Unix compilation commands
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
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.
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,
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.
-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
-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
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
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:
printffunction, then the definition of the
printffunction 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.
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
ld) after addding a number of complex pieces of information to
the command line. For example,
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
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 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
abc.c are recompiled each time the
command is run. But if you only changed
xyz.c, there's no need to
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 repayed many many times over in the time you'll save later when
debugging. I'd recommend starting with a GUI debugger like
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
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
-g compilation option does.
If you're done debugging, and you want to optimize your code, simply replace
-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 -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
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
-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
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
-O for both commands.
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
-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). 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:
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.
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
-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
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
-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
-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
-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
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.sa if you need to add a
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 -I/somewhere/include xyz.o -o xyz cc -g -L/usr/X11R6/lib -c xyz.c -o xyz
The irrelevant options are ignored; the above commands are exactly equivalent to this:
cc -g xyz.o -o xyz cc -g -c xyz.c -o xyz