2.2. A Line-by-Line Dissection
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.
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.
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().
This line is a
function call, the function is
already defined for you. When you call
printf()
you must pass it an
argument to tell it what to
display.
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() signals the end of the program it returns it
to the shell. The return value of a program is stored
by the shell, if you want to see it, type the following:
ciaran@pooh:~/book$ gcc -Wall -o hello hello.c
ciaran@pooh:~/book$ ./hello
hello, world
ciaran@pooh:~/book$ echo $?
0
ciaran@pooh:~/book$
