User Input

User input in C is, in short, a pain. There are three major input functions: getchar(), scanf(...), and fgets(...). To use any of these functions, you must include the stdio.h library.

getchar()

The getchar() function takes no arguments and returns the very next character in the standard input stream. If there are no more characters in the input stream, it returns the constant EOF.

Here’s an example that reads a student’s letter grade and then prints it back to the console.

char grade;
printf("Enter your grade: ");
grade = getchar();
printf("Your grade is %c\n", grade);

Note that if you are using getchar to read characters one at a time from standard input, you will NEVEr reach EOF – instead, the program will wait for you to type new input. If you have piped the contents of a file to be the input stream for the program, though, getchar will return EOF when you have reached the end of that file. If you want to use getchar to read all characters from standard input, a better check is to read until you reach \n (which would signify the user pressing Enter after supplying user input).

scanf(...)

The scanf(...) function allows us to read formatted input, like ints and doubles.

The first argument to scanf is the format string, which specifies the kind of data you expect to read. To specify the data types you expect, use the same control string characters you used for printf – %d, %f, %lf, %c, and %s. If you want to read an int, the first argument to scanf should be “%d”. If you want to read two ints, put “%d %d”.

The next arguments to scanf are the corresponding variables that you want to store the input in. We won’t go into details now, but scanf needs the memory address of these variables so it can modify their value. To get the address of a variable, put a & in front of the variable name.

Here’s a simple example that prompts the user for an integer, and then reads in the value:

int num;
printf("Enter a number: ");
scanf("%d", &num); 
//%d: we're reading in an integer
//&num: we're storing that integer in the num variable

Here’s an example that reads in an integer and a double:

int num1;
double num2;
printf("Enter an int and a double: ");
scanf("%d %lf", &num1, &num2);

The first number typed will get stored in num1, and the second number will get stored in num2. Our format string specified that these numbers should be separated by a space, but they can be separated by any amount of whitespace (multiple spaces, tabs, or newlines).

Suppose that the user is entering a fraction, like 9/5. Here’s how we could read in that information:

int numerator, denominator;
printf("Enter a fraction (like 9/5): ");
scanf("%d/%d", &numerator, &denominator);

By putting the “/” in the format string, we specify that we expect the input to have a / there, but we don’t wish to store it in a variable.

scanf returns the number of variables that were correctly read in.

Here is an (incomplete) list of subtleties when using scanf:

• In most cases, scanf skips whitespace. However, if you type a space (or tab or newline) where scanf expects a character, it will read the whitespace into your char variable.
• If you use scanf to read a single character, then the user will type an input character and then hit return. scanf will read the input character, but the newline will remain in the input buffer. This can cause problems if you call scanf a second time – the newline character will then be read, and not any new input. To fix this problem, add a call to getchar() after reading a char to read the extra newline character.
• If scanf reads input that it does not expect (for example, if it sees a character but is supposed to be reading an int), it will not discard the bad input. The bad input will still be in the input buffer if you call scanf again. To fix this, call getchar() until you reach \n. This will clear the input buffer.

In the next chapter, we will see that scanf can be very dangerous to use when reading strings from user input. In short, if the user enters a string that is longer than expected, scanf can accidentally overwrite other parts of memory. This can even be exploited using a buffer overflow attack by tricking scanf into reading in program instructions.

When we reach the section on strings (2.2), we will learn about the fgets function, which is a safer option for reading strings from both standard input and files.