title: Where to exact use NULL in C?
body: <p>I know NULL is a kind of pointer which points no location in memory but let me know that where to use this (NULL) exactly. I have seen it in case of File Handling as <code>id(fp!=NULL)</code>, passing it to <code>time(time_t *second)</code>as <code>time(NULL)</code>, sometimes like <code>int *p=NULL</code> and some other cases too. I also use these several times(imitating) but I am not able to understand the exact use of this pointer or directive whatever it is. Please explain the meaning of <code>NULL</code> vividly and apparently with examples. I am not telling to describe the above mentioned cases, they are just examples. Please exemplify in such a way that I may know the actual use of NULL and could find my answer of the mentioned cases.</p>