Sams Teach Yourself C in 21 Days

1. The gets()function accepts input from the keyboard until it reaches a newline
   character.


2. The string entered, minus the new line and with a trailing null character, is stored
   in the memory location pointed to by input.


3. The address of the string (the same value as input) is returned to the pointer ptr.


4. An assignment statementis an expression that evaluates to the value of the variable
   on the left side of the assignment operator. Therefore, the entire expression ptr =
   gets(input)evaluates to the value of ptr. By enclosing this expression in paren-
   theses and preceding it with the indirection operator (*), you obtain the value
   stored at the pointed-to address. This is, of course, the first character of the input
   string.
   5.NULLis a symbolic constant defined in the header file stdio.h. It has the value of the
   null character ( 0 ).


5. If the first character of the input string isn’t the null character (if a blank line hasn’t
   been entered), the comparison operator returns true, and the whileloop executes.
   If the first character is the null character (if a blank line has been entered), the
   comparison operator returns false, and the whileloop terminates.
   When you use gets()or any other function that stores data using a pointer, be sure that
   the pointer points to allocated space. It’s easy to make a mistake such as this:
   char *ptr;
   gets(ptr);
   The pointer ptrhas been declared but not initialized. It points somewhere, but you don’t
   know where. The gets()function doesn’t know that ptrhasn’t been initialized to point
   somewhere, so it simply goes ahead and stores the entered string at whatever address is
   contained in ptr. The string might overwrite something important, such as program code
   or the operating system. Most compilers won’t catch this type of mistakes, so you, the
   programmer, must be vigilant.

240 Day 10

FIGURE10.1

The components of a

whilestatement that

tests for input of a

blank line.

4

while ( *( ptr = gets(input)) != NULL)

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