Why should you learn to program in C?

• It is the de facto substandard of programming languages.
  • C runs on everything.
  • C lets you write programs that use very few resources.
  • C gives you near-total control over the system, down to the level of pushing around individual bits with your bare hands.
  • C imposes very few constraints on programming style: unlike higher-level languages, C doesn't have much of an ideology. There are very few programs you can't write in C.
  • Most of the programming languages people actually use (Visual Basic, perl, PHP, etc.) are executed by interpreters written in C (or C++, an extension to C).
• You will learn discipline.
  • C makes it easy to shoot yourself in the foot.
  • You can learn to avoid this by being careful about where you point it.
  • Pain is a powerful teacher of caution.

• You will fail CS323 if you don't learn C really well in CS223 (CS majors only).

On the other hand, there are many reasons why you might not want to use C later in life. It's missing a lot of features of modern program languages, including:

• A garbage collector.
• Minimal programmer-protection features like array bounds-checking.
• Non-trivial built-in data structures.
• Language support for exceptions, namespaces, object-oriented programming, etc.

For most problems where minimizing programmer time and maximizing robustness are more important than minimizing runtime, other languages are a better choice. But for CS223 we'll be using C.

If you want to read a lot of flaming about what C is or is not good for, see http://c2.com/cgi/wiki?CeeLanguage.

CategoryProgrammingNotes

See HowToUseTheComputingFacilities for details of particular commands. The basic steps are

• Creating the program with a text editor of your choosing. (I like vim for long programs and cat for very short ones.)

• Compiling it with gcc.

• Running it.

If any of these steps fail, the next step is debugging. We'll talk about debugging elsewhere.

3. Creating the program

Use your favorite text editor. The program file should have a name of the form foo.c; the .c at the end tells the C compiler the contents are C source code. Here is a typical C program:

   1 #include <stdio.h>
   2 
   3 /* print the numbers from 1 to 10 */
   4 
   5 int
   6 main(int argc, char **argv)
   7 {
   8     int i;
   9 
  10     puts("Now I will count from 1 to 10");
  11     for(i = 1; i <= 10; i++) {
  12         printf("%d\n", i);
  13     }
  14 
  15     return 0;
  16 }

4. Compiling and running a program

Here's what happens when I compile and run it on the Zoo:

$ gcc -o count count.c
$ ./count
Now I will count from 1 to 10
1
2
3
4
5
6
7
8
9
10
$

The first line is the command to compile the program. The second line runs the output file count. Calling it ./count is necessary because by default the shell (the program that interprets what you type) only looks for programs in certain standard system directories. To make it run a program in the current directory, we have to include the directory name.

5. Some notes on what the program does

Noteworthy features of this program include:

• The #include <stdio.h> in line 1. This is standard C boilerplate, and will appear in any program you see that does input or output. The meaning is to tell the compiler to include the text of the file /usr/include/stdio.h in your program as if you had typed it there yourself. This particular file contains declarations for the standard I/O library functions like puts (put string) and printf (print formatted), as used in the program. If you don't put it in, your program may or may not still compile. Do it anyway.

• Line 3 is a comment; its beginning and end is marked by the /* and */ characters. Comments are ignored by the compiler but can be helpful for other programmers looking at your code (including yourself, after you've forgotten why you wrote something).

• Lines 5 and 6 declare the main function. Every C program has to have a main function declared in exactly this way---it's what the operating system calls when you execute the program. The int on Line 3 says that main returns a value of type int (we'll describe this in more detail later in C/Functions), and that it takes two arguments: argc of type int, the number of arguments passed to the program from the command line, and argv, of a pointer type that we will get to eventually (C/Pointers), which is an array of the arguments (essentially all the words on the command line, including the program name). Note that it would also work to do this as one line (as KerniganRitchie typically does); the C compiler doesn't care about whitespace, so you can format things however you like, subject to the constraint that consistency will make it easier for people to read your code.

• Everything inside the curly braces is the body of the main function. This includes

  • The declaration int i;, which says that i will be a variable that holds an int (C/IntegerTypes).

  • Line 10, which prints an informative message using puts (C/InputOutput).

  • The for loop on Lines 11–13, which executes its body for each value of i from 1 to 10. We'll explain how for loops work later (C/Statements). Note that the body of the loop is enclosed in curly braces just like the body of the main function. The only statement in the body is the call to printf on Line 12; this includes a format string that specifies that we want a decimal-formatted integer followed by a newline (the \n).

  • The return 0; on Line 15 tells the operating system that the program worked (the convention in Unix is that 0 means success). If the program didn't work for some reason, we could have returned something else to signal an error.

CategoryProgrammingNotes

7. Using the Zoo

The best place for information about the Zoo is at http://zoo.cs.yale.edu/. Below are some points that are of particular relevance for CS223 students.

7.1. Getting an account

To get an account in the Zoo, follow the instructions at http://zoo.cs.yale.edu/cgi-bin/accounts.pl. You will need your NetID and password to sign up for an account.

Even if you already have an account, you still need to use this form to register as a CS 223 student, or you will not be able to submit assignments.

7.2. Getting into the room

The Zoo is located on the third floor of Arthur K Watson Hall, toward the front of the building. If you are a Yale student, your ID should get you into the building and the room. If you are not a student, you will need to get your ID validated in AKW 008a to get in after hours.

7.3. Remote use

See HowToUseTheZooRemotely.

8. Using Unix

The Zoo runs a Unix-like operating system called Linux. Most people run Unix with a command-line interface provided by a shell. Each line typed to the shell tells it what program to run (the first word in the line) and what arguments to give it (remaining words). The interpretation of the arguments is up to the program.

8.1. Getting a shell prompt in the Zoo

When you log in to a Zoo node directly, you may not automatically get a shell window. If you use the default login environment (which puts you into the KDE window manager), you need to click on the picture of the display with a shell in from of it in the toolbar at the bottom of the screen. If you run Gnome instead (you can change your startup environment using the popup menu in the login box), you can click on the foot in the middle of the toolbar. Either approach will pop up a terminal emulator from which you can run emacs, gcc, and so forth.

The default login shell in the Zoo is bash, and all examples of shell command lines given in these notes will assume bash. You can choose a different login shell on the account sign-up page if you want to, but you are probably best off just learning to like bash.

8.2. The Unix filesystem

Most of what one does with Unix programs is manipulate the filesystem. Unix files are unstructured blobs of data whose names are given by paths consisting of a sequence of directory names separated by slashes: for example /home/accts/some-user/cs223/hw1.c. At any time you are in a current working directory (type pwd to find out what it is and cd new-directory to change it). You can specify a file below the current working directory by giving just the last part of the pathname. The special directory names . and .. can also be used to refer to the current directory and its parent. So /home/accts/some-user/cs223/hw1.c is just hw1.c or ./hw1.c if your current working directory is /home/accts/some-user/cs223, cs223/hw1.c if your current working directory is /home/accts/some-user, and ../cs223/hw1.c if your current working directory is /home/accts/some-user/illegal-downloads.

All Zoo machines share a common filesystem, so any files you create or change on one Zoo machine will show up in the same place on all the others.

8.3. Unix command-line programs

Here are some handy Unix commands:

man

man program will show you the on-line documentation for a program (e.g., try man man or man ls). Handy if you want to know what a program does. On Linux machines like the ones in the Zoo you can also get information using info program, which has an Emacs-like interface.

ls

ls lists all the files in the current directory. Some useful variants:

• ls /some/other/dir; list files in that directory instead.

• ls -l; long output format showing modification dates and owners.

mkdir

mkdir dir will create a new directory in the current directory named dir.

rmdir

rmdir dir deletes a directory. It only works on directories that contain no files.

cd

cd dir changes the current working directory. With no arguments, cd changes back to your home directory.

pwd

pwd ("print working directory") shows what your current directory is.

mv

mv old-name new-name changes the name of a file. You can also use this to move files between directories.

cp

cp old-name new-name makes a copy of a file.

rm

rm file deletes a file. Deleted files cannot be recovered. Use this command carefully.

chmod

chmod changes the permissions on a file or directory. See the man page for the full details of how this works. Here are some common chmod's:

• chmod 644 file; owner can read or write the file, others can only read it.

• chmod 600 file; owner can read or write the file, others can't do anything with it.

• chmod 755 file; owner can read, write, or execute the file, others can read or execute it. This is typically used for programs or for directories (where the execute bit has the special meaning of letting somebody find files in the directory).

• chmod 700 file; owner can read, write, or execute the file, others can't do anything with it.

emacs, gcc, make, gdb, git

See corresponding sections.

8.4. Stopping and interrupting programs

Sometimes you may have a running program that won't die. Aside from costing you the use of your terminal window, this may be annoying to other Zoo users, especially if the process won't die even if you close the terminal window or log out.

There are various control-key combinations you can type at a terminal window to interrupt or stop a running program.

ctrl-C

Interrupt the process. Many processes (including any program you write unless you trap SIGINT using the sigaction system call) will die instantly when you do this. Some won't.

ctrl-Z

Suspend the process. This will leave a stopped process lying around. Type jobs to list all your stopped processes, fg to restart the last process (or fg %1 to start process %1 etc.), bg to keep running the stopped process in the background, kill %1 to kill process %1 politely, kill -KILL %1 to kill process %1 whether it wants to die or not.

ctrl-D

Send end-of-file to the process. Useful if you are typing test input to a process that expects to get EOF eventually or writing programs using cat > program.c (not really recommmended). For test input, you are often better putting it into a file and using input redirection (./program < test-input-file); this way you can redo the test after you fix the bugs it reveals.

ctrl-\

Quit the process. Sends a SIGQUIT, which asks a process to quit and dump core. Mostly useful if ctrl-C and ctrl-Z don't work.

If you have a runaway process that you can't get rid of otherwise, you can use ps g to get a list of all your processes and their process ids. The kill command can then be used on the offending process, e.g. kill -KILL 6666 if your evil process has process id 6666. Sometimes the killall command can simplify this procedure, e.g. killall -KILL evil kills all process with command name evil.

8.5. Running your own programs

If you compile your own program, you will need to prefix it with ./ on the command line to tell the shell that you want to run a program in the current directory (called '.') instead of one of the standard system directories. So for example, if I've just built a program called count, I can run it by typing

$ ./count

Here the "$ " is standing in for whatever your prompt looks like; you should not type it.

Any words after the program name (separated by whitespace---spaces and/or tabs) are passed in as arguments to the program. Sometimes you may wish to pass more than one word as a single argument. You can do so by wrapping the argument in single quotes, as in

$ ./count 'this is the first argument' 'this is the second argument'

8.6. Input and output

Some programs take input from standard input (typically the terminal). If you are doing a lot of testing, you will quickly become tired of typing test input at your program. You can tell the shell to redirect standard input from a file by putting the file name after a < symbol, like this:

$ ./count < huge-input-file

A '>' symbol is used to redirect standard output, in case you don't want to read it as it flies by on your screen:

$ ./count < huge-input-file > huger-output-file

A useful file for both input and output is the special file /dev/null. As input, it looks like an empty file. As output, it eats any characters sent to it:

$ ./sensory-deprivation-experiment < /dev/null > /dev/null

You can also pipe programs together, connecting the output of one to the input of the next. Good programs to put at the end of a pipe are head (eats all but the first ten lines), tail (eats all but the last ten lines), more (lets you page through the output by hitting the space bar, and tee (shows you the output but also saves a copy to a file). A typical command might be something like ./spew | more or ./slow-but-boring | tee boring-output. Pipes can consist of a long train of programs, each of which processes the output of the previous one and supplies the input to the next. A typical case might be:

$ ./do-many-experiments | sort | uniq -c | sort -nr

which, if ./do-many-experiments gives the output of one experiment on each line, produces a list of distinct experimental outputs sorted by decreasing frequency. Pipes like this can often substitute for hours of real programming.

9. Editing C programs

To write your programs, you will need to use a text editor, preferably one that knows enough about C to provide tools like automatic indentation and syntax highlighting. There are three reasonable choices for this in the Zoo: kate, emacs, and vim (which can also be run as vi). Kate is a GUI-style editor that comes with the KDE window system; it plays nicely with the mouse, but Kate skills will not translate well into other environements. Emacs and Vi have been the two contenders for the One True Editor since the 1970s—if you learn one (or both) you will be able to use the resulting skills everywhere. My personal preference is to use Vi, but Emacs has the advantage of using the same editing commands as the shell and gdb command-line interfaces.

9.1. Writing C programs with Emacs

To start Emacs, type emacs at the command line. If you are actually sitting at a Zoo node it should put up a new window. If not, Emacs will take over the current window. If you have never used Emacs before, you should immediately type C-h t (this means hold down the Control key, type h, then type t without holding down the Control key). This will pop you into the Emacs built-in tutorial.

9.1.1. My favorite Emacs commands

General note: C-x means hold down Control and press x; M-x means hold down Alt (Emacs calls it "Meta") and press x. For M-x you can also hit Esc and then x.

C-h

Get help. Everything you could possibly want to know about Emacs is available through this command. Some common versions: C-h t puts up the tutorial, C-h b lists every command available in the current mode, C-h k tells you what a particular sequence of keystrokes does, and C-h l tells you what the last 50 or so characters you typed were (handy if Emacs just garbled your file and you want to know what command to avoid in the future).

C-x u

Undo. Undoes the last change you made to the current buffer. Type it again to undo more things. A lifesaver. Note that it can only undo back to the time you first loaded the file into Emacs--- if you want to be able to back out of bigger changes, use git (described below).

C-x C-s

Save. Saves changes to the current buffer out to its file on disk.

C-x C-f

Edit a different file.

C-x C-c

Quit out of Emacs. This will ask you if you want to save any buffers that have been modified. You probably want to answer yes (y) for each one, but you can answer no (n) if you changed some file inside Emacs but want to throw the changes away.

C-f

Go forward one character.

C-b

Go back one character.

C-n

Go to the next line.

C-p

Go to the previous line.

C-a

Go to the beginning of the line.

C-k

Kill the rest of the line starting with the current position. Useful Emacs idiom: C-a C-k.

C-y

"Yank." Get back what you just killed.

TAB

Re-indent the current line. In C mode this will indent the line according to Emacs's notion of how C should be indented.

M-x compile

Compile a program. This will ask you if you want to save out any unsaved buffers and then run a compile command of your choice (see the section on compiling programs below). The exciting thing about M-x compile is that if your program has errors in it, you can type C-x ` to jump to the next error, or at least where gcc thinks the next error is.

9.2. Using Vi instead of Emacs

If you don't find yourself liking Emacs very much, you might want to try Vim instead. Vim is a vastly enhanced reimplementation of the classic vi editor, which I personally find easier to use than Emacs. Type vimtutor to run the tutorial. You can always get out by hitting the Escape key a few times and then typing :qa! .

For more details, see UsingVim.

10. Compiling programs

10.1. Using gcc

A C program will typically consist of one or more files whose names end with .c. To compile foo.c, you can type gcc foo.c. Assuming foo.c contains no errors egregious enough to be detected by the extremely forgiving C compiler, this will produce a file named a.out that you can then execute by typing ./a.out.

If you want to debug your program using gdb or give it a different name, you will need to use a longer command line. Here's one that compiles foo.c to foo (run it using ./foo) and includes the information that gdb needs: gcc -g3 -o foo foo.c

By default, gcc doesn't check everything that might be wrong with your program. But if you give it a few extra arguments, it will warn you about many (but not all) potential problems: gcc -g3 -Wall -std=c99 -pedantic -o foo foo.c

10.2. Using make

For complicated programs involving multiple source files, you are probably better off using make than calling gcc directly. Make is a "rule-based expert system" that figures out how to compile programs given a little bit of information about their components.

For example, if you have a file called foo.c, try typing make foo and see what happens.

In general you will probably want to write a Makefile, which is named Makefile or makefile and tells make how to compile programs in the same directory. Here's a typical Makefile:

# Any line that starts with a sharp is a comment and is ignored
# by Make.

# These lines set variables that control make's default rules.
# We STRONGLY recommend putting "-Wall -std=c99 -pedantic" in your CFLAGS.
CC=gcc
CFLAGS=-g3 -Wall -std=c99 -pedantic

# The next line is a dependency line.
# It says that if somebody types "make all"
# make must first make "hello-world".
# By default the left-hand-side of the first dependency is what you
# get if you just type "make" with no arguments.
all: hello-world

# How do we make hello-world?
# The dependency line says you need to first make hello-world.o
# and hello-library.o
hello-world: hello-world.o hello-library.o
        # Subsequent lines starting with a TAB character give
        # commands to execute.  Note the use of the CC and CFLAGS
        # variables.
        $(CC) $(CFLAGS) -o hello-world hello-world.o hello-library.o
        echo "I just built hello-world!  Hooray!"

# We can also declare that several things depend on one thing.
# Here we are saying that hello-world.o and hello-library.o
#  should be rebuilt whenever hello-library.h changes.
# There are no commands attached to this dependency line, so
#  make will have to figure out how to do that somewhere else
#  (probably from the builtin .c -> .o rule).
hello-world.o hello-library.o: hello-library.h

# Command lines can do more than just build things.  For example,
# "make test" will rebuild hello-world (if necessary) and then run it.
test: hello-world
        ./hello-world

# This lets you type "make clean" and get rid of anything you can
# rebuild.  The -f tells rm not to complain about files that aren't
# there.
clean:
        rm -f hello-world *.o

Given a Makefile, make looks at each dependency line and asks: (a) does the target on the left hand side exist, and (b) is it older than the files it depends on. If so, it looks for a set of commands for rebuilding the target, after first rebuilding any of the files it depends on; the commands it runs will be underneath some dependency line where the target appears on the left-hand side. It has built-in rules for doing common tasks like building .o files (which contain machine code) from .c files (which contain C source code). If you have a fake target like all above, it will try to rebuild everything all depends on because there is no file named all (one hopes).

10.2.1. Make gotchas

Make really really cares that the command lines start with a TAB character. TAB looks like eight spaces in Emacs and other editors, but it isn't the same thing. If you put eight spaces in (or a space and a TAB), Make will get horribly confused and give you an incomprehensible error message about a "missing separator". This misfeature is so scary that I avoided using make for years because I didn't understand what was going on. Don't fall into that trap--- make really is good for you, especially if you ever need to recompile a huge program when only a few source files have changed.

If you use GNU Make (on a zoo node), note that beginning with version 3.78, GNU Make prints a message that hints at a possible SPACEs-vs-TAB problem, like this:

$ make
Makefile:23:*** missing separator (did you mean TAB instead of 8 spaces?).  Stop.

If you need to repair a Makefile that uses spaces, one way of converting leading spaces into TABs is to use the unexpand program:

$ mv Makefile Makefile.old
$ unexpand Makefile.old > Makefile

11. Debugging

The standard debugger on the Zoo is gdb. See C/Debugging.

12. Version control

When you are programming, you will make mistakes. If you program long enough, these will eventually include true acts of boneheadedness like accidentally deleting all of your source files. You are also likely to spend some of your time trying out things that don't work, at the end of which you'd like to go back to the last version of your program that did work. All these problems can be solved by using a version control system.

There are six respectable version control systems installed on the Zoo: rcs, cvs, svn, bzr, hg, and git. If you are familiar with any of them, you should use that. If you have to pick one from scratch, I recommend using git. For details, see UsingGit, or look at the tutorials available at http://git-scm.org.

13. Submitting assignments

The submit command is is found in /c/cs223/bin on the Zoo. Here is the documentation (adapted from comments in the script):

submit    assignment-number file(s)
unsubmit  assignment-number file(s)
check     assignment-number
makeit    assignment-number [file]
protect   assignment-number file(s)
unprotect assignment-number file(s)
retrieve  assignment-number file[s]
testit    assignment-number test

The submit program can be invoked in eight different ways:

    /c/cs223/bin/submit  1  Makefile tokenize.c unique.c time.log

submits the named source files as your solution to Homework #1;

    /c/cs223/bin/check  2

lists the files that you have submitted for Homework #2;

    /c/cs223/bin/unsubmit  3  error.submit bogus.solution

deletes the named files that you had submitted previously for Homework #3
(i.e., withdraws them from submission, which is useful if you accidentally
submit the wrong file);

    /c/cs223/bin/makeit  4  tokenize unique

runs "make" on the files that you submitted previously for Homework #4;

    /c/cs223/bin/protect  5  tokenize.c time.log

protects the named files that you submitted previously for Homework #5 (so
they cannot be deleted accidentally); and

    /c/cs223/bin/unprotect  6  unique.c time.log

unprotects the named files that you submitted previously for Homework #6
(so they can be deleted); and

     /c/cs223/bin/retrieve  7  Csquash.c

retrieves copies of the named files that you submitted previously for Homework #7

     /c/cs223/bin/testit    8  BigTest

runs the test script /c/cs223/Hwk8/test.BigTest.

The submit program will only work if there is a directory with your name and login on it under /c/cs223/class. If there is no such directory, you need to make sure that you have correctly signed up for CS223 using the web form. Note that it may take up to an hour for this directory to appear after you sign up.

CategoryProgrammingNotes

15. Machine memory

Basic model: machine memory consists of many bytes of storage, each of which has an address which is itself a sequence of bits. Though the actual memory architecture of a modern computer is complex, from the point of view of a C program we can think of as simply a large address space that the CPU can store things in (and load things from), provided it can supply an address to the memory. Because we don't want to have to type long strings of bits all the time, the C compiler lets us give names to particular regions of the address space, and will even find free space for us to use.

16. Variables

A variable is a name given in a program for some region of memory. Each variable has a type, which tells the compiler how big the region of memory corresponding to it is and how to treat the bits stored in that region when performing various kinds of operations (e.g. integer variables are added together by very different circuitry than floating-point variables, even though both represent numbers as bits). In modern programming languages, a variable also has a scope (a limit on where the name is meaningful, which allows the same name to be used for different variables in different parts of the program) and an extent (the duration of the variable's existence, controlling when the program allocates and deallocates space for it).

16.1. Variable declarations

Before you can use a variable in C, you must declare it. Variable declarations show up in three places:

• Outside a function. These declarations declare global variables that are visible throughout the program (i.e. they have global scope). Use of global variables is almost always a mistake.

• In the argument list in the header of a function. These variables are parameters to the function. They are only visible inside the function body (local scope), exist only from when the function is called to when the function returns (bounded extent—note that this is different from what happens in some garbage-collected languages like Scheme), and get their initial values from the arguments to the function when it is called.

• At the start of any block delimited by curly braces. Such variables are visible only within the block (local scope again) and exist only when the containing function is active (bounded extent). The convention in C is has generally been to declare all such local variables at the top of a function; this is different from the convention in C++ or Java, which encourage variables to be declared when they are first used. This convention may be less strong in C99 code, since C99 adopts the C++ rule of allowing variables to be declared anywhere (which can be particularly useful for index variables in for loops).

Variable declarations consist of a type name followed by one or more variable names separated by commas and terminated by a semicolon (except in argument lists, where each declaration is terminated by a comma). I personally find it easiest to declare variables one per line, to simplify documenting them. It is also possible for global and local variables (but not function arguments) to assign an initial value to a variable by putting in something like = 0 after the variable name. It is good practice to put a comment after each variable declaration that explains what the variable does (with a possible exception for conventionally-named loop variables like i or j in short functions). Below is an example of a program with some variable declarations in it:

   1 #include <stdio.h>
   2 #include <ctype.h>
   3 
   4 /* This program counts the number of digits in its input. */
   5 
   6 /*
   7  *This global variable is not used; it is here only to demonstrate
   8  * what a global variable declaration looks like.
   9  */
  10 unsigned long SpuriousGlobalVariable = 127;
  11 
  12 int
  13 main(int argc, char **argv)
  14 {
  15     int c;              /* character read */
  16     int count = 0;      /* number of digits found */
  17 
  18     while((c = getchar()) != EOF) {
  19         if(isdigit(c)) {
  20             count++;
  21         }
  22     }
  23 
  24     printf("%d\n", count);
  25 
  26     return 0;
  27 }

16.2. Variable names

The evolution of variable names in different programming languages:

11101001001001

Physical addresses represented as bits.

#FC27

Typical assembly language address represented in hexadecimal to save typing (and because it's easier for humans to distinguish #A7 from #B6 than to distinguish 10100111 from 10110110.)

A1$

A string variable in BASIC, back in the old days where BASIC variables were one uppercase letter, optionally followed by a number, optionally followed by $ for a string variable and % for an integer variable. These type tags were used because BASIC interpreters didn't have a mechanism for declaring variable types.

IFNXG7

A typical FORTRAN variable name, back in the days of 6-character all-caps variable names. The I at the start means it's an integer variable. The rest of the letters probably abbreviate some much longer description of what the variable means. The default type based on the first letter was used because FORTRAN programmers were lazy, but it could be overridden by an explicit declaration.

i, j, c, count, top_of_stack, accumulatedTimeInFlight

Typical names from modern C programs. There is no type information contained in the name; the type is specified in the declaration and remembered by the compiler elsewhere. Note that there are two different conventions for representing multi-word names: the first is to replace spaces with underscores, and the second is to capitalize the first letter of each word (possibly excluding the first letter), a style called "camel case" (CamelCase). You should pick one of these two conventions and stick to it.

prgcGradeDatabase

An example of Hungarian notation, a style of variable naming in which the type of the variable is encoded in the first few character. The type is now back in the variable name again. This is not enforced by the compiler: even though iNumberOfStudents is supposed to be an int, there is nothing to prevent you from declaring float iNumberOfStudents if you expect to have fractional students for some reason. See http://web.umr.edu/~cpp/common/hungarian.html or HungarianNotation. Not clearly an improvement on standard naming conventions, but it is popular in some programming shops.

In C, variable names are called identifiers.¹ An identifier in C must start with a lower or uppercase letter or the underscore character _. Typically variables starting with underscores are used internally by system libraries, so it's dangerous to name your own variables this way. Subsequent characters in an identifier can be letters, digits, or underscores. So for example a, ____a___a_a_11727_a, AlbertEinstein, aAaAaAaAaAAAAAa, and ______ are all legal identifiers in C, but $foo and 01 are not.

The basic principle of variable naming is that a variable name is a substitute for the programmer's memory. It is generally best to give identifiers names that are easy to read and describe what the variable is used for, i.e., that are self-documenting. None of the variable names in the preceding list are any good by this standard. Better names would be total_input_characters, dialedWrongNumber, or stepsRemaining. Non-descriptive single-character names are acceptable for certain conventional uses, such as the use of i and j for loop iteration variables, or c for an input character. Such names should only be used when the scope of the variable is small, so that it's easy to see all the places where it is used at once.

C identifiers are case-sensitive, so aardvark, AArDvARK, and AARDVARK are all different variables. Because it is hard to remember how you capitalized something before, it is important to pick a standard convention and stick to it. The traditional convention in C goes like this:

• Ordinary variables and functions are lowercased or camel-cased, e.g. count, countOfInputBits.

• User-defined types (and in some conventions global variables) are capitalized, e.g. Stack, TotalBytesAllocated.

• Constants created with #define or enum are put in all-caps: MAXIMUM_STACK_SIZE, BUFFER_LIMIT.

17. Using variables

Ignoring pointers (C/Pointers) for the moment, there are essentially two things you can do to a variable: you can assign a value to it using the = operator, as in:

   1     x = 2;      /* assign 2 to x */
   2     y = 3;      /* assign 3 to y */

or you can use its value in an expression:

   1     x = y+1;    /* assign y+1 to x */

The assignment operator is an ordinary operator, and assignment expressions can be used in larger expressions:

    x = (y=2)*3; /* sets y to 2 and x to 6 */

This feature is usually only used in certain standard idioms, since it's confusing otherwise.

There are also shorthand operators for expressions of the form variable = variable operator expression. For example, writing x += y is equivalent to writing x = x + y, x /= y is the same as x = x / y, etc.

For the special case of adding or subtracting 1, you can abbreviate still further with the ++ and -- operators. These come in two versions, depending on whether you want the result of the expression (if used in a larger expression) to be the value of the variable before or after the variable is incremented:

   1     x = 0;
   2     y = x++;    /* sets x to 1 and y to 0 (the old value) */
   3     y = ++x;    /* sets x to 2 and y to 2 (the new value) */

The intuition is that if the ++ comes before the variable, the increment happens before the value of the variable is read (a preincrement; if it comes after, it happens after the value is read (a postincrement). This is confusing enough that it is best not to use the value of preincrement or postincrement operations except in certain standard idioms. But using x++ by itself as a substitute for x = x+1 is perfectly acceptable style.

CategoryProgrammingNotes

19. Integer types

In order to declare a variable, you have to specify a type, which controls both how much space the variable takes up and how the bits stored within it are interpreted in arithmetic operators.

The standard C integer types are:²

The typical size is for 32-bit architectures like the Intel i386. Some 64-bit machines might have 64-bit ints and longs, and some prehistoric computers had 16-bit ints. Particularly bizarre architectures might have even wilder bit sizes, but you are not likely to see this unless you program vintage 1970s supercomputers. Some compilers also support a long long type that is usually twice the length of a long (e.g. 64 bits on i386 machines); this may or may not be available if you insist on following the ANSI specification strictly. The general convention is that int is the most convenient size for whatever computer you are using and should be used by default.

Whether a variable is signed or not controls how its values are interpreted. In signed integers, the first bit is the sign bit and the rest are the value in 2's complement notation; so for example a signed char with bit pattern 11111111 would be interpreted as the numerical value -1 while an unsigned char with the same bit pattern would be 255. Most integer types are signed unless otherwise specified; an n-bit integer type has a range from -2^n-1 to 2^n-1-1 (e.g. -32768 to 32767 for a short.) Unsigned variables, which can be declared by putting the keyword unsigned before the type, have a range from 0 to 2ⁿ-1 (e.g. 0 to 65535 for an unsigned short).

For chars, whether the character is signed (-128..127) or unsigned (0..255) is at the whim of the compiler. If it matters, declare your variables as signed char or unsigned char. For storing actual characters that you aren't doing arithmetic on, it shouldn't matter.

19.1. C99 fixed-width types

C99 provides a stdint.h header file that defines integer types with known size independent of the machine architecture. So in C99, you can use int8_t instead of signed char to guarantee a signed type that holds exactly 8 bits, or uint64_t instead of unsigned long long to get a 64-bit unsigned integer type. The full set of types typically defined are int8_t, int16_t, int32_t, and int64_t for signed integers and the same starting with uint for signed integers. There are also types for integers that contain the fewest number of bits greater than some minimum (e.g., int_least16_t is a signed type with at least 16 bits, chosen to minimize space) or that are the fastest type with at least the given number of bits (e.g., int_fast16_t is a signed type with at least 16 bits, chosen to minimize time).

These are all defined using typedef; the main advantage of using stdint.h over defining them yourself is that if somebody ports your code to a new architecture, stdint.h should take care of choosing the right types automatically. The disadvantage is that, like many C99 features, stdint.h is not universally available on all C compilers.

If you need to print types defined in stdint.h, the larger inttypes.h header defines macros that give the corresponding format strings for printf.

20. Integer constants

Constant integer values in C can be written in any of four different ways:

• In the usual decimal notation, e.g. 0, 1, -127, 9919291, 97.

• In octal or base 8, when the leading digit is 0, e.g. 01 for 1, 010 for 8, 0777 for 511, 0141 for 97.

• In hexadecimal or base 16, when prefixed with 0x. The letters a through f are used for the digits 10 through 15. For example, 0x61 is another way to write 97.

• Using a character constant, which is a single ASCII character or an escape sequence inside single quotes. The value is the ASCII value of the character: 'a' is 97.³ Unlike languages with separate character types, C characters are identical to integers; you can (but shouldn't) calculate 97² by writing 'a'*'a'. You can also store a character anywhere.

Except for character constants, you can insist that an integer constant is unsigned or long by putting a u or l after it. So 1ul is an unsigned long version of 1. By default integer constants are (signed) ints. For long long constants, use ll, e.g., the unsigned long long constant 0xdeadbeef01234567ull. It is also permitted to write the l as L, which can be less confusing if the l looks too much like a 1.

21. Integer operators

21.1. Arithmetic operators

The usual + (addition), - (negation or subtraction), and * (multiplication) operators work on integers pretty much the way you'd expect. The only caveat is that if the result lies outside of the range of whatever variable you are storing it in, it will be truncated instead of causing an error:

   1     unsigned char c;
   2 
   3     c = -1;             /* sets c = 255 */
   4     c = 255 + 255;      /* sets c = 254 */
   5     c = 256 * 1772717;  /* sets c = 0 */

This can be a source of subtle bugs if you aren't careful. The usual giveaway is that values you thought should be large positive integers come back as random-looking negative integers.

Division (/) of two integers also truncates: 2/3 is 0, 5/3 is 1, etc. For positive integers it will always round down.

Prior to C99, if either the numerator or denominator is negative, the behavior was unpredictable and depended on what your processor does---in practice this meant you should never use / if one or both arguments might be negative. The C99 standard specified that integer division always removes the fractional part, effectively rounding toward 0; so (-3)/2 is -1, 3/-2 is -1, and (-3)/-2 is 1.

There is also a remainder operator % with e.g. 2%3 = 2, 5%3 = 2, 27 % 2 = 1, etc. The sign of the modulus is ignored, so 2%-3 is also 2. The sign of the dividend carries over to the remainder: (-3)%2 and (-3)%(-2) are both 1. The reason for this rule is that it guarantees that y == x*(y/x) + y%x is always true.

21.2. Bitwise operators

In addition to the arithmetic operators, integer types support bitwise logical operators that apply some Boolean operation to all the bits of their arguments in parallel. What this means is that the i-th bit of the output is equal to some operation applied to the i-th bit(s) of the input(s). The bitwise logical operators are ~ (bitwise negation: used with one argument as in ~0 for the all-1's binary value), & (bitwise AND), '|' (bitwise OR), and '^' (bitwise XOR, i.e. sum mod 2). These are mostly used for manipulating individual bits or small groups of bits inside larger words, as in the expression x & 0x0f, which strips off the bottom four bits stored in x.

Examples:

The shift operators << and >> shift the bit sequence left or right: x << y produces the value x⋅2^y (ignoring overflow); this is equivalent to shifting every bit in x y positions to the left and filling in y zeros for the missing positions. In the other direction, x >> y produces the value ⌊x⋅2^-y⌋, by shifting x y positions to the right. The behavior of the right shift operator depends on whether x is unsigned or signed; for unsigned values, it shifts in zeros from the left end always; for signed values, it shifts in additional copies of the leftmost bit (the sign bit). This makes x >> y have the same sign as x if x is signed.

If y is negative, it reverses the direction of the shift; so x << -2 is equivalent to x >> 2.

Examples (unsigned char x):

Examples (signed char x):

Shift operators are often used with bitwise logical operators to set or extract individual bits in an integer value. The trick is that (1 << i) contains a 1 in the i-th least significant bit and zeros everywhere else. So x & (1<<i) is nonzero if and only if x has a 1 in the i-th place. This can be used to print out an integer in binary format (which standard printf won't do):

   1 void
   2 print_binary(unsigned int n)
   3 {
   4     unsigned int mask = 0;
   5 
   6     /* this grotesque hack creates a bit pattern 1000... */
   7     /* regardless of the size of an unsigned int */
   8     mask = ~mask ^ (~mask >> 1);
   9 
  10     for(; mask != 0; mask >>= 1) {
  11         putchar((n & mask) ? '1' : '0');
  12     }
  13 }

(See test_print_binary.c for a program that uses this.)

In the other direction, we can set the i-th bit of x to 1 by doing x | (1 << i) or to 0 by doing x & ~(1 << i). See C/BitExtraction for applications of this to build arbitrarily-large bit vectors.

21.3. Logical operators

To add to the confusion, there are also three logical operators that work on the truth-values of integers, where 0 is defined to be false and anything else is defined by be true. These are && (logical AND), ||, (logical OR), and ! (logical NOT). The result of any of these operators is always 0 or 1 (so !!x, for example, is 0 if x is 0 and 1 if x is anything else). The && and || operators evaluate their arguments left-to-right and ignore the second argument if the first determines the answer (this is the only place in C where argument evaluation order is specified); so

   1     0 && execute_programmer();
   2     1 || execute_programmer();

is in a very weak sense perfectly safe code to run.

Watch out for confusing & with &&. The expression 1 & 2 evaluates to 0, but 1 && 2 evaluates to 1. The statement 0 & execute_programmer(); is also unlikely to do what you want.

Yet another logical operator is the ternary operator ?:, where x ? y : z equals the value of y if x is nonzero and z if x is zero. Like && and ||, it only evaluates the arguments it needs to:

   1     fileExists(badFile) ? deleteFile(badFile) : createFile(badFile);

Most uses of ?: are better done using an if-then-else statement (C/Statements).

21.4. Relational operators

Logical operators usually operate on the results of relational operators or comparisons: these are '==' (equality), '!=' (inequality), '<=' (less than or equal to) and '>=' (greater than or equal to). So, for example,

    if(size >= MIN_SIZE && size <= MAX_SIZE) {
        puts("just right");
    }

tests if size is in the (inclusive) range [MIN_SIZE..MAX_SIZE].

Beware of confusing == with =. The code

   1     /* DANGER! DANGER! DANGER! */
   2     if(x = 5) {
   3         ...

is perfectly legal C, and will set x to 5 rather than testing if it's equal to 5. Because 5 happens to be nonzero, the body of the if statement will always be executed. This error is so common and so dangerous that gcc will warn you about any tests that look like this if you use the -Wall option.

22. Input and output

To input or output integer values, you will need to convert them from or to strings. Converting from a string is easy using the atoi or atol functions declared in stdlib.h; these take a string as an argument and return an int or long, respectively.⁴

Output is usually done using printf (or sprintf if you want to write to a string without producing output). Use the %d format specifier for ints, shorts, and chars that you want the numeric value of, %ld for longs, and %lld for long longs.

A contrived program that uses all of these features is given below:

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 
   4 /* This program can be used to how atoi etc. handle overflow. */
   5 /* For example, try "overflow 1000000000000". */
   6 int
   7 main(int argc, char **argv)
   8 {
   9     char c;
  10     int i;
  11     long l;
  12     long long ll;
  13     
  14     if(argc != 2) {
  15         fprintf(stderr, "Usage: %s n\n", argv[0]);
  16         return 1;
  17     }
  18     
  19     c = atoi(argv[1]);
  20     i = atoi(argv[1]);
  21     l = atol(argv[1]);
  22     ll = atoll(argv[1]);
  23 
  24     printf("char: %d  int: %d  long: %ld  long long: %lld", c, i, l, ll);
  25 
  26     return 0;
  27 }

23. Alignment

Modern CPU architectures typically enforce alignment restrictions on multi-byte values, which mean that the address of an int or long typically has to be a multiple of 4. This is an effect of the memory being organized as groups of 32 bits that are written in parallel instead of 8 bits. Such restrictions are not obvious when working with integer-valued variables directly, but will come up when we talk about pointers in C/Pointers.

CategoryProgrammingNotes

Input and output from C programs is typically done through the standard I/O library, whose functions etc. are declared in stdio.h. A detailed descriptions of the functions in this library is given in Appendix B of KernighanRitchie. We'll talk about some of the more useful functions and about how input-output (I/O) works on Unix-like operating systems in general.

25. Character streams

The standard I/O library works on character streams, objects that act like long sequences of incoming or outgoing characters. What a stream is connected to is often not apparent to a program that uses it; an output stream might go to a terminal, to a file, or even to another program (appearing there as an input stream).

Three standard streams are available to all programs: these are stdin (standard input), stdout (standard output), and stderr (standard error). Standard I/O functions that do not take a stream as an argument will generally either read from stdin or write to stdout. The stderr stream is used for error messages. It is kept separate from stdout so that you can see these messages even if you redirect output to a file:

$ ls no-such-file > /tmp/dummy-output
ls: no-such-file: No such file or directory

26. Reading and writing single characters

To read a single character from stdin, use getchar:

   1     int c;
   2 
   3     c = getchar();

The getchar routine will return the special value EOF (usually -1; short for end of file) if there are no more characters to read, which can happen when you hit the end of a file or when the user types the end-of-file key control-D to the terminal. Note that the return value of getchar is declared to be an int since EOF lies outside the normal character range.

To write a single character to stdout, use putchar:

   1     putchar('!');

Even though putchar can only write single bytes, it takes an int as an argument. Any value outside the range 0..255 will be truncated to its last byte, as in the usual conversion from int to unsigned char.

Both getchar and putchar are wrappers for more general routines getc and putc that allow you to specify which stream you are using. To illustrate getc and putc, here's how we might define getchar and putchar if they didn't exist already:

   1 int
   2 getchar2(void)
   3 {
   4     return getc(stdin);
   5 }
   6 
   7 int
   8 putchar2(int c)
   9 {
  10     return putc(c, stdout);
  11 }

Note that putc, putchar2 as defined above, and the original putchar all return an int rather than void; this is so that they can signal whether the write succeeded. If the write succeeded, putchar or putc will return the value written. If the write failed (say because the disk was full), then putc or putchar will return EOF.

Here's another example of using putc to make a new function putcerr that writes a character to stderr:

   1 int
   2 putcerr(int c)
   3 {
   4     return putc(c, stderr);
   5 }

A rather odd feature of the C standard I/O library is that if you don't like the character you just got, you can put it back using the ungetc function. The limitations on ungetc are that (a) you can only push one character back, and (b) that character can't be EOF. The ungetc function is provided because it makes certain high-level input tasks easier; for example, if you want to parse a number written as a sequence of digits, you need to be able to read characters until you hit the first non-digit. But if the non-digit is going to be used elsewhere in your program, you don't want to eat it. The solution is to put it back using ungetc.

Here's a function that uses ungetc to peek at the next character on stdin without consuming it:

   1 /* return the next character from stdin without consuming it */
   2 int
   3 peekchar(void)
   4 {
   5     int c;
   6 
   7     c = getchar();
   8     if(c != EOF) ungetc(c, stdin);      /* puts it back */
   9     
  10     return c;
  11 }

27. Formatted I/O

Reading and writing data one character at a time can be painful. The C standard I/O library provides several convenient routines for reading and writing formatted data. The most commonly used one is printf, which takes as arguments a format string followed by zero or more values that are filled in to the format string according to patterns appearing in it.

Here are some typical printf statements:

   1     printf("Hello\n");          /* print "Hello" followed by a newline */
   2     printf("%c", c);            /* equivalent to putchar(c) */
   3     printf("%d", n);            /* print n (an int) formatted in decimal */
   4     printf("%u", n);            /* print n (an unsigned int) formatted in decimal */
   5     printf("%o", n);            /* print n (an unsigned int) formatted in octal */
   6     printf("%x", n);            /* print n (an unsigned int) formatted in hexadecimal */
   7     printf("%f", x);            /* print x (a float or double) */
   8 
   9     /* print total (an int) and average (a double) on two lines with labels */
  10     printf("Total: %d\nAverage: %f\n", total, average);

For a full list of formatting codes see Table B-1 in KernighanRitchie, or run man 3 printf.

The inverse of printf is scanf. The scanf function reads formatted data from stdin according to the format string passed as its first argument and stuffs the results into variables whose addresses are given by the later arguments. This requires prefixing each such argument with the & operator, which takes the address of a variable.

Format strings for scanf are close enough to format strings for printf that you can usually copy them over directly. However, because scanf arguments don't go through argument promotion (where all small integer types are converted to int and floats are converted to double), you have to be much more careful about specifying the type of the argument correctly.

   1     scanf("%c", &c);            /* like c = getchar(); c must be a char */
   2     scanf("%d", &n);            /* read an int formatted in decimal */
   3     scanf("%u", &n);            /* read an unsigned int formatted in decimal */
   4     scanf("%o", &n);            /* read an unsigned int formatted in octal */
   5     scanf("%x", &n);            /* read an unsigned int formatted in hexadecimal */
   6     scanf("%f", &x);            /* read a float */
   7     scanf("%lf", &x);           /* read a double */
   8 
   9     /* read total (an int) and average (a float) on two lines with labels */
  10     /* (will also work if input is missing newlines or uses other whitespace, see below) */
  11     scanf("Total: %d\nAverage: %f\n", &total, &average);

The scanf routine eats whitespace (spaces, tabs, newlines, etc.) in its input whenever it sees a conversion specification or a whitespace character in its format string. Non-whitespace characters that are not part of conversion specifications must match exactly. To detect if scanf parsed everything successfully, look at its return value; it returns the number of values it filled in, or EOF if it hits end-of-file before filling in any values.

The printf and scanf routines are wrappers for fprintf and fscanf, which take a stream as their first argument, e.g.:

   1     fprintf(stderr, "BUILDING ON FIRE, %d%% BURNT!!!\n", percentage);

Note the use of "%%" to print a single percent in the output.

28. Rolling your own I/O routines

Since we can write our own functions in C, if we don't like what the standard routines do, we can build our own on top of them. For example, here's a function that reads in integer values without leading minus signs and returns the result. It uses the peekchar routine we defined above, as well as the isdigit routine declared in ctype.h.

   1 /* read an integer written in decimal notation from stdin until the first
   2  * non-digit and return it.  Returns 0 if there are no digits. */
   3 int
   4 readNumber(void)
   5 {
   6     int accumulator;    /* the number so far */
   7     int c;              /* next character */
   8 
   9     accumulator = 0;
  10 
  11     while((c = peekchar()) != EOF && isdigit(c)) {
  12         c = getchar();                  /* consume it */
  13         accumulator *= 10;              /* shift previous digits over */
  14         accumulator += (c - '0');       /* add decimal value of new digit */
  15     }
  16 
  17     return accumulator;
  18 }

Here's another implementation that does almost the same thing:

   1 int
   2 readNumber2(void)
   3 {
   4     int n;
   5 
   6     if(scanf("%u", &n) == 1) {
   7         return n;
   8     } else {
   9         return 0;
  10     }
  11 }

The difference is that readNumber2 will consume any whitespace before the first digit, which may or may not be what we want.

More complex routines can be used to parse more complex input. For example, here's a routine that uses readNumber to parse simple arithmetic expressions, where each expression is either a number or of the form (expression+expression) or (expression*expression). The return value is the value of the expression after adding together or multiplying all of its subexpressions. (A complete program including this routine and the others defined earlier that it uses can be found in calc.c.)

   1 #define EXPRESSION_ERROR (-1)
   2 
   3 /* read an expression from stdin and return its value */
   4 /* returns EXPRESSION_ERROR on error */
   5 int
   6 readExpression(void)
   7 {
   8     int e1;             /* value of first sub-expression */
   9     int e2;             /* value of second sub-expression */
  10     int c;
  11     int op;             /* operation: '+' or '*' */
  12 
  13     c = peekchar();
  14 
  15     if(c == '(') {
  16         c = getchar();
  17 
  18         e1 = readExpression();
  19         op = getchar();
  20         e2 = readExpression();
  21 
  22         c = getchar();  /* this had better be ')' */
  23         if(c != ')') return EXPRESSION_ERROR;
  24 
  25         /* else */
  26         switch(op) {
  27         case '*':
  28             return e1*e2;
  29             break;
  30         case '+':
  31             return e1+e2;
  32             break;
  33         default:
  34             return EXPRESSION_ERROR;
  35             break;
  36         }
  37     } else if(isdigit(c)) {
  38         return readNumber();
  39     } else {
  40         return EXPRESSION_ERROR;
  41     }
  42 }

Because this routine calls itself recursively as it works its way down through the input, it is an example of a recursive descent parser. Parsers for more complicated languages (e.g. C) are usually not written by hand like this, but are instead constructed mechanically using a Parser generator.

29. File I/O

Reading and writing files is done by creating new streams attached to the files. The function that does this is fopen. It takes two arguments: a filename, and a flag that controls whether the file is opened for reading or writing. The return value of fopen has type FILE * and can be used in putc, getc, fprintf, etc. just like stdin, stdout, or stderr. When you are done using a stream, you should close it using fclose.

Here's a program that reads a list of numbers from a file whose name is given as argv[1] and prints their sum:

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 
   4 int
   5 main(int argc, char **argv)
   6 {
   7     FILE *f;
   8     int x;
   9     int sum;
  10 
  11     if(argc < 2) {
  12         fprintf(stderr, "Usage: %s filename\n", argv[0]);
  13         exit(1);
  14     }
  15 
  16     f = fopen(argv[1], "r");
  17     if(f == 0) {
  18         /* perror is a standard C library routine */
  19         /* that prints a message about the last failed library routine */
  20         /* prepended by its argument */
  21         perror(filename);
  22         exit(2);
  23     }
  24 
  25     /* else everything is ok */
  26     sum = 0;
  27     while(fscanf("%d", &x) == 1) {
  28         sum += x;
  29     }
  30 
  31     printf("%d\n", sum);
  32 
  33     /* not strictly necessary but it's polite */
  34     fclose(f);
  35 
  36     return 0;
  37 }

To write to a file, open it with fopen(filename, "w"). Note that as soon as you call fopen with the "w" flag, any previous contents of the file are erased. If you want to append to the end of an existing file, use "a" instead. You can also add + onto the flag if you want to read and write the same file (this will probably involve using fseek).

Some operating systems (Windows) make a distinction between text and binary files. For text files, use the same arguments as above. For binary files, add a b, e.g. fopen(filename, "wb") to write a binary file.

   1 /* leave a greeting in the current directory */
   2 
   3 #include <stdio.h>
   4 #include <stdlib.h>
   5 
   6 #define FILENAME "hello.txt"
   7 #define MESSAGE "hello world"
   8 
   9 int
  10 main(int argc, char **argv)
  11 {
  12     FILE *f;
  13 
  14     f = fopen(FILENAME, "w");
  15     if(f == 0) {
  16         perror(FILENAME);
  17         exit(1);
  18     }
  19 
  20     /* unlike puts, fputs doesn't add a newline */
  21     fputs(MESSAGE, f);
  22     putc('\n', f);
  23 
  24     fclose(f);
  25 
  26     return 0;
  27 }

CategoryProgrammingNotes

The bodies of C functions (including the main function) are made up of statements. These can either be simple statements that do not contain other statements, or compound statements that have other statements inside them. Control structures are compound statements like if/then/else, while, for, and do..while that control how or whether their component statements are executed.

31. Simple statements

The simplest kind of statement in C is an expression (followed by a semicolon, the terminator for all simple statements). Its value is computed and discarded. Examples:

   1     x = 2;              /* an assignment statement */
   2     x = 2+3;            /* another assignment statement */
   3     2+3;                /* has no effect---will be discarded by smart compilers */
   4     puts("hi");         /* a statement containing a function call */
   5     root2 = sqrt(2);    /* an assignment statement with a function call */

Most statements in a typical C program are simple statements of this form.

Other examples of simple statements are the jump statements return, break, continue, and goto. A return statement specifies the return value for a function (if there is one), and when executed it causes the function to exit immediately. The break and continue statements jump immediately to the end of a loop (or switch; see below) or the next iteration of a loop; we'll talk about these more when we talk about loops. The goto statement jumps to another location in the same function, and exists for the rare occasions when it is needed. Using it in most circumstances is a sin.

32. Compound statements

Compound statements come in two varieties: conditionals and loops.

32.1. Conditionals

These are compound statements that test some condition and execute one or another block depending on the outcome of the condition. The simplest is the if statement:

   1     if(houseIsOnFire) {
   2         /* ouch! */
   3         scream();
   4         runAway();
   5     }

The body of the if statement is executed only if the expression in parentheses at the top evaluates to true (which in C means any value that is not 0).

The braces are not strictly required, and are used only to group one or more statements into a single statement. If there is only one statement in the body, the braces can be omitted:

   1     if(programmerIsLazy) omitBraces();

This style is recommended only for very simple bodies. Omitting the braces makes it harder to add more statements later without errors.

   1     if(underAttack)
   2         launchCounterAttack();   /* executed only when attacked */
   3         hideInBunker();          /* ### DO NOT INDENT LIKE THIS ### executed always */

In the example above, the lack of braces means that the hideInBunker() statement is not part of the if statement, despite the misleading indentation. This sort of thing is why I generally always put in braces in an if.

An if statement may have an else clause, whose body is executed if the test is false (i.e. equal to 0).

   1     if(happy) {
   2         smile();
   3     } else {
   4         frown();
   5     }

A common idiom is to have a chain of if and else if branches that test several conditions:

   1     if(temperature < 0) {
   2         puts("brrr");
   3     } else if(temperature < 100) {
   4         puts("hooray");
   5     } else {
   6         puts("ouch!");
   7     }

This can be inefficient if there are a lot of cases, since the tests are applied sequentially. For tests of the form <expression> == <small constant>, the switch statement may provide a faster alternative. Here's a typical switch statement:

   1     /* print plural of cow, maybe using the obsolete dual number construction */
   2     switch(numberOfCows) {
   3     case 1:
   4         puts("cow");
   5         break;
   6     case 2:
   7         puts("cowen");
   8         break;
   9     default:
  10         puts("cows");
  11         break;
  12     }

This prints the string "cow" if there is one cow, "cowen" if there are two cowen, and "cows" if there are any other number of cows. The switch statement evaluates its argument and jumps to the matching case label, or to the default label if none of the cases match. Cases must be constant integer values.

The break statements inside the block jump to the end of the block. Without them, executing the switch with numberOfCows equal to 1 would print all three lines. This can be useful in some circumstances where the same code should be used for more than one case:

   1     switch(c) {
   2     case 'a':
   3     case 'e':
   4     case 'i':
   5     case 'o':
   6     case 'u':
   7         type = VOWEL;
   8         break;
   9     default:
  10         type = CONSONANT;
  11         break;
  12     }

or when a case "falls through" to the next:

   1     switch(countdownStart) {
   2     case 3:
   3         puts("3");
   4     case 2:
   5         puts("2");
   6     case 1:
   7         puts("1")
   8     case 0:
   9         puts("KABLOOIE!");
  10         break;
  11     default:
  12         puts("I can't count that high!");
  13         break;
  14     }

Note that it is customary to include a break on the last case even though it has no effect; this avoids problems later if a new case is added. It is also customary to include a default case even if the other cases supposedly exhaust all the possible values, as a check against bad or unanticipated inputs.

   1     switch(oliveSize) {
   2     case JUMBO():
   3         eatOlives(SLOWLY);
   4         break;
   5     case COLLOSSAL:
   6         eatOlives(QUICKLY);
   7         break;
   8     case SUPER_COLLOSSAL:
   9         eatOlives(ABSURDLY);
  10         break;
  11     default:
  12         /* unknown size! */
  13         abort();
  14         break;
  15     }

Though switch statements are better than deeply nested if/else-if constructions, it is often even better to organize the different cases as data rather than code. We'll see examples of this when we talk about function pointers C/FunctionPointers.

Nothing in the C standards prevents the case labels from being buried inside other compound statements. One rather hideous application of this fact is Duff's device.

32.2. Loops

There are three kinds of loops in C.

32.2.1. The while loop

A while loop tests if a condition is true, and if so, executes its body. It then tests the condition is true again, and keeps executing the body as long as it is. Here's a program that deletes every occurence of the letter e from its input.

   1 #include <stdio.h>
   2 
   3 int
   4 main(int argc, char **argv)
   5 {
   6     int c;
   7 
   8     while((c = getchar()) != EOF) {
   9         switch(c) {
  10         case 'e':
  11         case 'E':
  12             break;
  13         default:
  14             putchar(c);
  15             break;
  16         }
  17     }
  18 
  19     return 0;
  20 }

Note that the expression inside the while argument both assigns the return value of getchar to c and tests to see if it is equal to EOF (which is returned when no more input characters are available). This is a very common idiom in C programs. Note also that even though c holds a single character, it is declared as an int. The reason is that EOF (a constant defined in stdio.h) is outside the normal character range, and if you assign it to a variable of type char it will be quietly truncated into something else. Because C doesn't provide any sort of exception mechanism for signalling unusual outcomes of function calls, designers of library functions often have to resort to extending the output of a function to include an extra value or two to signal failure; we'll see this a lot when the null pointer shows up in C/Pointers.

32.2.2. The do..while loop

The do..while statement is like the while statement except the test is done at the end of the loop instead of the beginning. This means that the body of the loop is always executed at least once.

Here's a loop that does a random walk until it gets back to 0 (if ever). If we changed the do..while loop to a while loop, it would never take the first step, because pos starts at 0.

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <time.h>
   4 
   5 int
   6 main(int argc, char **argv)
   7 {
   8     int pos = 0;       /* position of random walk */
   9 
  10     srandom(time(0));  /* initialize random number generator */
  11 
  12     do {
  13         pos += random() & 0x1 ? +1 : -1;
  14         printf("%d\n", pos);
  15     } while(pos != 0);
  16 
  17     return 0;
  18 }

The do..while loop is used much less often in practice than the while loop. Note that it is always possible to convert a do..while loop to a while loop by making an extra copy of the body in front of the loop.

32.2.3. The for loop

The for loop is a form of SyntacticSugar that is used when a loop iterates over a sequence of values stored in some variable (or variables). Its argument consists of three expressions: the first initializes the variable and is called once when the statement is first reached. The second is the test to see if the body of the loop should be executed; it has the same function as the test in a while loop. The third sets the variable to its next value. Some examples:

   1     /* count from 0 to 9 */
   2     for(i = 0; i < 10; i++) {
   3         printf("%d\n", i);
   4     }
   5     
   6     /* and back from 10 to 0 */
   7     for(i = 10; i >= 0; i--) {
   8         printf("%d\n", i);
   9     }
  10 
  11     /* this loop uses some functions to move around */
  12     for(c = firstCustomer(); c != END_OF_CUSTOMERS; c = customerAfter(c)) {
  13         helpCustomer(c);
  14     }
  15 
  16     /* this loop prints powers of 2 that are less than n*/
  17     for(i = 1; i < n; i *= 2) {
  18         printf("%d\n", i);
  19     }
  20 
  21     /* this loop does the same thing with two variables by using the comma operator */
  22     for(i = 0, power = 1; power < n; i++, power *= 2) {
  23         printf("2^%d = %d\n", i, power);
  24     }
  25 
  26     /* Here are some nested loops that print a times table */
  27     for(i = 0; i < n; i++) {
  28         for(j = 0; j < n; j++) {
  29             printf("%d*%d=%d ", i, j, i*j);
  30         }
  31         putchar('\n');
  32     }

A for loop can always be rewritten as a while loop.

   1     for(i = 0; i < 10; i++) {
   2         printf("%d\n", i);
   3     }
   4 
   5     /* is exactly the same as */
   6 
   7     i = 0;
   8     while(i < 10) {
   9         printf("%d\n", i);
  10         i++;
  11     }

32.2.4. Loops with break, continue, and goto

The break statement immediately exits the innermmost enclosing loop or switch statement.

   1     for(i = 0; i < n; i++) {
   2         openDoorNumber(i);
   3         if(boobyTrapped()) {
   4             break;
   5         }
   6     }

The continue statement skips to the next iteration. Here is a program with a loop that iterates through all the integers from -10 through 10, skipping 0:

   1 #include <stdio.h>
   2 
   3 /* print a table of inverses */
   4 #define MAXN (10)
   5 
   6 int
   7 main(int argc, char **argv)
   8 {
   9     int n;
  10 
  11     for(n = -MAXN; n <= MAXN; n++) {
  12         if(n == 0) continue;
  13         printf("1.0/%3d = %+f\n", n, 1.0/n);
  14     }
  15 
  16     return 0;
  17 }

Occasionally, one would like to break out of more than one nested loop. The way to do this is with a goto statement.

   1     for(i = 0; i < n; i++) {
   2         for(j = 0; j < n; j++) {
   3             doSomethingTimeConsumingWith(i, j);
   4             if(checkWatch() == OUT_OF_TIME) {
   5                 goto giveUp;
   6             }
   7         }
   8     }
   9 giveUp:
  10     puts("done");

The target for the goto is a label, which is just an identifier followed by a colon and a statement (the empty statement ; is ok).

The goto statement can be used to jump anywhere within the same function body, but breaking out of nested loops is widely considered to be its only genuinely acceptable use in normal code.

32.3. Choosing where to put a loop exit

Choosing where to put a loop exit is usually pretty obvious: you want it after any code that you want to execute at least once, and before any code that you want to execute only if the termination test fails.

If you know in advance what values you are going to be iterating over, you will most likely be using a for loop:

   1 for(i = 0; i < n; i++) {
   2     a[i] = 0;
   3 }

Most of the rest of the time, you will want a while loop:

   1 while(!done()) {
   2     doSomething();
   3 }

The do..while loop comes up mostly when you want to try something, then try again if it failed:

   1 do {
   2     result = fetchWebPage(url);
   3 } while(result == 0);

Finally, leaving a loop in the middle using break can be handy if you have something extra to do before trying again:

   1 for(;;) {
   2     result = fetchWebPage(url);
   3     if(result != 0) {
   4         break;
   5     }
   6     /* else */
   7     fprintf(stderr, "fetchWebPage failed with error code %03d\n", result);
   8     sleep(retryDelay);  /* wait before trying again */
   9 }

(Note the empty for loop header means to loop forever; while(1) also works.)

CategoryProgrammingNotes

Real numbers are represented in C by the floating point types float, double, and long double. Just as the integer types can't represent all integers because they fit in a bounded number of bytes, so also the floating-point types can't represent all real numbers. The difference is that the integer types can represent values within their range exactly, while floating-point types almost always give only an approximation to the correct value, albeit across a much larger range. The three floating point types differ in how much space they use (32, 64, or 80 bits on x86 CPUs; possibly different amounts on other machines), and thus how much precision they provide. Most math library routines expect and return doubles (e.g., sin is declared as double sin(double), but there are usually float versions as well (float sinf(float)).

34. Floating point basics

The core idea of floating-point representations (as opposed to fixed point representations as used by, say, ints), is that a number x is written as m*b^e where m is a mantissa or fractional part, b is a base, and e is an exponent. On modern computers the base is almost always 2, and for most floating-point representations the mantissa will be scaled to be between 1 and b. This is done by adjusting the exponent, e.g.

etc.

The mantissa is usually represented in base b, as a binary fraction. So (in a very low-precision format), 1 would be 1.000*2⁰, 2 would be 1.000*2¹, and 0.375 would be 1.100*2^-2, where the first 1 after the decimal point counts as 1/2, the second as 1/4, etc. Note that for a properly-scaled (or normalized) floating-point number in base 2 the digit before the decimal point is always 1. For this reason it is usually dropped (although this requires a special representation for 0).

Negative values are typically handled by adding a sign bit that is 0 for positive numbers and 1 for negative numbers.

35. Floating-point constants

Any number that has a decimal point in it will be interpreted by the compiler as a floating-point number. Note that you have to put at least one digit after the decimal point: 2.0, 3.75, -12.6112. You can specific a floating point number in scientific notation using e for the exponent: 6.022e23.

36. Operators

Floating-point types in C support most of the same arithmetic and relational operators as integer types; x > y, x / y, x + y all make sense when x and y are floats. If you mix two different floating-point types together, the less-precise one will be extended to match the precision of the more-precise one; this also works if you mix integer and floating point types as in 2 / 3.0. Unlike integer division, floating-point division does not discard the fractional part (although it may produce round-off error: 2.0/3.0 gives 0.66666666666666663, which is not quite exact). Be careful about accidentally using integer division when you mean to use floating-point division: 2/3 is 0. Casts can be used to force floating-point division (see below).

Some operators that work on integers will not work on floating-point types. These are % (use modf from the math library if you really need to get a floating-point remainder) and all of the bitwise operators ~, <<, >>, &, ^, and |.

37. Conversion to and from integer types

Mixed uses of floating-point and integer types will convert the integers to floating-point.

You can convert floating-point numbers to and from integer types explicitly using casts. A typical use might be:

   1 /* return the average of a list */
   2 double
   3 average(int n, int a[])
   4 {
   5     int sum = 0;
   6     int i;
   7 
   8     for(i = 0; i < n; i++) {
   9         sum += a[i];
  10     }
  11 
  12     return (double) sum / n;
  13 }

If we didn't put in the (double) to convert sum to a double, we'd end up doing integer division, which would truncate the fractional part of our average.

In the other direction, we can write:

   1    i = (int) f;

to convert a float f to int i. This conversion loses information by throwing away the fractional part of f: if f was 3.2, i will end up being just 3.

38. The IEEE-754 floating-point standard

The IEEE-754 floating-point standard is a standard for representing and manipulating floating-point quantities that is followed by all modern computer systems. It defines several standard representations of floating-point numbers, all of which have the following basic pattern (the specific layout here is for 32-bit floats):

bit  31 30    23 22                    0
     S  EEEEEEEE MMMMMMMMMMMMMMMMMMMMMMM

The bit numbers are counting from the least-significant bit. The first bit is the sign (0 for positive, 1 for negative). The following 8 bits are the exponent in excess-127 binary notation; this means that the binary pattern 01111111 = 127 represents an exponent of 0, 1000000 = 128, represents 1, 01111110 = 126 represents -1, and so forth. The mantissa fits in the remaining 24 bits, with its leading 1 stripped off as described above.

Certain numbers have a special representation. Because 0 cannot be represented in the standard form (there is no 1 before the decimal point), it is given the special representation 0 00000000 00000000000000000000000. (There is also a -0 = 1 00000000 00000000000000000000000, which looks equal to +0 but prints differently.) Numbers with exponents of 11111111 = 255 = 2¹²⁸ represent non-numeric quantities such as "not a number" (NaN), returned by operations like (0.0/0.0) and positive or negative infinity. A table of some typical floating-point numbers (generated by the program float.c) is given below:

         0 =                        0 = 0 00000000 00000000000000000000000
        -0 =                       -0 = 1 00000000 00000000000000000000000
     0.125 =                    0.125 = 0 01111100 00000000000000000000000
      0.25 =                     0.25 = 0 01111101 00000000000000000000000
       0.5 =                      0.5 = 0 01111110 00000000000000000000000
         1 =                        1 = 0 01111111 00000000000000000000000
         2 =                        2 = 0 10000000 00000000000000000000000
         4 =                        4 = 0 10000001 00000000000000000000000
         8 =                        8 = 0 10000010 00000000000000000000000
     0.375 =                    0.375 = 0 01111101 10000000000000000000000
      0.75 =                     0.75 = 0 01111110 10000000000000000000000
       1.5 =                      1.5 = 0 01111111 10000000000000000000000
         3 =                        3 = 0 10000000 10000000000000000000000
         6 =                        6 = 0 10000001 10000000000000000000000
       0.1 =      0.10000000149011612 = 0 01111011 10011001100110011001101
       0.2 =      0.20000000298023224 = 0 01111100 10011001100110011001101
       0.4 =      0.40000000596046448 = 0 01111101 10011001100110011001101
       0.8 =      0.80000001192092896 = 0 01111110 10011001100110011001101
     1e+12 =             999999995904 = 0 10100110 11010001101010010100101
     1e+24 =   1.0000000138484279e+24 = 0 11001110 10100111100001000011100
     1e+36 =   9.9999996169031625e+35 = 0 11110110 10000001001011111001110
       inf =                      inf = 0 11111111 00000000000000000000000
      -inf =                     -inf = 1 11111111 00000000000000000000000
       nan =                      nan = 0 11111111 10000000000000000000000

What this means in practice is that a 32-bit floating-point value (e.g. a float) can represent any number between 1.17549435e-38 and 3.40282347e+38, where the e separates the (base 10) exponent. Operations that would create a smaller value will underflow to 0 (slowly—IEEE 754 allows "denormalized" floating point numbers with reduced precision for very small values) and operations that would create a larger value will produce inf or -inf instead.

For a 64-bit double, the size of both the exponent and mantissa are larger; this gives a range from 1.7976931348623157e+308 to 2.2250738585072014e-308, with similar behavior on underflow and overflow.

Intel processors internally use an even larger 80-bit floating-point format for all operations. Unless you declare your variables as long double, this should not be visible to you from C except that some operations that might otherwise produce overflow errors will not do so, provided all the variables involved sit in registers (typically the case only for local variables and function parameters).

39. Error

In general, floating-point numbers are not exact: they are likely to contain round-off error because of the truncation of the mantissa to a fixed number of bits. This is particularly noticeable for large values (e.g. 1e+12 in the table above), but can also be seen in fractions with values that aren't powers of 2 in the denominator (e.g. 0.1). Round-off error is often invisible with the default float output formats, since they produce fewer digits than are stored internally, but can accumulate over time, particularly if you subtract floating-point quantities with values that are close (this wipes out the mantissa without wiping out the error, making the error much larger relative to the number that remains).

The easiest way to avoid accumulating error is to use high-precision floating-point numbers (this means using double instead of float). On modern CPUs there is little or no time penalty for doing so, although storing doubles instead of floats will take twice as much space in memory.

Note that a consequence of the internal structure of IEEE 754 floating-point numbers is that small integers and fractions with small numerators and power-of-2 denominators can be represented exactly—indeed, the IEEE 754 standard carefully defines floating-point operations so that arithmetic on such exact integers will give the same answers as integer arithmetic would (except, of course, for division that produces a remainder). This fact can sometimes be exploited to get higher precision on integer values than is available from the standard integer types; for example, a double can represent any integer between -2⁵³ and 2⁵³ exactly, which is a much wider range than the values from 2^-31^ to 2^31^-1 that fit in a 32-bit int or long.  (A 64-bit long long does better.)  So double` should be considered for applications where large precise integers are needed (such as calculating the net worth in pennies of a billionaire.)

One consequence of round-off error is that it is very difficult to test floating-point numbers for equality, unless you are sure you have an exact value as described above. It is generally not the case, for example, that (0.1+0.1+0.1) == 0.3 in C. This can produce odd results if you try writing something like for(f = 0.0; f <= 0.3; f += 0.1): it will be hard to predict in advance whether the loop body will be executed with f = 0.3 or not. (Even more hilarity ensues if you write for(f = 0.0; f != 0.3; f += 0.1), which after not quite hitting 0.3 exactly keeps looping for much longer than I am willing to wait to see it stop, but which I suspect will eventually converge to some constant value of f large enough that adding 0.1 to it has no effect.) Most of the time when you are tempted to test floats for equality, you are better off testing if one lies within a small distance from the other, e.g. by testing fabs(x-y) <= fabs(EPSILON * y), where EPSILON is usually some application-dependent tolerance. This isn't quite the same as equality (for example, it isn't transitive), but it usually closer to what you want.

40. Reading and writing floating-point numbers

Any numeric constant in a C program that contains a decimal point is treated as a double by default. You can also use e or E to add a base-10 exponent (see the table for some examples of this.) If you want to insist that a constant value is a float for some reason, you can append F on the end, as in 1.0F.

For I/O, floating-point values are most easily read and written using scanf (and its relatives fscanf and sscanf) and printf. For printf, there is an elaborate variety of floating-point format codes; the easiest way to find out what these do is experiment with them. For scanf, pretty much the only two codes you need are "%lf", which reads a double value into a double *, and "%f", which reads a float value into a float *. Both these formats are exactly the same in printf, since a float is promoted to a double before being passed as an argument to printf (or any other function that doesn't declare the type of its arguments). But you have to be careful with the arguments to scanf or you will get odd results as only 4 bytes of your 8-byte double are filled in, or—even worse—8 bytes of your 4-byte float are.

41. Non-finite numbers in C

The values nan, inf, and -inf can't be written in this form as floating-point constants in a C program, but printf will generate them and scanf seems to recognize them. With some machines and compilers you may be able to use the macros INFINITY and NAN from <math.h> to generate infinite quantities. The macros isinf and isnan can be used to detect such quantities if they occur.

42. The math library

(See also KernighanRitchie Appendix B4.)

Many mathematical functions on floating-point values are not linked into C programs by default, but can be obtained by linking in the math library. Examples would be the trigonometric functions sin, cos, and tan (plus more exotic ones), sqrt for taking square roots, pow for exponentiation, log and exp for base-e logs and exponents, and fmod for when you really want to write x%y but one or both variables is a double. The standard math library functions all take doubles as arguments and return double values; most implementations also provide some extra functions with similar names (e.g., sinf) that use floats instead, for applications where space or speed is more important than accuracy.

There are two parts to using the math library. The first is to include the line

   1 #include <math.h>
   2 

somewhere at the top of your source file. This tells the preprocessor to paste in the declarations of the math library functions found in /usr/include/math.h.

The second step is to link to the math library when you compile. This is done by passing the flag -lm to gcc after your C program source file(s). A typical command might be:

gcc -o program program.c -lm

If you don't do this, you will get errors from the compiler about missing functions. The reason is that the math library is not linked in by default, since for many system programs it's not needed.

CategoryProgrammingNotes

A function, procedure, or subroutine encapsulates some complex computation as a single operation. Typically, when we call a function, we pass as arguments all the information this function needs, and any effect it has will be reflected in either its return value or (in some cases) in changes to values pointed to by the arguments. Inside the function, the arguments are copied into local variables, which can be used just like any other local variable---they can even be assigned to without affecting the original argument.

44. Function definitions

A typical function definition looks like this: