1.4. Introducing GCC
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.
Example 1-2. tiny.c
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.
Example 1-3. tiny2.c
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$
