문자열 조작 함수 strchr 구현

/*
   The strchr() and strrchr() functions return a pointer to the matched character or NULL 
   if  the  character  is  not found.   
 */
char *strchr_r(const char *str, int c)
{
    if(!str)
       return NULL;

    char ch = (char)c;

    while((*str) && ((*str) != ch))
        str++;
    if((*str) == ch)
        return (char *)str;

    return NULL;
}