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A C programming tutorial for users of the GNU operating system.
Copyright © 2002, 2003, 2008 Ciaran O'Riordan
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the licence is included in the section entitled “GNU Free Documentation License”.
Welcome to Learning GNU C. The aim of this book is to teach GNU users how to write software in C. It is written primarily as a tutorial for beginners but it should be thorough enough to be useful as a reference by intermediate programmers. The basics are layed down in full in the first few chapters, beginners will read these chapters carefully while those with prior experience can skim through them. All the information is there, no prior knowledge of programming is assumed. Because the goal of this book is to make you a software developer, we'll then move on to some very practical usages, the likes of which normally aren't taught until much later in other books.
It is assumed that you have access to a computer with a GNU system installed. Users of FreeBSD, OpenBSD, or NetBSD should also find this manual useful. Some familiarity with using your computer from the command line shell would be helpful but all neccessary commands will be shown along side programming examples. The only piece of software you do need experience with is a text editor. Any text editor will do. GNU Emacs is an especially good one for programmers. It has been in development for over twenty years and contains thousands of useful features. GNU Nano is a simple text editor you could use, some programmers like to use ViM or vi (pronounced vee eye). If you already have a favourite text editor, you can use that. There are also graphical text editors geared towards programmers such as Anjuta and KDevelop but most programmers prefer text based editors.
Most GNU systems use Linux as a kernel. These systems are often known as GNU/Linux systems.
The contents of this book can be divided into two topics: the core C language, and the standard functionality made available to the programmer. The standard functionality I mention is provided by GNU Libc, this is a standard library of C functionality that is part of every GNU system. Neither of these topics is of much use without the other but there is a focus on the core language near the beginning and more discussion on Libc near the end. The ordering of topics is designed to teach C programming in an incremental fashion where each chapter builds on the previous one. Some aspects of the core language are only really of use to experienced programmers and so appear near the end.
The C language on it's own can make decisions, repeat commands, store data, and perform mathematics. Equally importantly, it provides a method to make use of extra functionality such as Libc.
Libc provides functionality such as reading and writing files, sorting and searching data, getting input from the user, displaying data to the user, communicating across networks, creating easily translatable programs, and many other things.
C is a standard, it is the programmers programming language. The majority of of software that makes up GNU and BSD systems and the applications that run on them, is written in C.
C was developed over thirty years ago for writing operating systems and applications. It's small, extensible design has allowed it to evolve with the computer industry. Because of it's age and popularity, C is a very well supported language. Many tools exist to make C programming easier and these tools are often very mature and of a high standard. All the software we will use in this book is written in C.
Software is Free Software when the user has the freedom to: Use the software for any purpose, make changes to the software, share the software with others, and distribute modified versions of the software. Today, copyright law allows a software developer to restrict what a user can do with a piece of software. In 1984, Richard Stallman started the Free Software movement to reject this behaviour.
Free Software empowers it's users by allowing them to help themselves and each other by making changes they want to the software or getting someone else to make these changes. Software that is not Free Software is often called proprietary software. Proprietary software says that cooperating is wrong. Producers of proprietary software use copyright law to make it a criminal offense to help others by sharing software.
The goal of proprietary software is profit maximisation for one entity. In contrast, the goal of Free Software is to be as useful as possible to as many people as possible. The GNU project was started to reject proprietary software, thus freeing computer users from having to choose between being a criminal and betraying their community.
Finally there are the technical benefits. Free Software is free from marketing plots. It doesn't restrict itself to force users to buy extra pieces of software. Each piece of GNU is designed to be as useful as possible. As a programmer you can use the same C programming software that is used in major projects.
Proprietary software is generally distributed in a machine readable form only. This means that the user cannot see what what a program is really doing. Free Software is required to come with source code in a human readable format. As a programmer you can read the source code for any piece os Free Software you like. If there are errors in a program, you can fix them.
The peer review, and collaboration of thousands of programmers around the world has created a lot of high quality Free Software. The GNU/Linux operating system is one of it's greatest achievments.
Free Software will change the world for the better.
GNU is a complete, Unix-like operating system that has been in development for just over twenty years. GNU software is known for it's stability, standard compliance, and the freedom it gives to computer users.
A programming language is a way to give a computer a sequence of instructions to do something. Programming languages can be sorted into three three categories: machine languages, Assembly languages, and High-Level languages. Of these types only machine languages can be understood directly by a computer.
A machine language is the set of instructions that a computer's central processing unit (CPU) understands. All instructions and information are represented by numbers; very fast for computers, very hard for human brains to read or write. To ease the task of computer programming, people designed easier languages called assembly languages. An assembly language is one which provides textual names for the available machine language commands. This, along with the fact that assembly languages allowed programmers to add spaces and tabs to their code, made assembly languages far easier to program with. Assembly code can then be fed to an assembler which translates it into the machine language of the target computer's CPU.
The use of assembly languages spread very fast, they became known as second generation languages but there were still two problems with assembly languages. Firstly, each command does only a very basic task such as add two numbers or load a value from memory. Using these small commands was quite tedious. The second problem was much bigger. Programs written in an assembly language are bound to a particular type of CPU. Each type of CPU has it's own machine language and, therefore, it's own assembly language.
Finally, machine independant languages were developed, known as third generation or high-level languages. Designed to be easy to read, these languages were made up of English words, basic mathematical symbols, and a few punctuation characters. These languages allow simple statements to be expressioned concisely, for example, adding two numbers and storing the result in memory could be expressed as:
data = 10 + 200;
Instead of:
Load R1, 10
Load R2, 200
Addi R1, R2
Store R2, L1
C is one such high-level language. A tool called a compiler is used to convert C into machine code. A program can be written in C and compiled for any computer, it's up to the compiler to get the hardware-specific instructions right.
To see just how readable C is compared to Assembly language, take a look at the following tiny program written in each:
.section .rodata
.LC0:
.string "Tax Due: %d\n"
.text
.align 2
.globl main
.type main,@function
main:
pushl %ebp
movl %esp, %ebp
subl $24, %esp
andl $-16, %esp
movl $0, %eax
subl %eax, %esp
movl $1000, %eax
movl $400, %edx
movl $0x3e6147ae, -12(%ebp)
subl %edx, %eax
pushl %eax
fildl (%esp)
leal 4(%esp), %esp
fmuls -12(%ebp)
fnstcw -18(%ebp)
movw -18(%ebp), %ax
movb $12, %ah
movw %ax, -20(%ebp)
fldcw -20(%ebp)
fistpl -16(%ebp)
fldcw -18(%ebp)
subl $8, %esp
pushl -16(%ebp)
pushl $.LC0
call printf
addl $16, %esp
movl $1, %eax
leave
ret
.Lfe1:
.size main,.Lfe1-main
And the same program in C:
#include <stdio.h>
int
main()
{
int wages = 1000;
int tax_allowance = 400;
float tax_rate = 0.22;
int tax_due;
tax_due = (wages - tax_allowance) * tax_rate;
printf("Tax Due: %d euro\n", tax_due);
return 0;
}
The output of both programs is the same: Tax Due: 131 euro. The readability winner is obvious. What's more, the Assembly code shown is written in the 80386 instruction set, it will not work on machines that use a different instruction set. The C code can be compiled for almost any computer.
GNU comes with a compiler called GCC. Originally this stood for GNU C Compiler but since it can now compile languages other than C it's name was changed to GNU Compiler Collection. To check if you have GCC installed, type “gcc –version” at a command line. If you get a message like:
ciaran@pooh:~/book$ gcc --version
bash: gcc: command not found
ciaran@pooh:~/book$
Then you don't have GCC installed. If you do, you'll see a version number and maybe some copyright and warranty information, like this:
ciaran@pooh:~/book$ gcc --version
gcc (GCC) 4.2.3 (Debian 4.2.3-1)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
ciaran@pooh:~/book$
The version of GCC you have installed may be different, anything similar, such as 2.95.2 or 3.3.0, is ok. If you got an error message saying command not found then you don't have GCC installed. If you installed GNU from a CD, you should find GCC there. If you don't know how to install applications from a CD then get a friend or the person who installed your GNU system to do it for you.
Now we're going to show you a tiny bit of C code and how to compile it. The point here is to show you how to use GCC so we won't explain the C code yet. Here's the smallest C program that GCC will compile. It does nothing.
main()
{
}
Type this piece of code into your text editor and save it to a file called tiny.c. You can choose any name so long as it ends with .c, this is the extension used by C programs, GCC checks for this extension when compiling a program. With the file saved, you can now compile it into an executable program by typing:
ciaran@pooh:~/book$ gcc tiny.c
ciaran@pooh:~/book$
This command should succeed with no output. If you got any error messages check that you typed the program in correctly. Weighing in at eight characters we'll assume you've gotten this much correct and move on. A file called a.out should have appeared in your directory. This is the machine language program created from the above code, if you run it you will see that it really does nothing at all. The name a.out exists for historical reasons, it is short for assembler output.
Although GCC will compile this code, it isn't strictly complete. If we enable GCC's warnings we will be told what is missing. You are not expected to understand the warning messages right now, we show this only to demonstrate GCC's warnings. You enable warnings by adding the -Wall switch to your compilation command.
ciaran@pooh:~/book$ gcc -Wall tiny.c
tiny.c:2: warning: return-type defaults to `int'
tiny.c: In function `main':
tiny.c:3: warning: control reaches end of non-void function
ciaran@pooh:~/book$
These warnings appear because our program is not strictly complete. To get rid of these warnings we must add two more lines. So here's the smallest valid C program.
int
main()
{
return 0;
}
When we compile this with the -Wall option we will get no warnings. Another option: -o filename can be used to specify the name you want to give to your program (instead of a.out).
ciaran@pooh:~/book$ gcc -Wall -o tiny-program tiny2.c
ciaran@pooh:~/book$ ls
tiny2.c tiny-program
ciaran@pooh:~/book$ ./tiny-program
ciaran@pooh:~/book$
The “return” command ends the program's execution because it tell the program to return to whoever called it. Functions always return with a value. Unless otherwise defined, a return value of zero means there were no errors encountered. You can check this at the command line with the command “echo $?”. If you change “return 0” to “return 7”, and then recompile and run the program, then an “echo $?” command will diplay 7. Not very exiting, but that's proof that your first program did something.
This concludes our introduction. Hopefully you now have a grasp of what programming is. In the next chapter we'll start writing basic programs that actually do something and explain how they do it.
If you're interested in more background and compiler details, you might like to read the appendixes at the end of this book. Otherwise, let's go straight to writing your first program.
All C programs are broken down into blocks of code called functions. Say you wanted to write a simple program to add three numbers supplied by the user; you would need a function to get a value from the user, a function to perform the math, and a function to display the answer. C programs are mostly made up of two things: functions and data. Functions are blocks of code that complete a task while data is information stored in the program. are pieces of memory that store information. This information can be changed at any time within the program hence the name variable. The purpose of programs can usually be broken down into three actions: accept some data as input, manipulate that data, output it. A simple example would be the command ls you type into your shell. ls is a program, written in C, that gathers directory information from your operating system, converts it to a human readable format, and outputs it to your terminal.
The first thing we will look at is outputing data. Data is a very broad term, but in C is all comes down to numbers. Rather than get bogged down in the theory we'll jump into a quick program that displays a message to your screen.
The first piece of useful code we will look at is a classic. When compiled and run it will display a simple greeting to your screen. This program defines a function called main() and calls (uses) a function called printf(). printf() is a function provided for us by the Standard Device Input/Output library. This library comes with every GNU system. Here's our little program:
#include <stdio.h>
int
main()
{
printf("hello, world\n");
return 0;
}
Compile and run this program before moving on. If all goes well, it will display the text string “hello, world” to your terminal (the standard output device). Here's the compilation command just in case you've forgotten:
ciaran@pooh:~/book$ gcc -Wall -o hello hello.c
ciaran@pooh:~/book$ ./hello
hello, world
ciaran@pooh:~/book$
If you got any error or warning messages check that your code matches the code in this book exactly. Any messages you got should tell you the line of code where your mistake is. If you've typed the code in correctly you will get no such messages.
In this example we placed the text string "hello, world" in the parenthesis following the printf(). A printing function wouldn't be much good if you couldn't tell it what to print. Many standard functions require you to pass arguments to them that will tell them how you want their job done.
We'll do a quick description of what each line does. Don't worry if your not sure about some parts, we'll do plenty more examples.
#include <stdio.h>
This line tells GCC to include information about how to use the functions from the Standard Device Input/Output library. Usually the standard input device is your keyboard and the standard output device a terminal (which is displayed on your monitor). This library is very widely used, we'll come across a lot of functions from it in this book.
int
main()
These two lines begin the definition of the function
main. We'll explain the first of these two lines
later.
{
The open curly braces signals the beginning of a block of code. All
code between this curly brace and it's matching closing brace is part
of the function main.
printf("hello, world\n");
This line is a function call, the function is already defined for you.
return 0;
The return statement ends execution of the function main, any
statements after this line would not be executed. When main() ends,
your program exits. When a function ends, it can pass a value back to
whoever called it, this is done by placing the value after return.
main always returns an integer (a positive or negative
number with no decimal point). We tell the compiler to expect this by
preceding the definition of main with int. When returning from
main() it is convention to return zero if no problems were
encountered.
}
The closing curly brace signals the end of the block of code that
makes up main.
The two lines that make up the body of main are known as
statements. More specifically they are simple statements
(as opposed to compound statements which we will encounter in chapter
4). Statements are to C what sentences are to spoken languages. A
semi-colon ends a simple statement. The blank lines in the program
are optional, C never requires a blank line but they make code much
easier to read.
We mentioned that our function main returns the value
zero. For most functions the return value can be used within the
program but since returning from main ends the program, it
returns the value to the shell. Your shell stores this value, if
you'd like to see it just type:
ciaran@pooh:~/book$ gcc -Wall -o hello hello.c
ciaran@pooh:~/book$ ./hello
hello, world
ciaran@pooh:~/book$ echo $?
0
ciaran@pooh:~/book$
To verify this you can change the value in the program from zero to ninety-nine, recompile and re-run the program. You'll see 99 on your screen.
Comments are a way to add explanatory text to your program. Just like
blank lines, they are ignored by the compiler so they don't affect
your program in any way. As the programs you write get larger you
will find it helpful to have comments in your code to remind you what
you were thinking. In the examples in this book we will use comments
to explain what is going on. There are two ways to insert a comment
into your program, the most common way is to start and end your
comments with /* and */ respectively. Comments of this
sort can span multiple lines. The second way is by placing //
at the start of your comment. Comments of this sort are terminated at
the end of a line. Here's our hello, world program with comments.
/* The purpose of this program is to
* display some text to the screen
* and then exit.
*/
#include <stdio.h>
int
main()
{
/* printf displays a string */
printf("hello, world\n");
return 0; //zero indicates there were no errors
}
When compiled, this code will produce exactly the same executable. Lines 2 and 3 of the comment at the top start with an asterisk, this is not necessary but it makes it clear that the comment extends for four lines.
variables to store data. C programs can change the data stored
in a variable at any time, hence the name. Every variable has an
identifier which you can use to refer to it's data when you want to
use or change it's value. An expression is anything that can be
evaluated i.e. 1 + 1 is an expression of the value 2.
In this expression, the plus sign is a binary operator; it
operates on two values to create a single value.
The rules for naming a variable are the same as for naming a function, you can use letters, numbers, and the underscore character and the first character must not be a number. Also like functions, variables must be declared before they can be used. The identifier you give to a variable should say what the the variable will be used for, this makes you code much easier to read. You can define your own variables or you can use one of the types already defined for you. Before we get bogged down in terminology let's look at a quick code example to show how simple it all is. In this example we will use two variables of the pre-defined type int.
#include <stdio.h>
int
main()
{
int number_of_bicycles;
int number_of_wheels;
number_of_bicycles = 6;
number_of_wheels = number_of_bicycles * 2;
printf("I have %d bicycles\n", number_of_bicycles);
printf("So I have %d wheels\n", number_of_wheels);
return 0;
}
There are a few new things to look at here, we'll break the program into chunks to explain them.
int number_of_bicycles;
int number_of_wheels;
These two lines each declare a variable. int is one of the built-in data types of the C language. Variables of type int can store positive or negative whole numbers.
number_of_bicycles = 6;
This line stores the value 6 in the variable
number_of_bicycles. The equals sign is known as the assignment
operator, it assigns the value on the right hand side of it to the
variable on the left hand side.
number_of_wheels = number_of_bicycles * 2;
Again, this line uses the assignment operator but it also uses the
multiplication operator. The asterisk is another binary operator, it
multiplies two values to create a single value. In this case it
creates the value 12 which is then stored in number_of_wheels.
printf("I have %d bicycles\n", number_of_bicycles);
printf("So I have %d wheels\n", number_of_wheels);
Here we see printf again but it's being used unlike we have
seen before. Here it is taking two arguments which are
separated by a comma. The first argument to printf is known as
the format string. When a %d is encountered in the format
string printf knows to expect an extra argument. The %d is
replaced by the value of this extra argument. One addition argument
is expected for each %d encountered.
With this new knowledge it should be no surprise that when we compile and run this piece of code we get the following:
I have 6 bicycles
So I have 12 wheels
As always, don't worry if you are unsure about certain parts. We'll do plenty more examples.
All the data types defined by C are made up of units of memory called bytes. On most computer architectures a byte is made up of eight bits, each bit stores a one or a zero. These eight bits with two states give 256 combinations (28). So an integer which takes up two bytes can store a number between 0 and 65535 (0 and 216. Usually however, integer variables use the first bit to store whether the number is positive or negative so their value will be between -32768 and +32767.
As we mentioned, there are eight basic data types defined in the C language. Five types for storing integers of varying sizes and three types for storing floating point values (values with a decimal point). C doesn't provide a basic data type for text. Text is made up of individual characters and characters are represented by numbers. In the last example we used one of the integer types: int. This is the most commonly used type in the C language.
The majority of data used in computer programs is made up of the integer types, we'll discuss the floating point types a little later. In order of size, starting with the smallest, the integer types are char, short, int, long and long long. The smaller types have the advantage of taking up less memory, the larger types incur a performance penalty. Variables of type int store the largest possible integer which does not incur this performance penalty. For this reason, int variables can be different depending what type of computer you are using.
The char data type is usually one byte, it is so called because they are commonly used to store single characters. The size of the other types is dependent on the hardware of your computer. Most desktop machines are 32-bit, this refers to the size of data that they are designed for processing. On 32-bit machines the int data type takes up 4 bytes (232). The short is usually smaller, the long can be larger or the same size as an int and finally the long long is for handling very large numbers.
The type of variable you use generally doesn't have a big impact on the speed or memory usage of your application. Unless you have a special need you can just use int variables. We will try to point out the few cases where it can be important in this book. A decade ago, most machines had 16-bit processors, this limited the size of int variables to 2 bytes. At the time, short variables were usually also 2 bytes and long would be 4 bytes. Nowadays, with 32-bit machines, the default type (int) is usually large enough to satisfy what used to require a variable of type long. The long long type was introduced more recently to handle very large numeric values.
Some computers are better at handling really big numbers so the size
of the data types will be bigger on these machines. To find out the
size of each data type on your machine compile and run this piece of
code. It uses one new language construct sizeof. This tells
you how many bytes a data type takes up.
int
main()
{
printf("sizeof(char) == %d\n", sizeof(char));
printf("sizeof(short) == %d\n", sizeof(short));
printf("sizeof(int) == %d\n", sizeof(int));
printf("sizeof(long) == %d\n", sizeof(long));
printf("sizeof(long long) == %d\n", sizeof(long long));
return 0;
}
Time for another example. This bit of code demonstrates a few more new things which we'll explain in a minute.
#include <stdio.h>
int
main ()
{
short first_number = -5;
long second_number, third_number;
second_number = 20000 + 10000;
printf ("the value of first_number is %hd\n", first_number);
printf ("the value of second_number is %ld\n", second_number);
printf ("the value of third_number is %ld\n", third_number);
return 0;
}
We've used a short and two long variables. We could have used int
variables but chose to use other types to show how similar they are.
In the first line of main we declare a variable and give it a
value all in one line. This is pretty normal. The second line
declares two variables at once by separating them with a comma. This
can be handily but code is often more readable when variable
declarations get a line to themselves.
The third line is very like some code from the first example, the
addition operator produces the value 30000 which gets stored in
second_number. The last thing to point out is that instead of %d, the
format string of printf contains %hd for the short variable and
%ld for the long variables. These little groupings of characters are
called conversion specifiers. Each type of variable has it's
own conversion specifier. If you want to print a single percent sign
(%) you must write %%.
When you compile and run this you will see the value of your variables. The value of third_number will be strange. This is because it was never assigned a value. When you declare a variable, the operating system allocates some memory for it. You have no way of know what this memory was used for previously. Until you give your variable a value, the data stored in it is essentially random. Forgetting to assign a value to a variable is a common mistake among beginning programmers.
You may have noticed two characters near the end of our printf
statements \n. These don't get displayed to the screen, they are the
notation printf uses to represent newline. '\' is the c
escape character when it is encountered within quotes the
following character usually has a special meaning. Another example is
\t which is used to represent a TAB.
Another special character that printf looks out for is '%',
this tells it to look at the next few characters and be ready to
replace them with the value of a variable. %d is the character
sequence that represents a variable of type int to be displayed using
the decimal counting system (0 .. 9). For every %d in the format
string you must tell printf what variable you want it replaced
with. Here's some more use of printf in code:
int
main()
{
int one = 1;
int two = 2;
int three = 4; /* the values are unimportant here */
printf ("one ==\t%d\ntwo ==\t%d\nthree ==\t%d\n", one, two, three);
return 0;
}
We mentioned at the start of this chapter that point of a program usually involves performing actions on data. By using standard mathematical symbols, arithmetic in C is easily readable.
int
main()
{
int hours_per_day;
int days_per_week;
hours_per_day = 8;
days_per_week = 5;
printf ("I work %d hours a week.\n", (days_per_week * hours_per_day) );
printf ("%d %d hour days\n", days_per_week, hours_per_day);
return 0;
}
These have there place but are often used to fix badly written code. If two functions need to operate on a variable you should use pointers to share this variable rather than make it available to every function.
text.
A good rule to follow is to never use numbers other than 1 and 0 in
your code. If you require another numeric constant you should make it
a const variable; this way it gets a nice meaningful name. The number
40 has little meaning, however, the identifier
HOURS_WORKED_PER_WEEK tells us something about what a function
is doing. Another benefit is that you can change the value of a const
variable in one place rather than having to change all occurrences of
40. Using the latter method it is easy to make a mistake by changing
an unrelated occurrence of 40 or forgetting to change an occurrence.
C programs are generally broken down into tasks. To calculate the bill at a resteraunt you need some code to ask for the costs of each dish, calculate the total, and then display the answer. Each of these tasks is taken care of by a function.
In the last chapter we wrote a do-nothing program, this program
consisted on one function, main. When writing real software
your programs will be made up of tens or hundreds of functions. When
you write a program you will write many functions to perform the tasks
you need. There are, however, a lot of common tasks such as
displaying text to the screen that a lot of programmers will need.
Instead of having everyone reinventing the wheel, GNU systems come
with libraries of pre-defined functions for many of these tasks.
Over the years, thousands of such functions have accumulated. If you
were writing a program that plays the game, BINGO, you would have to
write the game specific functions yourself but you would find that
others have already written functions for generating random numbers,
displaying results to the screen, getting input from the player etc.
Every C program must have a function called main, this is where
execution of the program begins. The code of a program could
be completely contained in main but it is more usual to split a
program into many small functions.
In that last example we defined just one function. To add another
function you must generally do two things. First you must
define the function, just like we defined main. Also
you you must declare it. Declaring a function is like telling
GCC to expect it, we didn't have to declare main because it is
a special function and GCC knows to expect it. The name, or
identifier, you give to a function must appear in both the definition
and the declaration.
Functions identifiers can be made up of the alphabetic characters a-z
and A-Z, the numeric characters 0-9 and the underscore character _.
These can be used in any order so long as the first character of the
identifier is not a number. As we said earlier, C is case-sensitive
so My_Function is completely different to my_function.
A functions identifier must be unique. Identifiers can safely be up
to 63 characters long or as short as 1 character.
Along with it's identifier you must give each function a type
and a block of code. The type tells the compiler what sort of
data it returns. The return value of a function can be
ignored, printf returns an integer saying how many character it
displayed to the terminal. This information wasn't important to us so
we ignored it in our program. In the next chapter we'll discuss types
of data in detail, until then we'll gloss over return values.
Here's a program that defines three functions:
#include <stdio.h>
/* function declarations */
int first_function (void);
int goodbye (void);
int
main () // function definition
{
printf ("the program begins...\n");
first_function ();
goodbye ();
return 0;
}
int
first_function () // function definition
{
/* this function does nothing */
return 0;
}
int
goodbye () // function definition
{
printf ("...and the program ends.\n");
return 0;
}
In the above example we wrote first_function which does nothing
and goodbye which displays a message. Functions must be
declared before they can be called are called, in our case this
means they must appear our definition of main. In practice,
function declarations are generally grouped at the top of a file after
any #include lines and before any function definitions.
Programs do not have to be written in just one file, your
code can split up into as many files as you want, if a program is
comprised of forty functions you could put each
function into a separate file. This is a bit extreme though. Often
functions are grouped by topic and put into separate files. Say you
were writing a program that worked out the price of a pizza and
displayed the result, you could put the calculation functions into one
file, the display functions into another and have
main in a third one. The command you would use
to compile your program would look something like this:
ciaran@pooh:~/book$ gcc -o pizza_program main.c prices.c display.c
Remember: If you define a function in prices.c and you want to call this function in main.c you must declare the function in main.c.
Keeping track of function declarations can get messy, for this reason
Header files are used to house C code that you wish to appear in
multiple files. You have actually already used a header file.
stdio.h is a header file which contains many function
declarations, it contains the function declarations for printf
and printf. Once you have placed the function declarations you
wish to share into a header file you can #include your header
in each C file that needs the information. The only difference being
that you surround your filename in quotes instead of angle brackets
("my_header.h" instead of <system_header.h>). To
illustrate these points we'll write that pizza program I mentioned
earlier.
The small amount of programming we have shown so far isn't enough to make a decent interactive program. To keep it simple, we will write just a skeleton program so you can see the structure and usage of header files without getting bogged down in new concepts. In Chapter 3 we will write a full version of this program. The code here can be compiled and run but it will not ask the user for any input or calculate the price.
First we have main.c, this will only contain the function
main. main will call some of the functions we define in
other files. Note that main.c doesn't have a line
#include <stdio.h> as it does not use any of the
functions in the Standard Device I/O library.
#include "display.h"
#include "prices.h"
int
main()
{
display_options ();
calculate_price ();
display_price ();
return 0;
}
Next we have display.c. This contains two functions, both of
which are called from main and so we put there declarations in
a header file display.h.
#include <stdio.h>
int
display_options ()
{
printf ("Welcome to the pizza parlor\n");
printf ("What size pizza would you like? (in inches)");
return 0;
}
int
display_price ()
{
printf ("Your pizza will cost 0.00\n");
return 0;
}
/* header file just contains function declarations, an file that wants
* to use either of these functions just has to #include this file */
int display_options (void);
int display_price (void);
Finally we have prices.c which contains the functions for
getting input from the user and calculating the total cost of the
pizza. Only one of these functions is called from main, the
declarations for the other two are therefore put at the top of the
file. We'll fill in the code for these functions in Chapter 3.
int get_size (void);
int get_toppings (void);
int
calculate_price ()
{
/* insert code here. Will call get_size() and get_toppings(). */
return 0;
}
int
get_size ()
{
/* insert code here */
return 0;
}
int get_toppings ()
{
/* insert code here */
return 0;
}
int calculate_price (void);
This can then be compiled with the command: gcc -Wall -o
pizza_program main.c prices.c display.c. When run, it will display a
greeting and announce that your pizza costs E0.00.
Before we move on, let's take a look at one more function from the
Standard Device I/O Library: printf. The Print Formatted
command is an advanced form of printf. The string you pass to
printf can contain character sequences which have special
meanings. Unlike printf, there is no automatic new-line at the
end of a string displayed by printf to insert a new-line you
add the characters \n.
What we've covered so far shouldn't be too hard. If you'd like to experiment, try writing similar programs that output a few lines. Split your program into a couple of functions and divide them into two files.
Always enable GCC's warnings when compiling your program. Warnings mean your code is unclear or incomplete, GCC will guess at the correct meaning and will usually get it right but you should not rely on this. Looking at and correcting the warnings will help you get used to the language. Most warnings are accompanied by the line number where the problem is. If you can't see anything wrong with that line check the line above it; if a statement is incomplete GCC won't notice that it is an error until it encounters the beginning of the following statement. Don't forget your semi-colons.
Taking actions based on decisions
C provides two sytles of decision making: branching and looping. Branching is deciding what actions to take and looping is deciding how many times to take a certain action.
Branching is so called because the program chooses to follow one branch or another. The if statement is the most simple of the branching statements. It takes an expression in parenthesis and an statement or block of statements (surrounded by curly braces). if the expression is true (evaluates to non-zero) then the statement or block of statements gets executed. Otherwise these statements are skipped. if statements take the following form:
if (expression)
statement;
or
if (expression)
{
statement1;
statement2;
statement3;
}
Here's a quick code example:
#include <stdio.h>
int
main()
{
int cows = 6;
if (cows > 1)
printf("We have cows\n");
if (cows > 10)
printf("loads of them!\n");
return 0;
}
When compiled and run this program will display:
ciaran@pooh:~/book$ gcc -Wall -Werror -o cows using_if.c
ciaran@pooh:~/book$ ./cows
We have cows
ciaran@pooh:~/book$
The second printf statement does not get
executed because it's expression is false (evaluates to zero).
A second form of if statement exists which allows you to also specify a block of code to execute if the test expression is false. This is known as an if ... else statement and is formed by placing the reserved word else and another block of code after the usual if statment. You program will execute one of the two blocks of code based on the test condition after the if. Here's what it looks like:
int
main()
{
int cows = 0;
if (cows > 1)
{
printf("We have cows\n");
printf("%d cows to be precise\n", cows);
}
else
{
if (cows == 0)
printf("We have no cows at all\n");
else
printf("We have only one cow\n");
}
if (cows > 10)
printf("Maybe too many cows.\n");
return 0;
}
You should be able to guess the output by now:
ciaran@pooh:~/book$ ./cows2
We have no cows at all
ciaran@pooh:~/book$
In the last example there was an if .. else statement inside
another if .. else statement. This is perfectly legal in C and
is quite common. There is another form of branching you can use but
it's a little more complex so we'll leave it to to end of the chapter.
Loops provide a way to repeat commands and control how many times they
are repeated. Say you wanted to print the alphabet to the screen, you
could do this with a call to printf. This is one solution but
it doesn't scale very well, what if you wanted to print all the
numbers between one and one thousand in a column? this could be
handled by one big printf or loads of printf calls but
repetitive work should be done by the computer, leaving you more time
to work on the interesting parts of your program.
The most basic loop in C is the while loop. A while statement is like a repeating if statement. Like an If statement, if the test condition is true: the statments get executed. The difference is that after the statements have been executed, the test condition is checked again. If it is still true the statements get executed again. This cycle repeats until the test condition evaluates to false. If the test condition is false the first time, the statments don't get executed at all. On the other hand if it's test condition never evaluates to false it may continue looping infinitely. To control the number of times a loop executes it's code you usually have at least one variable in the test condition that gets altered in the subsequent block of code. This allows the test condition to become false at some point.
Here's the quick example you are probably expecting. It's a simple guessing game, very simple for the person who is writing the code as they know the answer. When testing this program remember to guess the wrong answer a few times.
#include <stdio.h>
int
main()
{
const int MAGIC_NUMBER = 6;
int guessed_number;
printf("Try to guess what number I'm thinking of\n");
printf("HINT: It's a number between 1 and 10\n");
printf ("enter your guess: ");
scanf ("%d", &guessed_number);
while (guessed_number != MAGIC_NUMBER)
{
printf ("enter your guess: ");
scanf ("%d", &guessed_number);
}
printf ("you win.\n");
return 0;
}
The block of code following the while statement will be executed repeatedly until the player guesses the number six.
for is similar to while, it's just written differently. for
statements are often used to proccess lists such a range of numbers:
#include <stdio.h>
int
main()
{
int i;
/* display the numbers from 0 to 9 */
for (i = 0; i < 10; i++)
printf ("%d\n", i);
return 0;
}
do .. while is just like a while loop except that the test condition is checked at the end of the loop rather than the start. This has the effect that the content of the loop are always executed at least once.
#include <stdio.h>
int
main()
{
const int MAGIC_NUMBER = 6;
int guessed_number;
printf ("Try to guess what number I'm thinking of\n");
printf ("HINT: It's a number between 1 and 10\n");
do
{
printf ("enter your guess: ");
scanf ("%d", &guessed_number);
}
while (guessed_number != MAGIC_NUMBER);
printf ("you win.\n");
return 0;
}
The switch statement is much like a nested if .. else
statement. Its mostly a matter of preference which you use, switch
statement can be slightly more efficient and easier to read.
The ?: operator is just like an if .. else statement
except that because it is an operator you can use it within
expressions.
Blah, blah, here's an example:
#include <stdio.h>
int
main()
{
int apples = 6;
printf ("I have %d apple%s\n", apples, (apples == 1) ? "" : "s");
return 0;
}
?: is a ternary operator in that it takes three values,
this is the only ternary operator C has.
You've seen break already, we ended each case of our switch
statement with one. break exits out of a loop.
continue is similar to break in that it short circuits the
execution of a code block but continue brings execution back to
the start of a loop.
sorting the programmers from the students
A limitation you may have noticed is that functions can only affect your program via their return value, so what do you do when you want a function to alter more than one variable? You use pointers. A pointer is a special kind of variable. Pointers are designed for storing memory address i.e. the address of another variable. Declaring a pointer is the same as declaring a normal variable except you stick an asterisk '*' in front of the variables identifier. There are two new operators you will need to know to work with pointers. The address of operator '&' and the dereferencing operator '*'. Both are prefix unary operators. When you place an ampersand in front of a variable you will get it's address, this can be store in a pointer. When you place an asterisk in front of a pointer you will get the value at the memory address pointed to. As usual, we'll look at a quick code example to show how simple this is.
#include <stdio.h>
int
main()
{
int my_variable = 6, other_variable = 10;
int *my_pointer;
printf ("the address of my_variable is : %p\n", &my_variable);
printf ("the address of other_variable is : %p\n", &other_variable);
my_pointer = &my_variable;
printf ("\nafter \"my_pointer = &my_variable\":\n");
printf ("\tthe value of my_pointer is %p\n", my_pointer);
printf ("\tthe value at that address is %d\n", *my_pointer);
my_pointer = &other_variable;
printf ("\nafter \"my_pointer = &other_variable\":\n");
printf ("\tthe value of my_pointer is %p\n", my_pointer);
printf ("\tthe value at that address is %d\n", *my_pointer);
return 0;
}
The output shows you the address of the two variables, the addresses
your system assigns to the variables will be different to mine. In
printf you'll notice we used %p to display the
addresses. This is the conversion specifier for all pointers.
Anyway, here's the output I got:
the address of my_variable is : 0xbffffa18
the address of other_variable is : 0xbffffa14
after "my_pointer = &my_variable":
the value of my_pointer is 0xbffffa18
the value at that address is 6
after "my_pointer = &other_variable":
the value of my_pointer is 0xbffffa14
the value at that address is 10
There. That's not too complicated. Once you are comfortable with pointers you're well on your way to mastering C.
When your program is running and a variable declaration is encountered, you program makes a request for some memory. The operating system finds a spare piece of memory that is large enough and tells your program the address of this piece of memory. Any time your program wants to read the data stored in that variable, it looks at it's memory address and reads the number of bytes equal to the size of the data type of that variable.
If you run the example from the start of this chapter a second time you may or may not get the same result for the addresses, this depends on your system but even if you repeatably get the same addresses right now there is no guarantee that you will get the same result tomorrow, in fact it's rather unlikely.
One of the best things about pointers is that they allow functions to alter variables outside of there own scope. By passing a pointer to a function you can allow that function to read and write to the data stored in that variable. Say you want to write a function that swaps the values of two variables. Without pointers this would be practically impossible, here's how you do it with pointers:
#include <stdio.h>
int swap_ints (int *first_number, int *second_number);
int
main()
{
int a = 4, b = 7;
printf ("pre-swap values are: a == %d, b == %d\n", a, b);
swap_ints (&a, &b);
printf ("post-swap values are: a == %d, b == %d\n", a, b)
return 0;
}
int
swap_ints (int *first_number, int *second_number)
{
int temp;
/* temp = "what is pointed to by" first_number; etc... */
temp = *first_number;
*first_number = *second_number;
*second_number = temp;
return 0;
}
As you can see, the function declaration of swap_ints tells GCC
to expect two pointers (address of variables). Also, the
address-of operator (&) is used to pass the address of the two
variables rather than their values. swap_ints then reads
Arithmetic can be performed on pointers just like any other variable, this is only useful in a few cases though. If you were (for some reason) to divide a pointer by two it would then point to an area of your computers memory that would probably not belong to your program. If your program tried to read or write to this area of memory the text segmentation fault will display and your program will abort. A segmentation fault occurs when a program tries to access a segment of memory that it does not have permission to access.
There are times however when simple addition can be used on a pointer. We'll see this in the next chapter when we discuss arrays (multiple variables at consecutive memory addresses). In the case of addition (and subtraction), arithmetic is performed in units equal to the size of the pointers data type.
When a variable is declared as being a pointer to type void it is known as a generic pointer. Since you cannot have a variable of type void, the pointer will not point to any data and therefore cannot be dereferenced. It is still a pointer though, to use it you just have to cast it to another kind of pointer first. Hence the term Generic pointer.
This is very useful when you want a pointer to point to data of different types at different times.
Here is some code using a void pointer:
int
main()
{
int i;
char c;
void *the_data;
i = 6;
c = 'a';
the_data = &i;
printf ("the_data points to the integer value %d\n", *(int*) the_data);
the_data = &c;
printf ("the_data now points to the character %c\n", *(char*) the_data);
return 0;
}
contiguous and structured data
C provides two methods for defining structured, or aggregate data types: arrays and structs. Both can contain any of the standard data types including pointers as other structs and arrays. Arrays contain many variables of the same type while structs can contain any mixture of types.
An array is a data type which contains many variables of the same type. Each element of the array is given a number by which you can access that element. For an array of 100 elements, the first element is 0 (zero) and the last is 99. This indexed access makes it very convenient to loop through each element of the array.
Declaring an array is much the same as declaring any other variable except that you must specify the array size. The size (or number of elements) is an integer value placed in square brackets after the arrays identifier.
int
main ()
{
int person[10];
float hourly_wage[4] = {2, 4.9, 10, 123.456};
int index;
index = 4;
person[index] = 56;
printf ("the %dth person is number %d and earns $%f an hour\n",
(index + 1), person[index], hourly_wage[index]);
return 0;
}
NOTE: it is up to you to make sure you don't try to access an element that is not in the array such as the eleventh element of a ten element array. Attempting to access a value past the end of an array will either crash your program or worse, it could retrieve garbage data without telling you that an error occurred.
In the above example we initialised the array hourly_wage by placing a comma separated list of values in curly braces. Using this method you can initialise as few or as many array elements as you like however you cannot initialise an element without initialising all the previous elements. If you initialise some but not all elements of an array the remaining elements will be automatically initialised to zero.
To get around this inconvenience, a GNU extension to the C language allows you to initialise array elements selectively by number. When initialised by number, the elements can be placed in any order withing the curly braces preceded by [index]=value. Like so:
#include <stdio.h>
int
main()
{
int i;
int first_array[100] = { [90]=4, [0]=5, [98]=6 };
double second_array[5] = { [3] = 1.01, [4] = 1.02 };
printf ("sure enough, first_array[90] == %d\n\n", first_array[90]);
printf ("sure enough, first_array[99] == %d\n\n", first_array[99]);
for (i = 0; i < 5; i++)
printf ("value of second_array[%d] is %f\n", i, second_array[i]);
return 0;
}
The array we used in the last example was a one dimensional array. Arrays can have more than one dimension, these arrays-of-arrays are called multidimensional arrays. They are very similar to standard arrays with the exception that they have multiple sets of square brackets after the array identifier. A two dimensional array can be though of as a grid of rows and columns.
#include <stdio.h>
const int num_rows = 7;
const int num_columns = 5;
int
main()
{
int box[num_rows][num_columns];
int row, column;
for (row = 0; row < num_rows; row++)
for (column = 0; column < num_columns; column++)
box[row][column] = column + (row * num_columns);
for (row = 0; row < num_rows; row++)
{
for(column = 0; column < num_columns; column++)
{
printf("%4d", box[row][column]);
}
printf("\n");
}
return 0;
}
If you compile and run this example you'll get a box of numbers like this:
0 1 2 3 4
5 6 7 8 9
10 11 12 13 14
15 16 17 18 19
20 21 22 23 24
25 26 27 28 29
30 31 32 33 34
The above array has two dimensions and can be called a doubly subscripted array. GCC allows arrays of up to 29 dimensions although actually using an array of more than three dimensions is very rare.
Text in C is represented by a number of consecutive variables of type char terminated with the null character '\0'.
The C language provides only the most basic, commonly used types, many languages provide a larger set of types but this is only for convenience. C's way of handling text strings is a good example of this. At times you may think it would be handy if there were other data types which could store complex data. C allows you to define your own.
In C you can create a new type e.g. Person. Person can store an
int called age, a string called name and another int
called height_in_cm. Here's the code to make this new type:
struct Person
{
char[40] name;
int age;
int height_in_cm;
};
This code creates a variable called struct Person. You can declare
variable and pointers to variables of this type in the usual way. Say
you declared a variable john of type struct Person. To access
the age field you would use john.age. I'll make this clearer
with a quick example using the previous definition of struct Person:
int
main()
{
struct Person hero = { 20, "Robin Hood", 191 };
struct Person sidekick;
john.age = 31;
john.name = "John Little"
john.height_in_cm = 237;
printf ("%s is %d years old and stands %dcm tall in his socks\n",
sidekick.name, sidekick.age, sidekick.height_in_cm);
printf ("He is often seen with %s.\n", hero.name);
return 0;
}
When compiled and executed this will display:
John Little is 31 years old and stands 237cm tall in his socks
He is often seen with Robin Hood.
C also supports types that can have dynamic types, a variable that can be and int at one point, a double later and an unsigned long long after that. These data types are declared just like a struct except they use the union keyword. Their behavior is completely different to a struct.
Requesting memory at run-time
Often when you write a program you don't actually know how much data is will have to store or process. In previous examples we've read in some text from the user. We've used large character arrays to store this data but what happens if the user enters more text than we can handle? your program crashes. Disaster. At run-time an application can make a request for more memory.
Glibc provides functions for requesting extra memory, malloc is
the fist one we will show you. You must have a pointer to start with.
Forgetting to free memory when your finished with it is one of
the worst programming mistakes you can make. Is losing pointers a
common problem? pointer falls out of scope?
Reading and writing to files
Place Holder
Changing the behavior of variables
You will have noticed that variables in functions lose their values every time the function exists, this is done for reasons of efficiency, the operating system doesn't know if you will need the data again so it releases the memory allocated to your program back to the system.
By default, variables in C use the auto storage class. This is so called because the variables are automatically created when needed and deleted when they fall out of scope.
You can specify a variable to have the auto storage class by prefixing the variables declaration with the auto keyword but this has no effect, the keyword was introduced into the language for symmetry with the other storage specifiers.
static variables are variables that don't get deleted when they
fall out of scope, they are permanent and retain their value between
calls to the function. Here's an example:
#include <stdio.h>
int get_next_square(void);
int
main()
{
int i;
for (i = 0; i < 10; i++)
printf ("%6d\n", get_next_square ());
printf ("and %6d\n", get_next_square ());
return 0;
}
int
get_next_square ()
{
static int count = 1;
count += 1;
return count * count;
}
This will list the squares of the numbers from zero to ten. Zero to nine are printed by the loop and the square of ten is printed afterwards just to show it still has it's value.
When you declare a variable as extern your program doesn't actually reserve any memory for it, extern means that the variable already exits external to the function or file.
If you want to make a variable available to every file in a project you declare it globally in one file, that is, not inside a function, and add an extern declaration of that variable to a header file that is included in all the other files.
The register storage class is used as a hint to the compiler that a variable is heavily used and access to it should be optimised if possible. Variables are usually stored in normal memory (RAM) and passed back and forth to the computers processor as needed, the speed the data is sent at is pretty fast but can be improved on. Almost all computer processors contain cpu registers, these are memory slots on the actual processor, storing data there gets rid of the overhead of retrieving the data from normal memory. This memory is quite small compared to normal memory though so only a few variables can be stored there. GCC will always make use of registers by deciding what variables it thinks will be accessed often, this works well but will never be perfect because GCC doesn't know the purpose of your program. By using the register keyword you can tell GCC what needs to be optimised.
One problem with placing a variable into a cpu register is that you can't get a pointer to it, pointers can only point to normal memory. Because of this restriction GCC will ignore the register keyword on variables whos address is taken at any point in the program.
The resulting program will contain a request, on creation of the variable that it be placed in a cpu register, the operating system may ignore or honour this request.
This is something to do with pointers, I think it tells the compiler that a specific pointer is the only pointer to a section of memory, the compiler can optimise code better with this knowledge. I think.
typedef isn't much like the others, it's used to give a
variable type a new name. There are two main reasons for doing this.
The most common is to give a name to a struct you have defined so that
you can use your new data type without having to always precede it
with the struct keyword.
The second use for typedef is for compatibility. Say you want to store a 32-bit number. If you use int you are not guaranteed that it will be 32-bit on every machine. To get around this you can use preprocessor directives to selectively typedef a new type to the right size.
#include <stdio.h>
/* type, position coordinates and armament */
struct _ship
{
int type;
int x;
int y;
int missiles;
};
typedef struct _ship ship;
int
main ()
{
ship battle_ship_1;
ship battle_ship_2 = {1, 60, 66, 8};
battle_ship_1.type = 63;
battle_ship_1.x = 54;
battle_ship_1.y = 98;
battle_ship_1.missiles = 12;
/* More code to actually use this data would go here */
return 0;
}
When & how to use them
The C Preprocessor is a simple macro-expander that is run on source code files before passing them to the compiler. Lines that begin with the hash symbol '#' are directives to the C preprocessor.
When you create a macro you assign a name to a C expression. You can then use this name in your code just as you would have used the expression. The preprocessor replaces all occurences of that name with the expression.
Macros are snippets of code that get processed before compilation.
This is done by the C preprocessor, #define statements
are macros. Take a look at this piece of code:
#define SIZE_OF_SQUARE 4
int
main ()
{
int i, j;
for (i = 0; i < SIZE_OF_SQUARE; i++)
{
for (j = 0; j < SIZE_OF_SQUARE; j++)
{
printf ("*"); // print an asterisk for each column
}
printf ("\n"); // and a newline at the end of each row
}
}
The output of this code will be a box:
****
****
****
****
The C preprocessor simply replaces the macro SIZE_OF_BOX with
the value “4”. This very useful for two reasons:
SIZE_OF_BOX but in larger programs this make life much
easier and removes the possibility of forgetting to change one of the
values.
#define PI 3.142857143.
Some of the small function in glibc are implemented as
macros, getc is one
Macros can be miss-used and it's hard to catch the bugs because the macro no longer exists when the code gets to the compiler. The most error is the macro argument with side effect. Take the this small example:
#define MAX (a, b) (a > b ? a : b)
int
main()
{
int cows = 10, sheep = 12;
printf ("we have %d of our most common animal\n", MAX (cows, sheep));
return 0;
}
We compile and execute this code and get:
ciaran@pooh:~/book$ ./a.out
we have 12 of our most common animal
ciaran@pooh:~/book$
Yup, everything looks good. Try this next example:
#define MAX (a, b) (a > b ? a : b)
int
main ()
{
int cows = 10, sheep = 12;
printf ("We have %d of our most common animal\n", MAX(cows, sheep));
printf ("Hang on, we just bought another one.\n");
printf ("Now we have %d.\n", MAX(cows, ++sheep));
return 0;
}
Can you see what's going to happen?
ciaran@pooh:~/book$ ./a.out
We have 12 of our most common animal
Hang on, we just bought another one.
Now we have 14.
ciaran@pooh:~/book$
When the text substitution occurs there will be two instances of
++sheep. Another more sinister way for this bug may manifest
itself is when you pass use a function as an argument to a macro. If
the function modifies a global or static variable then this
modification may occur multiple times. These bugs can be very
hard to find, the code is perfectly valid so the compiler has nothing
to complain about, the bug will only be noticed at run time and wont
occur every time the macro is called, only when it is called with an
argument that has a side effect.
The preprocessor was commonly used to make up for small deficiencies of the language, however, as the language has evolved these defiances have be all but done away. It's still good to know how to use and understand preprocessor macros, they are very common. Macros have been part of the language for longer than their replacements and people have gotten used to them.
If you are thinking that a const int global variable could replace a
simple #define you are right. const variables have some
advantages, one small advantage is that you can get their address when
you need to pass around a pointer to their value, in this way they are
more flexible than macros. If you don't take the address of the const
variable then GCC can optimise it to a level similar to a
#define.
Complex macros, that is functions implemented as macros, can be replaced by inline functions.
The VA_ARGS macros etc.
Variable length argument lists are something you have already come
across, think of printf, how would you write the prototype for
this function when you don't know how many arguments will be passed to
it.
pointers to functions
Another use of the void data type is for making pointers to functions. This is a fairly advanced programming technique but a very useful one once you become comfortable with it.
Here is an example of a function pointer:
int
main()
{
/* oh, crap, better go write one... */
return 0;
}
GCC permits functions to be defined within other functions. Functions defined like this are known as nested functions and obey the same scoping rules as variables. When the parent function exits, the child function falls out of scope and is unavailable.
Probably the main reason for nested functions being allowed by GCC for flexibilities sake although small performance increases can be gained by using them correctly. Nested functions obey Lexical scoping, they have access to the variables of the function that contains them.
For this reason, they can accomplish tasks that would usually require functions taking pointers as arguments. There is a slight performance loss when pointers are used because a variable that has a pointer cannot be stored in a machine register. Pointers never point to machine registers so how would the pointer work?
If you are defining a function at the
#include <stdio.h>
int
main ()
{
int swap (int *a, int *b)
{
int c;
c = *a;
*a = *b;
*b = c;
return 0;
}
int first = 12, second = 34;
printf ("f is %d and s is %d\n", first, second);
swap (&first, &second);
printf ("f is %d and s is %d\n", first, second);
return 0;
}
You don't have to declare nested functions like you do normal functions however you can if you like. The only reason for doing so would be for the sake of readability, you might like the function definition to appear near where it is used. It's up to you, but if you do decide to declare you nested function you must explicitly declare it as auto.
Nested functions have local scope, declaring a nested function as extern is will cause an error. static and inline are both valid however the meaning of static escapes me.
A program starts by the operating system calling a programs
main function. Every one of your programs so far have defined
main as a function taking no arguments but this is not always
the case. main is the only function in C that can be defined
in multiple ways. It can take no arguments, two arguments or three
arguments. The two and three argument forms allow it to receive
arguments from the shell. The three argument form is not particularly
useful and is never necessary, we'll cover it briefly at the end of
this chapter.
The two argument form takes an int and an array of strings. When
defining main you can give these arguments any name but it is
convention to call them argc and argv[]. The first
argument holds a count of how many elements there are in the array of
strings passed as the second argument. The array is always null
terminated so argv[argc] == NULL.
Here's a short program demonstrating the use of
int
main (int argc, char *argv[])
{
int i;
for (i = 0; i < argc; i++)
printf ("argv[%d] == %s\n", i, argv[i]);
return 0;
}
to be passed to main via two arguments: an int and a
*char[] (an array of strings). The int is usually called argc
which is short for argument count, as the name suggests, it stores the
number of arguments passed to main. The second argument,
usually called argv is an array of strings.
C's method of getting command line arguments is pretty simple but when your program has a lot of options it can get complex. To solve this, Glibc provides a series of functions to perform command tasks for you. The argp_* functions perform much of the work for you and they do it in a standard way which makes you program more familiar to users. Here's an short program using argp:
/* put a tiny argp program here */
When you run this program you will see...
Here's a longer program, it uses four global variables to store information about your program:
#include <stdlib.h>
#include <argp.h>
const char *argp_program_version = "simple_argp 0.1";
const char *argp_program_bug_address =
"<some_email_address@you_care_about.com>";
static char doc[] =
"short program to show the use of argp\nThis program does little";
static char args_doc[] = "ARG1 ARG2";
/* initialise an argp_option struct with the options we except */
static struct argp_option options[] =
{
{"verbose", 'v', 0, 0, "Produce verbose output" },
{"output", 'o', "FILE", 0, "Output to FILE" },
{ 0 }
};
/* Used by `main' to communicate with `parse_opt'. */
struct arguments
{
char *args[2]; /* ARG1 & ARG2 */
int silent, verbose;
char *output_file;
};
/* Parse a single option. */
static error_t
parse_opt (int key, char *arg, struct argp_state *state)
{
/* Get the INPUT argument from `argp_parse', which we
know is a pointer to our arguments structure. */
struct arguments *arguments = state->input;
switch (key)
{
case 'q': case 's':
arguments->silent = 1;
break;
case 'v':
arguments->verbose = 1;
break;
case 'o':
arguments->output_file = arg;
break;
case ARGP_KEY_ARG:
if (state->arg_num >= 2)
/* Too many arguments. */
argp_usage (state);
arguments->args[state->arg_num] = arg;
break;
case ARGP_KEY_END:
if (state->arg_num < 2)
/* Not enough arguments. */
argp_usage (state);
break;
default:
return ARGP_ERR_UNKNOWN;
}
return 0;
}
/* Our argp parser. */
static struct argp argp = { options, parse_opt, args_doc, doc };
int main (int argc, char **argv)
{
struct arguments arguments;
/* Default values. */
arguments.silent = 0;
arguments.verbose = 0;
arguments.output_file = "-";
/* Parse our arguments; every option seen by `parse_opt' will
be reflected in `arguments'. */
argp_parse (&argp, argc, argv, 0, 0, &arguments);
printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
"VERBOSE = %s\nSILENT = %s\n",
arguments.args[0], arguments.args[1],
arguments.output_file,
arguments.verbose ? "yes" : "no",
arguments.silent ? "yes" : "no");
exit (0);
}
This is pretty simple. no?
BLAH, talk about how to use the three argument form and the other way of getting at environment variables, show a toy example.
Reusable compiled code
Libraries are like programs except they don't contain a main
function for execution to begin at. The functions in libraries can be
used in other applications by linking the application to the library.
To link with a library you must use the -l switch followed by
the library name. Glibc's math library is called libm.so, you
don't need the lib or .so arts to to use this library
you add the switch -lm o your compilation line.
Compiling a program involves many tools, GCC takes care of this by calling other programs to handle each stage of the process. The three main stages are preprocessing, compilation and linking. In C code, lines that begin with the hash symbol “#” are commands for the preprocessor, GCC includes a preprocessor called CPP (C Preprocessor). #define and #include are by far the most common preprocessor commands The compilation process is broken down into many smaller stages. One of these stages is confusingly called compilation. Compilation is the process of converting source code to object code. If you invoke GCC with “-c” it will When programs become large it can take time to compile them, by splitting a program into smaller files you can re-compile only the files that you have changed. First you must tell gcc to only compile the source files. This
Writing a library is similar to writing a program. The first obvious
difference is that there is no main. Libraries are very handy
for functions you use regularly or functions you think others may find
useful.
Libraries can be linked in a dynamic or static way. A staticly linked library gets compiled into your application. Dynamic linking is a newer method that allows an application to link with a library at run-time, this has many advantages. For a start, the library can be updated without having to recompile the application and vice versa.
Considerations for important projects
This is one of the most important qualities of good code, luckily, it is an art that becomes natural with practice. One of the biggest detractors from readability is clever code. Scrunching multiple operations into one line or statement may have seemed like an achievement but when someone else tries to read it it will slow them down if they have to unravel these operations in there head.
Easy optimisations: Low hanging fruit
C is well know as the fastest high-level language available
There are two times you will optimise your code: while you're writing it and after it's performance disappoints you.
As a rule, it is said that ninety percent of your an applications running time is taken up by ten percent of it's code. There is little point in optimising a function that is rarely called.
Function attributes are a GNU extension to the C language. They allow you to give GCC more information about a function. This information can be used for many purposes including optimisation and stricter checking.
Function attributes are specified as part of a function declaration.
After the closing parenthesis of the functions arguments the keyword
__attribute__ followed by the desired attributes in a set of
double parenthesis. Here's a function with the pure attribute.
int my_func(int first, int second) __attribute__ ((pure));
Functions can have multiple attributes, to do this, separate the attributes with commas inside the double parenthesis.
A pure function is one which do not affect anything outside of it's own scope. This means it may read global variables or variables to which it was passed a pointer but it may not write to such variables. It should not read from volatile variables or external resources (such as files).
const is a stricter version of pure, it tells GCC that a
function will not read any data other that of variables that are
passed to it. Data cannot be read by dereferencing a pointer passed
to a const function.
The only effect a pure or const function can have on your program is it's return value. Having such a function return void would make it pointless.
GCC can use this information to perform common subexpression elimination (!). This means it may call the function fewer times than it was told to as it knows the outcome will be the same each time. For example: if you had a function which converted Celsius to Fahrenheit, and it was placed in a loop calculating the same value each time, GCC would could replace this function call with the value returned. GCC knows this is safe if the conversion function is const.
For you, the programmer, valid C is defined by the compiler. There are many dialects of C in existence, thankfully they are all very similar. There are also other languages that are based on C such as Objective C and C++. These languages are very like C in there appearance but there usage is quite different. GCC understands many dialects of C as well as many other languages (including Objective C and C++).
C was created by Dennis Ritchie between 1969 and 1973. In 1978 he published, along with Brian Kernighan, an excellent C manual: The C programming language. This was the first formal definition of the language. Unfortunately the book left many aspects of the language undefined, this meant that people writing compilers had to make decisions as to how to handle these aspects. The result was that a piece of code would behave differently depending on what compiler was used. This dialect is no longer used, GCC supports it only for compiling very old programs. We mention it here purely for historical purposes. Being the original C dialect it is sometimes called Traditional C.
In 1983 the American National Standards Institute (ANSI) set up a committee to draw up a more exact standard and fix a few shortcomings they saw in the language. In 1989 they finalised this standard which was accepted by the International Standards Organisation (ISO). This new dialect became known as C89. It is also called ISO C or ANSI C. GCC is one of the most conforming compilers available.
The ANSI C committee meets infrequently to update the standard. The latest updated standard was released in 1999 and is known as C99. Few compilers fully support C99 yet; making changes to one of the most important pieces of software to an operating system takes time. GCC's C99 support is mostly complete (as of 2008) but the developers are still working on it.
GNU C is most similar to C89 but has a lot of the new features of C99 added and a few other extensions. These extensions have been added conservatively by the developers as problems are found that C99 doesn't provide good solutions to. GNU C is the default dialect of GCC and is the dialect we will use in this book. We will try our best to point out GNU extensions when we use them but in general, it is better to make full use GNU C. Use of ISO C is limiting your programs to the lowest common denominator and should only be used in special cases.
If you would like to use a dialect other than the default, you can
specify your choice with the -std= switch followed by name of the
dialect. The names are: c89, c99, gnu89 and,
gnu99. gnu89 is the current default but gnu99 will become the
default when C99 support is complete. The change will not be very
noticeable.
Extensions such as those added by GCC are the main source of inspiration for new ISO C standards. When the ANSI C group see a lot of compilers implementing an extension they review the necessity of that feature and if they decide it would be of benefit they work out a standard way to implement it. Some of GCC's extensions may make it into the next standard, some will not.
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