Question

You've been using printf to print things, and that's great and all, but you need more. In this exercise program you're using the functions fscanf and fgets to build information about a person in a structure. After this simple introduction to reading input, you'll get a full list of the functions that C has for I/O. Some of these you've already seen and used, so this will be another memorization exercise.

#include <stdio.h>
#include "dbg.h"

#define MAX_DATA 100

typedef enum EyeColor {
    BLUE_EYES, GREEN_EYES, BROWN_EYES,
    BLACK_EYES, OTHER_EYES
} EyeColor;

const char *EYE_COLOR_NAMES[] = {
    "Blue", "Green", "Brown", "Black", "Other"
};

typedef struct Person {
    int age;
    char first_name[MAX_DATA];
    char last_name[MAX_DATA];
    EyeColor eyes;
    float income;
} Person;

int main(int argc, char *argv[])
{
    Person you = {.age = 0};
    int i = 0;
    char *in = NULL;

    printf("What's your First Name? ");
    in = fgets(you.first_name, MAX_DATA-1, stdin);
    check(in != NULL, "Failed to read first name.");

    printf("What's your Last Name? ");
    in = fgets(you.last_name, MAX_DATA-1, stdin);
    check(in != NULL, "Failed to read last name.");

    printf("How old are you? ");
    int rc = fscanf(stdin, "%d", &you.age);
    check(rc > 0, "You have to enter a number.");

    printf("What color are your eyes:\n");
    for(i = 0; i <= OTHER_EYES; i++) {
        printf("%d) %s\n", i+1, EYE_COLOR_NAMES[i]);
    }
    printf("> ");

    int eyes = -1;
    rc = fscanf(stdin, "%d", &eyes);
    check(rc > 0, "You have to enter a number.");

    you.eyes = eyes - 1;
    check(you.eyes <= OTHER_EYES && you.eyes >= 0, "Do it right, that's not an option.");

    printf("How much do you make an hour? ");
    rc = fscanf(stdin, "%f", &you.income);
    check(rc > 0, "Enter a floating point number.");

    printf("----- RESULTS -----\n");

    printf("First Name: %s", you.first_name);
    printf("Last Name: %s", you.last_name);
    printf("Age: %d\n", you.age);
    printf("Eyes: %s\n", EYE_COLOR_NAMES[you.eyes]);
    printf("Income: %f\n", you.income);

    return 0;
error:

    return -1;
}

This program is deceptively simple, and introduces a function called fscanf which is the "file scanf". The scanf family of functions are the inverse of the printf versions. Where printf printed out data based on a format, scanf reads (or scans) input based on a format.

There's nothing original in the beginning of the file, so here's what the main is doing:

ex24.c:24-28

Set up some variables we'll need.

ex24.c:30-32

Get your first name using the fgets function, which reads a string from the input (in this case stdin ) but makes sure it doesn't overflow the given buffer.

ex24.c:34-36

Same thing for you.last_name , again using fgets .

ex24.c:38-39

Uses fscanf to read an integer from stdin and put it into you.age . You can see that the same format string is used as printf to print an integer. You should also see that you have to give the address of you.age so that fscanf has a pointer to it and can modify it. This is a good example of using a pointer to a piece of data as an "out parameter".

ex24.c:41-45

Print out all the options available for eye color, with a matching number that works with the EyeColor enum above.

ex24.c:47-50

Using fscanf again, get a number for the you.eyes , but make sure the input is valid. This is important because someone can enter a value outside the EYE_COLOR_NAMES array and cause a segfault.

ex24.c:52-53

Get how much you make as a float for the you.income .

ex24.c:55-61

Print everything out so you can see if you have it right. Notice that EYE_COLOR_NAMES is used to print out what the EyeColor enum is actually called.

What You Should See

When you run this program you should see your inputs being properly converted. Make sure you try to give it bogus input too so you can see how it protects against the input.

$ make ex24
cc -Wall -g -DNDEBUG    ex24.c   -o ex24
$ ./ex24
What's your First Name? Zed
What's your Last Name? Shaw
How old are you? 37
What color are your eyes:
1) Blue
2) Green
3) Brown
4) Black
5) Other
> 1
How much do you make an hour? 1.2345
----- RESULTS -----
First Name: Zed
Last Name: Shaw
Age: 37
Eyes: Blue
Income: 1.234500

How To Break It

This is all fine and good, but the real important part of this exercise is how scanf actually sucks. It's fine for simple conversion of numbers, but fails for strings because it's difficult to tell scanf how big a buffer is before you read. There's also a problem with a function like gets (not fgets , the non-f version) which we avoided. That function has no idea how big the input buffer is at all and will just trash your program.

To demonstrate the problems with fscanf and strings, change the lines that use fgets so they are fscanf(stdin, "%50s", you.first_name) and then try to use it again. Notice it seems to read too much and then eat your enter key? This doesn't do what you think it does, and really rather than deal with weird scanf issues, just use fgets .

Next, change the fgets to use gets , then bust out your valgrind and do this: valgrind ./ex24 < /dev/urandom to feed random garbage into your program. This is called "fuzzing" your program, and it is a good way to find input bugs. In this case, you're feeding garbage from the /dev/urandom file, and then watching it crash. On some platforms you may have to do this a few times, or even adjust the MAX_DATA define so it's small enough.

The gets function is so bad that some platforms actually warn you when the program runs that you're using gets . You should never use this function, ever.

Finally, take the input for you.eyes and remove the check that the number given is within the right range. Then feed it bad numbers like -1 or 1000. Do this under Valgrind too so you can see what happens.

The I/O Functions

This is a short list of various I/O functions that you should look up and create index cards that have the function name, what it does, and all the variants similar to it.

• fscanf
• fgets
• fopen
• freopen
• fdopen
• fclose
• fcloseall
• fgetpos
• fseek
• ftell
• rewind
• fprintf
• fwrite
• fread

Go through these and memorize the different variants and what they do. For example, for the card on fscanf you'll have scanf , sscanf , vscanf , etc. and then what each of those do on the back.

Finally, to get the information you need for these cards, use man to read the help for it. For example, the page for fscanf comes from man fscanf .

Extra Credit

• Rewrite this to not use fscanf at all. You'll need to use functions like atoi to convert the input strings to numbers.
• Change this to use plain scanf instead of fscanf to see what the difference is.
• Fix it so that the input names get stripped of the trailing newline characters and any whitespace.
• Use scanf to write a function that reads a character at a time and files in the names but doesn't go past the end. Make this function generic so it can take a size for the string, and make sure you end the string with '\0' no matter what.