Ubuntu Manpage: makepp_tutorial_compilation -- Unix compilation commands

NAME

       makepp_tutorial_compilation -- Unix compilation commands

DESCRIPTION

Skip this this manual page if you have a good grasp on what the compilation commands do.

I find that distressingly few people seem to be taught in their programming classes is how to go about
compiling programs once they've written them. Novices rely either on a single memorized command, or else
on the builtin rules in make. I have been surprised by extremely computer literate people who learned to
compile without optimization because they simply never were told how important it is. Rudimentary
knowledge of how compilation commands work may make your programs run twice as fast or more, so it's
worth at least five minutes. This page describes just about everything you'll need to know to compile C
or C++ programs on just about any variant of Unix.

The examples will be mostly for C, since C++ compilation is identical except that the name of the
compiler is different. Suppose you're compiling source code in a file called "xyz.c" and you want to
build a program called "xyz". What must happen?

You may know that you can build your program in one step, using a command like this:

cc -g xyz.c -o xyz

This will work, but it conceals a two-step process that you must understand if you are writing makefiles.
(Actually, there are more than two steps, but you only have to understand two of them.) For a program of
more than one module, the two steps are usually explicitly separated.

Compilation
The first step is the translation of your C or C++ source code into a binary file called an object file.
Object files usually have an extension of ".o". (For some more recent projects, ".lo" is also used for a
slightly different kind of object file.)

The command to produce an object file on Unix looks something like this:

cc -g -c xyz.c -o xyz.o

"cc" is the C compiler. Sometimes alternate C compilers are used; a very common one is called "gcc". A
common C++ compiler is the GNU compiler, usually called "g++". Virtually all C and C++ compilers on Unix
have the same syntax for the rest of the command (at least for basic operations), so the only difference
would be the first word.

We'll explain what the "-g" option does later.

The "-c" option tells the C compiler to produce a ".o" file as output. (If you don't specify "-c", then
it performs the second compilation step automatically.)

The "-o xyz.o" option tells the compiler what the name of the object file is. You can omit this, as long
as the name of the object file is the same as the name of the source file except for the ".o" extension.

For the most part, the order of the options and the file names does not matter. One important exception
is that the output file must immediately follow "-o".

Linking
The second step of building a program is called linking. An object file cannot be run directly; it's an
intermediate form that must be linked to other components in order to produce a program. Other
components might include:

• Libraries. A library, roughly speaking, is a collection of object modules that are included as
necessary. For example, if your program calls the "printf" function, then the definition of the
"printf" function must be included from the system C library. Some libraries are automatically
linked into your program (e.g., the one containing "printf") so you never need to worry about them.

• Object files derived from other source files in your program. If you write your program so that it
actually has several source files, normally you would compile each source file to a separate object
file and then link them all together.

The linker is the program responsible for taking a collection of object files and libraries and linking
them together to produce an executable file. The executable file is the program you actually run.

The command to link the program looks something like this:

cc -g xyz.o -o xyz

It may seem odd, but we usually run the same program ("cc") to perform the linking. What happens under
the surface is that the "cc" program immediately passes off control to a different program (the linker,
sometimes called the loader, or "ld") after adding a number of complex pieces of information to the
command line. For example, "cc" tells "ld" where the system library is that includes the definition of
functions like "printf". Until you start writing shared libraries, you usually do not need to deal
directly with "ld".

If you do not specify "-o xyz", then the output file will be called "a.out", which seems to me to be a
completely useless and confusing convention. So always specify "-o" on the linking step.

If your program has more than one object file, you should specify all the object files on the link
command.

Why you need to separate the steps
Why not just use the simple, one-step command, like this:

cc -g xyz.c -o xyz

instead of the more complicated two-stage compilation

cc -g -c xyz.c -o xyz.o
cc -g xyz.o -o xyz

if internally the first is converted into the second? The difference is important only if there is more
than one module in your program. Suppose we have an additional module, "abc.c". Now our compilation
looks like this:

# One-stage command.
cc -g xyz.c abc.c -o xyz

# Two-stage command.
cc -g -c xyz.c -o xyz.o
cc -g -c abc.c -o abc.o
cc -g xyz.o abc.o -o xyz

The first method, of course, is converted internally into the second method. This means that both
"xyz.c" and "abc.c" are recompiled each time the command is run. But if you only changed "xyz.c",
there's no need to recompile "abc.c", so the second line of the two-stage commands does not need to be
done. This can make a huge difference in compilation time, especially if you have many modules. For
this reason, virtually all makefiles keep the two compilation steps separate.

That's pretty much the basics, but there are a few more little details you really should know about.

Debugging vs. optimization
Usually programmers compile a program either either for debug or for speed. Compilation for speed is
called optimization; compiling with optimization can make your code run up to 5 times faster or more,
depending on your code, your processor, and your compiler.

With such dramatic gains possible, why would you ever not want to optimize? The most important answer is
that optimization makes use of a debugger much more difficult (sometimes impossible). (If you don't know
anything about a debugger, it's time to learn. The half hour or hour you'll spend learning the basics
will be repaid many many times over in the time you'll save later when debugging. I'd recommend starting
with a GUI debugger like "kdbg", "ddd", or "gdb" run from within emacs (see the info pages on gdb for
instructions on how to do this).) Optimization reorders and combines statements, removes unnecessary
temporary variables, and generally rearranges your code so that it's very tough to follow inside a
debugger. The usual procedure is to write your code, compile it without optimization, debug it, and then
turn on optimization.

In order for the debugger to work, the compiler has to cooperate not only by not optimizing, but also by
putting information about the names of the symbols into the object file so the debugger knows what things
are called. This is what the "-g" compilation option does.

If you're done debugging, and you want to optimize your code, simply replace "-g" with "-O". For many
compilers, you can specify increasing levels of optimization by appending a number after "-O". You may
also be able to specify other options that increase the speed under some circumstances (possibly trading
off with increased memory usage). See your compiler's man page for details. For example, here is an
optimizing compile command that I use frequently with the "gcc" compiler:

gcc -O6 -malign-double -c xyz.c -o xyz.o

You may have to experiment with different optimization options for the absolute best performance. You
may need different options for different pieces of code. Generally speaking, a simple optimization flag
like "-O6" works with many compilers and usually produces pretty good results.

Warning: on rare occasions, your program doesn't actually do exactly the same thing when it is compiled
with optimization. This may be due to (1) an invalid assumption you made in your code that was harmless
without optimization, but causes problems because the compiler takes the liberty of rearranging things
when you optimize; or (2) sadly, compilers have bugs too, including bugs in their optimizers. For a
stable compiler like "gcc" on a common platform like an Pentium, optimization bugs are seldom a problem
(as of the year 2000--there were problems a few years ago).

If you don't specify either "-g" or "-O" in your compilation command, the resulting object file is
suitable neither for debugging nor for running fast. For some reason, this is the default. So always
specify either "-g" or "-O".

On some systems, you must supply "-g" on both the compilation and linking steps; on others (e.g. Linux),
it needs to be supplied only on the compilation step. On some systems, "-O" actually does something
different in the linking phase, while on others, it has no effect. In any case, it's always harmless to
supply "-g" or "-O" for both commands.

Warnings
Most compilers are capable of catching a number of common programming errors (e.g., forgetting to return
a value from a function that's supposed to return a value). Usually, you'll want to turn on warnings.
How you do this depends on your compiler (see the man page), but with the "gcc" compiler, I usually use
something like this:

gcc -g -Wall -c xyz.c -o xyz.o

(Sometimes I also add "-Wno-uninitialized" after "-Wall" because of a warning that is usually wrong that
crops up when optimizing.)

These warnings have saved me many many hours of debugging.

Other useful compilation options
Often, necessary include files are stored in some directory other than the current directory or the
system include directory (/usr/include). This frequently happens when you are using a library that comes
with include files to define the functions or classes.

Suppose, for example, you are writing an application that uses the Qt libraries. You've installed a
local copy of the Qt library in /home/users/joe/qt, which means that the include files are stored in the
directory /home/users/joe/qt/include. In your code, you want to be able to do things like this:

#include <qwidget.h>

instead of

#include "/home/users/joe/qt/include/qwidget.h"

You can tell the compiler to look for include files in a different directory by using the "-I"
compilation option:

g++ -I/home/users/joe/qt/include -g -c mywidget.cpp -o mywidget.o

There is usually no space between the "-I" and the directory name.

When the C++ compiler is looking for the file qwidget.h, it will look in /home/users/joe/qt/include
before looking in the system include directory. You can specify as many "-I" options as you want.

Using libraries
You will often have to tell the linker to link with specific external libraries, if you are calling any
functions that aren't part of the standard C library. The "-l" (lowercase L) option says to link with a
specific library:

cc -g xyz.o -o xyz -lm

"-lm" says to link with the system math library, which you will need if you are using functions like
"sqrt".

Beware: if you specify more than one "-l" option, the order can make a difference on some systems. If
you are getting undefined variables when you know you have included the library that defines them, you
might try moving that library to the end of the command line, or even including it a second time at the
end of the command line.

Sometimes the libraries you will need are not stored in the default place for system libraries. "-labc"
searches for a file called libabc.a or libabc.so or libabc.sa in the system library directories (/usr/lib
and usually a few other places too, depending on what kind of Unix you're running). The "-L" option
specifies an additional directory to search for libraries. To take the above example again, suppose
you've installed the Qt libraries in /home/users/joe/qt, which means that the library files are in
/home/users/joe/qt/lib. Your link step for your program might look something like this:

g++ -g test_mywidget.o mywidget.o -o test_mywidget -L/home/users/joe/qt/lib -lqt

(On some systems, if you link in Qt you will need to add other libraries as well (e.g.,
"-L/usr/X11R6/lib -lX11 -lXext"). What you need to do will depend on your system.)

Note that there is no space between "-L" and the directory name. The "-L" option usually goes before any
"-l" options it's supposed to affect.

How do you know which libraries you need? In general, this is a hard question, and varies depending on
what kind of Unix you are running. The documentation for the functions or classes you are using should
say what libraries you need to link with. If you are using functions or classes from an external
package, there is usually a library you need to link with; the library will usually be a file called
"libabc.a" or "libabc.so" or "libabc.sa" if you need to add a "-labc" option.

Some other confusing things
You may have noticed that it is possible to specify options which normally apply to compilation on the
linking step, and options which normally apply to linking on the compilation step. For example, the
following commands are valid:

cc -g -L/usr/X11R6/lib -c xyz.c -o xyz.o
cc -g -I/somewhere/include xyz.o -o xyz

The irrelevant options are ignored; the above commands are exactly equivalent to this:

cc -g -c xyz.c -o xyz.o
cc -g xyz.o -o xyz

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