdigit(pszName[nIndex]))
        {
            break;
        }
        nResult = nResult * 10 + (pszName[nIndex] - '0');
    }
    if (!bFlag)
    {
        nResult = -nResult;
    }
    
    return nResult;
}

int main()
{
    char* pszNum = "456";
    int nNum = atoi(pszNum);
    printf("%d\n", nNum);
    return 0;
}