Atoi

int atoi( char* pStr ) {  
int iRetVal = 0;    
if (pStr)  
{    
 while ( *pStr && *pStr <= '9' && *pStr >= '0' )     
 {      
  iRetVal = (iRetVal * 10) + (*pStr - '0');      
  pStr++;    
 }  
}   
return iRetVal; 
} 


If we allow -ve and + signs we need to take care of them as well in the out put and check for invalid characters.