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What is the difference between EOF and NULL

EOF and NULL are standard defines which normally equate to an integer value,

NULL is always 0

EOF   is compiler dependent         most commonly its -1 (in gcc & g++ it is -1).

Functions that are defined to return a pointer (like fgets()) typically return NULL to indicate that an exceptional condition has prevented them from doing their usual job. NULL is used because it is not a value that the function could ever return in the normal, successful case. The only time the function will return NULL is when something strange happened.

Functions that are defined to return an integer (like fscanf()) typically return EOF when something exceptional happens. Functions that do this do it because EOF is a value that can not possibly occur in the normal case where the function was able to succeed at doing whatever it was supposed to do. The EOF value is unambiguous.

So, generally you can figure out whether you should be looking for NULL or for EOF by considering the return type of the function. However, it's always a good idea to check the documentation because it's possible that there could be several conditions that might cause the function to return one of these distinctive values. End-of-file is not the only possible cause.

BTW, a scary thing about those code fragments is that they do not check the result of fopen(). (Incidentally, since fopen() returns a pointer you'd expect it to return NULL if it fails. That's exactly what it does.) In a real program you would always, always check that return value.


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