learn-c-the-hard-waych25.txt

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Chapter 25
Exercise 24: Input, Output, Files

   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.
     __________________________________________________________________

   Source 63: ex24.c
   1  #include <stdio.h>
   2  #include "dbg.h"
   3
   4  #define MAX_DATA 100
   5
   6  typedef enum EyeColor {
   7      BLUE_EYES, GREEN_EYES, BROWN_EYES,
   8      BLACK_EYES, OTHER_EYES
   9  } EyeColor;
   10
   11  const char *EYE_COLOR_NAMES[] = {
   12      "Blue", "Green", "Brown", "Black", "Other"
   13  };
   14
   15  typedef struct Person {
   16      int age;
   17      char first_name[MAX_DATA];
   18      char last_name[MAX_DATA];
   19      EyeColor eyes;
   20      float income;
   21  } Person;
   22
   23
   24  int main(int argc, char *argv[])
   25  {
   26      Person you = {.age = 0};
   27      int i = 0;
   28      char *in = NULL;
   29
   30      printf("What's your First Name? ");
   31      in = fgets(you.first_name, MAX_DATA-1, stdin);
   32      check(in != NULL, "Failed to read first name.");
   33
   34      printf("What's your Last Name? ");
   35      in = fgets(you.last_name, MAX_DATA-1, stdin);
   36      check(in != NULL, "Failed to read last name.");
   37
   38      printf("How old are you? ");
   39      int rc = fscanf(stdin, "%d", &you.age);
   40      check(rc > 0, "You have to enter a number.");
   41
   42      printf("What color are your eyes:\n");
   43      for(i = 0; i <= OTHER_EYES; i++) {
   44          printf("%d) %s\n", i+1, EYE_COLOR_NAMES[i]);
   45      }
   46      printf("> ");
   47
   48      int eyes = -1;
   49      rc = fscanf(stdin, "%d", &eyes);
   50      check(rc > 0, "You have to enter a number.");
   51
   52      you.eyes = eyes - 1;
   53      check(you.eyes <= OTHER_EYES && you.eyes >= 0, "Do it right, th
   at's not an option.");
   54
   55      printf("How much do you make an hour? ");
   56      rc = fscanf(stdin, "%f", &you.income);
   57      check(rc > 0, "Enter a floating point number.");
   58
   59      printf("----- RESULTS -----\n");
   60
   61      printf("First Name: %s", you.first_name);
   62      printf("Last Name: %s", you.last_name);
   63      printf("Age: %d\n", you.age);
   64      printf("Eyes: %s\n", EYE_COLOR_NAMES[you.eyes]);
   65      printf("Income: %f\n", you.income);
   66
   67      return 0;
   68  error:
   69
   70      return -1;
   71  }
     __________________________________________________________________

   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.

25.1 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.
     __________________________________________________________________

   Source 64: ex24 output
   1$ make ex24
   2cc -Wall -g -DNDEBUG    ex24.c   -o ex24
   3$ ./ex24
   4What's your First Name? Zed
   5What's your Last Name? Shaw
   6How old are you? 37
   7What color are your eyes:
   81) Blue
   92) Green
   103) Brown
   114) Black
   125) Other
   13> 1
   14How much do you make an hour? 1.2345
   15----- RESULTS -----
   16First Name: Zed
   17Last Name: Shaw
   18Age: 37
   19Eyes: Blue
   20Income: 1.234500
     __________________________________________________________________

25.2 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.

25.3 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.

    1. fscanf
    2. fgets
    3. fopen
    4. freopen
    5. fdopen
    6. fclose
    7. fcloseall
    8. fgetpos
    9. fseek
   10. ftell
   11. rewind
   12. fprintf
   13. fwrite
   14. 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.

25.4 Extra Credit

    1. Rewrite this to not use fscanf at all. You'll need to use functions
       like atoi to convert the input strings to numbers.
    2. Change this to use plain scanf instead of fscanf to see what the
       difference is.
    3. Fix it so that the input names get stripped of the trailing newline
       characters and any whitespace.
    4. 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.

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