Compilation

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

• 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

    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

or

    # 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

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

    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

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

    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

    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