makepp_tutorial_compilation -- Unix compilation commands
Skip this this manual page if you have a good grasp on what the compilation
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
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.
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"
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
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
- 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.
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 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
# 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.
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
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.
Often, necessary include files are stored in some directory other than the
current directory or the system include directory ( /usr/include
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
. In your code, you want to be able to do
things like this:
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
before looking in the system include
directory. You can specify as many "-I" options as you want.
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".
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
in the system library directories
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.
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
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
cc -g -c xyz.c -o xyz.o
cc -g xyz.o -o xyz