What does isalnum (int) do?

Returns a nonzero integer if the argument is any lower- or uppercase letter or 0-9 digit. Otherwise, it returns the integer 0.

What is BUFSIZ?

BUFSIZ is a constant macro representing the size of the input buffer. It is defined in stdio.h and represents the size for your C implementation. ANSI C requires that it be at least 256 bytes. A common size is 512. It gives us the maximum number of characters that can be transferred through our standard input stream, stdin.

What would be the result of the expression -6/5?

The ANSI C standard says that the result is "implementation defined." This means that ANSI C has no hard and fast rule, therefore, the result depends on the compiler that you use. Check your compiler; it should give either -1 or -2 as a result.

Note that the "correct" answer is -1.2. The compiler therefore has a choice of rounding up or down.

What happens if we try to assign an integer type value to a float type variable?

C puts a decimal point at the end and converts the method of storage to exponential binary form and store the result in the variable's memory cell. Thus,

p = 6/4;

takes the integer value (6/4, which becomes 1 due to the cutting off of the fractional part), converts this to 1.0 (with a decimal point meaning it is in exponential binary form), and stores this in the memory cell for the variable.