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<article>
<artheader>
<title>A User's Guide to FreeBSD Programming Tools</title>
<authorgroup>
<author>
<firstname>James</firstname>
<surname>Raynard</surname>
<affiliation>
<address>
<email>jraynard@FreeBSD.org</email>
</address>
</affiliation>
</author>
</authorgroup>
<pubdate>August 17, 1997</pubdate>
<copyright>
<year>1997</year>
<holder>James Raynard</holder>
</copyright>
<abstract>
<para>This document is an introduction to using some of the
programming tools supplied with FreeBSD, although much of it
will be applicable to many other versions of Unix. It does
<emphasis>not</emphasis> attempt to describe coding in any
detail. Most of the document assumes little or no previous
programming knowledge, although it is hoped that most
programmers will find something of value in it</para>
</abstract>
</artheader>
<sect1>
<title>Introduction<anchor id=foo></title>
<para>FreeBSD offers an excellent development environment.
Compilers for C, C++, and Fortran and an assembler come with the
basic system, not to mention a Perl interpreter and classic Unix
tools such as <command>sed</command> and <command>awk</command>. If that is
not enough, there are many more compilers and interpreters in
the Ports collection. FreeBSD is very compatible with standards
such as <acronym>POSIX</acronym> and <acronym>ANSI</acronym> C, as well with
its own BSD heritage, so it is possible to write applications
that will compile and run with little or no modification on a
wide range of platforms.</para>
<para>However, all this power can be rather overwhelming at first
if you've never written programs on a Unix platform before.
This document aims to help you get up and running, without
getting too deeply into more advanced topics. The intention is
that this document should give you enough of the basics to be
able to make some sense of the documentation.</para>
<para>Most of the document requires little or no knowledge of
programming, although it does assume a basic competence with
using Unix and a willingness to learn!</para>
</sect1>
<sect1>
<title>Introduction to Programming</title>
<para>A program is a set of instructions that tell the computer to
do various things; sometimes the instruction it has to perform
depends on what happened when it performed a previous
instruction. This section gives an overview of the two main
ways in which you can give these instructions, or
<quote>commands</quote> as they are usually called. One way
uses an <firstterm>interpreter</firstterm>, the other a
<firstterm>compiler</firstterm>. As human languages are too difficult for
a computer to understand in an unambiguous way, commands are
usually written in one or other languages specially designed for
the purpose.</para>
<sect2>
<title>Interpreters</title>
<para>With an interpreter, the language comes as an environment,
where you type in commands at a prompt and the environment
executes them for you. For more complicated programs, you can
type the commands into a file and get the interpreter to load
the file and execute the commands in it. If anything goes
wrong, many interpreters will drop you into a debugger to help
you track down the problem.</para>
<para>The advantage of this is that you can see the results of
your commands immediately, and mistakes can be corrected
readily. The biggest disadvantage comes when you want to
share your programs with someone. They must have the same
interpreter, or you must have some way of giving it to them,
and they need to understand how to use it. Also users may not
appreciate being thrown into a debugger if they press the
wrong key! From a performance point of view, interpreters can
use up a lot of memory, and generally do not generate code as
efficiently as compilers.</para>
<para>In my opinion, interpreted languages are the best way to
start if you have not done any programming before. This kind
of environment is typically found with languages like Lisp,
Smalltalk, Perl and Basic. It could also be argued that the
Unix shell (<command>sh</command>, <command>csh</command>) is itself an
interpreter, and many people do in fact write shell
<quote>scripts</quote> to help with various
<quote>housekeeping</quote> tasks on their machine. Indeed, part
of the original Unix philosophy was to provide lots of small
utility programs that could be linked together in shell
scripts to perform useful tasks.</para>
</sect2>
<sect2>
<title>Interpreters available with FreeBSD</title>
<para>Here is a list of interpreters that are available as
<ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/">FreeBSD
packages</ulink>, with a brief discussion of some of the
more popular interpreted languages.</para>
<para>To get one of these packages, all you need to do is to
click on the hotlink for the package, then run</para>
<screen>&prompt.root; <userinput>pkg_add <replaceable>package name</></userinput>
</screen>
<para>as root. Obviously, you will need to have a fully
functional FreeBSD 2.1.0 or later system for the package to
work!</para>
<variablelist>
<varlistentry>
<term><acronym>BASIC</acronym></term>
<listitem>
<para>Short for Beginner's All-purpose Symbolic
Instruction Code. Developed in the 1950s for teaching
University students to program and provided with every
self-respecting personal computer in the 1980s,
<acronym>BASIC</acronym> has been the first programming
language for many programmers. It's also the foundation
for <trademark>Visual Basic</trademark>.</para>
<para>The <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/bwbasic-2.10.tgz">Bywater
Basic Interpreter</ulink> and the <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/pbasic-2.0.tgz">Phil
Cockroft's Basic Interpreter</ulink> (formerly Rabbit
Basic) are available as FreeBSD <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/">FreeBSD
packages</ulink></para>
</listitem>
</varlistentry>
<varlistentry>
<term>Lisp</term>
<listitem>
<para>A language that was developed in the late 1950s as
an alternative to the <quote>number-crunching</quote>
languages that were popular at the time. Instead of
being based on numbers, Lisp is based on lists; in fact
the name is short for <quote>List Processing</quote>.
Very popular in AI (Artificial Intelligence)
circles.</para>
<para>Lisp is an extremely powerful and sophisticated
language, but can be rather large and unwieldy.</para>
<para>FreeBSD has <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/gcl-2.0.tgz">GNU
Common Lisp</ulink> available as a package.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Perl</term>
<listitem>
<para>Very popular with system administrators for writing
scripts; also often used on World Wide Web servers for
writing <acronym>CGI</acronym> scripts.</para>
<para>The latest version (version 5) comes with FreeBSD.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Scheme</term>
<listitem>
<para>A dialect of Lisp that is rather more compact and
cleaner than Common Lisp. Popular in Universities as it
is simple enough to teach to undergraduates as a first
language, while it has a high enough level of
abstraction to be used in research work.</para>
<para>FreeBSD has packages of the <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/elk-3.0.tgz">Elk
Scheme Interpreter</ulink>, the <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/mit-scheme-7.3.tgz">MIT
Scheme Interpreter</ulink> and the <ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/scm-4e1.tgz">SCM
Scheme Interpreter</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Icon</term>
<listitem>
<para><ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/icon-9.0.tgz">The
Icon Programming Language</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Logo</term>
<listitem>
<para><ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/ucblogo-3.3.tgz">Brian
Harvey's LOGO Interpreter</ulink>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Python</term>
<listitem>
<para><ulink
URL="ftp://ftp.FreeBSD.org:pub/FreeBSD/packages/lang/python-1.2">The
Python Object-Oriented Programming
Language</ulink></para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
<title>Compilers</title>
<para>Compilers are rather different. First of all, you write
your code in a file (or files) using an editor. You then run
the compiler and see if it accepts your program. If it did
not compile, grit your teeth and go back to the editor; if it
did compile and gave you a program, you can run it either at a
shell command prompt or in a debugger to see if it works
properly.
<footnote>
<para>If you run it in the shell, you may get a core
dump.</para>
</footnote></para>
<para>Obviously, this is not quite as direct as using an
interpreter. However it allows you to do a lot of things
which are very difficult or even impossible with an
interpreter, such as writing code which interacts closely with
the operating system—or even writing your own operating
system! It's also useful if you need to write very efficient
code, as the compiler can take its time and optimise the code,
which would not be acceptable in an interpreter. And
distributing a program written for a compiler is usually more
straightforward than one written for an interpreter—you
can just give them a copy of the executable, assuming they
have the same operating system as you.</para>
<para>Compiled languages include Pascal, C and C++. C and C++
are rather unforgiving languages, and best suited to more
experienced programmers; Pascal, on the other hand, was
designed as an educational language, and is quite a good
language to start with. Unfortunately, FreeBSD doesn't have
any Pascal support, except for a Pascal-to-C converter in the
ports.</para>
<para>As the edit-compile-run-debug cycle is rather tedious when
using separate programs, many commercial compiler makers have
produced Integrated Development Environments
(<acronym>IDE</acronym>s for short). FreeBSD does not have an
<acronym>IDE</acronym> as such; however it is possible to use Emacs
for this purpose. This is discussed in <xref
linkend="emacs">.</para>
</sect2>
</sect1>
<sect1>
<title>Compiling with <command>cc</command></title>
<para>This section deals only with the GNU compiler for C and C++,
since that comes with the base FreeBSD system. It can be
invoked by either <command>cc</command> or <command>gcc</command>. The
details of producing a program with an interpreter vary
considerably between interpreters, and are usually well covered
in the documentation and on-line help for the
interpreter.</para>
<para>Once you've written your masterpiece, the next step is to
convert it into something that will (hopefully!) run on FreeBSD.
This usually involves several steps, each of which is done by a
separate program.</para>
<procedure>
<step>
<para>Pre-process your source code to remove comments and do
other tricks like expanding macros in C.</para>
</step>
<step>
<para>Check the syntax of your code to see if you have obeyed
the rules of the language. If you have not, it will
complain!</para>
</step>
<step>
<para>Convert the source code into assembly
language—this is very close to machine code, but still
understandable by humans. Allegedly.
<footnote>
<para>To be strictly accurate, <command>cc</command> converts the
source code into its own, machine-independent
<firstterm>p-code</firstterm> instead of assembly language at
this stage.</para>
</footnote></para>
</step>
<step>
<para>Convert the assembly language into machine
code—yep, we are talking bits and bytes, ones and
zeros here.</para>
</step>
<step>
<para>Check that you have used things like functions and
global variables in a consistent way. For example, if you
have called a non-existent function, it will
complain.</para>
</step>
<step>
<para>If you are trying to produce an executable from several
source code files, work out how to fit them all
together.</para>
</step>
<step>
<para>Work out how to produce something that the system's
run-time loader will be able to load into memory and
run.</para>
</step>
<step>
<para>Finally, write the executable on the file system.</para>
</step>
</procedure>
<para>The word <firstterm>compiling</firstterm> is often used to refer to
just steps 1 to 4—the others are referred to as
<firstterm>linking</firstterm>. Sometimes step 1 is referred to as
<firstterm>pre-processing</firstterm> and steps 3-4 as
<firstterm>assembling</firstterm>.</para>
<para>Fortunately, almost all this detail is hidden from you, as
<command>cc</command> is a front end that manages calling all these
programs with the right arguments for you; simply typing</para>
<screen>&prompt.user; <userinput>cc foobar.c</>
</screen>
<para>will cause <filename>foobar.c</filename> to be compiled by all the
steps above. If you have more than one file to compile, just do
something like</para>
<screen>&prompt.user; <userinput>cc foo.c bar.c</>
</screen>
<para>Note that the syntax checking is just that—checking
the syntax. It will not check for any logical mistakes you may
have made, like putting the program into an infinite loop, or
using a bubble sort when you meant to use a binary
sort.
<footnote>
<para>In case you didn't know, a binary sort is an efficient
way of sorting things into order and a bubble sort
isn't.</para>
</footnote></para>
<para>There are lots and lots of options for <command>cc</command>, which
are all in the man page. Here are a few of the most important
ones, with examples of how to use them.</para>
<variablelist>
<varlistentry>
<term><option>-o <replaceable>filename</replaceable></option></term>
<listitem>
<para>The output name of the file. If you do not use this
option, <command>cc</command> will produce an executable called
<filename>a.out</filename>.
<footnote>
<para>The reasons for this are buried in the mists of
history.</para>
</footnote></para>
<informalexample>
<screen>&prompt.user; <userinput>cc foobar.c</> <lineannotation>executable is <filename>a.out</></>
&prompt.user; <userinput>cc -o foobar foobar.c</> <lineannotation>executable is <filename>foobar</></>
</screen>
</informalexample>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-c</option></term>
<listitem>
<para>Just compile the file, do not link it. Useful for toy
programs where you just want to check the syntax, or if
you are using a <filename>Makefile</filename>.</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -c foobar.c</userinput>
</screen>
</informalexample>
<para>This will produce an <firstterm>object file</firstterm> (not an
executable) called <filename>foobar.o</filename>. This
can be linked together with other object files into an
executable.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-g</option></term>
<listitem>
<para>Create a debug version of the executable. This makes
the compiler put information into the executable about
which line of which source file corresponds to which
function call. A debugger can use this information to show
the source code as you step through the program, which is
<emphasis>very</emphasis> useful; the disadvantage is that
all this extra information makes the program much bigger.
Normally, you compile with <option>-g</option> while you
are developing a program and then compile a <quote>release
version</quote> without <option>-g</option> when you're
satisfied it works properly.</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -g foobar.c</userinput>
</screen>
</informalexample>
<para>This will produce a debug version of the
program.
<footnote>
<para>Note, we didn't use the <option>-o</option> flag
to specify the executable name, so we will get an
executable called <filename>a.out</filename>.
Producing a debug version called
<filename>foobar</filename> is left as an exercise for
the reader!</para>
</footnote></para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-O</option></term>
<listitem>
<para>Create an optimised version of the executable. The
compiler performs various clever tricks to try and produce
an executable that runs faster than normal. You can add a
number after the <option>-O</option> to specify a higher
level of optimisation, but this often exposes bugs in the
compiler's optimiser. For instance, the version of
<command>cc</command> that comes with the 2.1.0 release of
FreeBSD is known to produce bad code with the
<option>-O2</option> option in some circumstances.</para>
<para>Optimisation is usually only turned on when compiling
a release version.</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -O -o foobar foobar.c</userinput>
</screen>
</informalexample>
<para>This will produce an optimised version of
<filename>foobar</filename>.</para>
</listitem>
</varlistentry>
</variablelist>
<para>The following three flags will force <command>cc</command>
to check that your code complies to the relevant international
standard, often referred to as the <acronym>ANSI</acronym>
standard, though strictly speaking it is an
<acronym>ISO</acronym> standard.</para>
<variablelist>
<varlistentry>
<term><option>-Wall</option></term>
<listitem>
<para>Enable all the warnings which the authors of
<command>cc</command> believe are worthwhile. Despite the
name, it will not enable all the warnings
<command>cc</command> is capable of.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-ansi</option></term>
<listitem>
<para>Turn off most, but not all, of the
non-<acronym>ANSI</acronym> C features provided by
<command>cc</command>. Despite the name, it does not
guarantee strictly that your code will comply to the
standard.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>-pedantic</option></term>
<listitem>
<para>Turn off <emphasis>all</emphasis>
<command>cc</command>'s non-<acronym>ANSI</acronym> C
features.</para>
</listitem>
</varlistentry>
</variablelist>
<para>Without these flags, <command>cc</command> will allow you to
use some of its non-standard extensions to the standard. Some
of these are very useful, but will not work with other
compilers—in fact, one of the main aims of the standard is
to allow people to write code that will work with any compiler
on any system. This is known as <firstterm>portable
code</firstterm>.</para>
<para>Generally, you should try to make your code as portable as
possible, as otherwise you may have to completely re-write the
program later to get it to work somewhere else—and who
knows what you may be using in a few years time?</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -Wall -ansi -pedantic -o foobar foobar.c</userinput>
</screen>
</informalexample>
<para>This will produce an executable <filename>foobar</filename>
after checking <filename>foobar.c</filename> for standard
compliance.</para>
<variablelist>
<varlistentry>
<term><option>-l<replaceable>library</replaceable></option></term>
<listitem>
<para>Specify a function library to be used during when
linking.</para>
<para>The most common example of this is when compiling a
program that uses some of the mathematical functions in C.
Unlike most other platforms, these are in a separate
library from the standard C one and you have to tell the
compiler to add it.</para>
<para>The rule is that if the library is called
<filename>lib<replaceable>something</replaceable>.a</filename>,
you give <command>cc</command> the argument
<option>-l<replaceable>something</replaceable></option>.
For example, the math library is
<filename>libm.a</filename>, so you give
<command>cc</command> the argument <option>-lm</option>.
A common <quote>gotcha</quote> with the math library is
that it has to be the last library on the command
line.</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -o foobar foobar.c -lm</userinput>
</screen>
</informalexample>
<para>This will link the math library functions into
<filename>foobar</filename>.</para>
<para>If you are compiling C++ code, you need to add
<option>-lg++</option>, or <option>-lstdc++</option> if
you are using FreeBSD 2.2 or later, to the command line
argument to link the C++ library functions.
Alternatively, you can run <command>c++</command> instead
of <command>cc</command>, which does this for you.
<command>c++</command> can also be invoked as
<command>g++</command> on FreeBSD.</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -o foobar foobar.cc -lg++</userinput> <lineannotation>For FreeBSD 2.1.6 and earlier</>
&prompt.user; <userinput>cc -o foobar foobar.cc -lstdc++</userinput> <lineannotation>For FreeBSD 2.2 and later</>
&prompt.user; <userinput>c++ -o foobar foobar.cc</userinput>
</screen>
</informalexample>
<para>Each of these will both produce an executable
<filename>foobar</filename> from the C++ source file
<filename>foobar.cc</filename>. Note that, on Unix
systems, C++ source files traditionally end in
<filename>.C</filename>, <filename>.cxx</filename> or
<filename>.cc</filename>, rather than the
<trademark>MS-DOS</trademark> style
<filename>.cpp</filename> (which was already used for
something else). <command>gcc</command> used to rely on
this to work out what kind of compiler to use on the
source file; however, this restriction no longer applies,
so you may now call your C++ files
<filename>.cpp</filename> with impunity!</para>
</listitem>
</varlistentry>
</variablelist>
<sect2>
<title>Common <command>cc</command> Queries and Problems</title>
<qandaset>
<qandaentry>
<question>
<para>I am trying to write a program which uses the
<function>sin()</function> function and I get an error
like this. What does it mean?</para>
<informalexample>
<screen>/var/tmp/cc0143941.o: Undefined symbol `_sin' referenced from text segment
</screen>
</informalexample>
</question>
<answer>
<para>When using mathematical functions like
<function>sin()</function>, you have to tell
<command>cc</command> to link in the math library, like
so:</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -o foobar foobar.c -lm</userinput>
</screen>
</informalexample>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>All right, I wrote this simple program to practice
using <option>-lm</option>. All it does is raise 2.1 to
the power of 6.</para>
<informalexample>
<programlisting>#include <stdio.h>
int main() {
float f;
f = pow(2.1, 6);
printf("2.1 ^ 6 = %f\n", f);
return 0;
}
</programlisting>
</informalexample>
<para>and I compiled it as:</para>
<informalexample>
<screen>&prompt.user; <userinput>cc temp.c -lm</userinput>
</screen>
</informalexample>
<para>like you said I should, but I get this when I run
it:</para>
<informalexample>
<screen>&prompt.user; <userinput>./a.out</userinput>
2.1 ^ 6 = 1023.000000
</screen>
</informalexample>
<para>This is <emphasis>not</emphasis> the right answer!
What is going on?</para>
</question>
<answer>
<para>When the compiler sees you call a function, it
checks if it has already seen a prototype for it. If it
has not, it assumes the function returns an
<type>int</type>, which is definitely not what you want
here.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>So how do I fix this?</para>
</question>
<answer>
<para>The prototypes for the mathematical functions are in
<filename>math.h</filename>. If you include this file,
the compiler will be able to find the prototype and it
will stop doing strange things to your
calculation!</para>
<informalexample>
<programlisting>#include <math.h>
#include <stdio.h>
int main() {
...
</programlisting>
</informalexample>
<para>After recompiling it as you did before, run
it:</para>
<informalexample>
<screen>&prompt.user; <userinput>./a.out</userinput>
2.1 ^ 6 = 85.766121
</screen>
</informalexample>
<para>If you are using any of the mathematical functions,
<emphasis>always</emphasis> include
<filename>math.h</filename> and remember to link in the
math library.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>I compiled a file called
<filename>foobar.c</filename> and I cannot find an
executable called <filename>foobar</filename>. Where's
it gone?</para>
</question>
<answer>
<para>Remember, <command>cc</command> will call the
executable <filename>a.out</filename> unless you tell it
differently. Use the
<option>-o <replaceable>filename</replaceable></option>
option:</para>
<informalexample>
<screen>&prompt.user; <userinput>cc -o foobar foobar.c</userinput>
</screen>
</informalexample>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>OK, I have an executable called
<filename>foobar</filename>, I can see it when I run
<command>ls</command>, but when I type in
<command>foobar</command> at the command prompt it tells
me there is no such file. Why can it not find
it?</para>
</question>
<answer>
<para>Unlike <trademark>MS-DOS</trademark>, Unix does not
look in the current directory when it is trying to find
out which executable you want it to run, unless you tell
it to. Either type <command>./foobar</command>, which
means <quote>run the file called
<filename>foobar</filename> in the current
directory</quote>, or change your <systemitem
class=environvar>PATH</systemitem> environment
variable so that it looks something like</para>
<informalexample>
<screen>bin:/usr/bin:/usr/local/bin:.
</screen>
</informalexample>
<para>The dot at the end means <quote>look in the current
directory if it is not in any of the
others</quote>.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>I called my executable <filename>test</filename>,
but nothing happens when I run it. What is going
on?</para>
</question>
<answer>
<para>Most Unix systems have a program called
<command>test</command> in <filename>/usr/bin</filename>
and the shell is picking that one up before it gets to
checking the current directory. Either type:</para>
<informalexample>
<screen>&prompt.user; <userinput>./test</userinput>
</screen>
</informalexample>
<para>or choose a better name for your program!</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>I compiled my program and it seemed to run all right
at first, then there was an error and it said something
about <errorname>core dumped</errorname>. What does that
mean?</para>
</question>
<answer>
<para>The name <firstterm>core dump</firstterm> dates back
to the very early days of Unix, when the machines used
core memory for storing data. Basically, if the program
failed under certain conditions, the system would write
the contents of core memory to disk in a file called
<filename>core</filename>, which the programmer could
then pore over to find out what went wrong.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>Fascinating stuff, but what I am supposed to do
now?</para>
</question>
<answer>
<para>Use <command>gdb</command> to analyse the core (see
<xref linkend="debugging">).</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>When my program dumped core, it said something about
a <errorname>segmentation fault</errorname>. What's
that?</para>
</question>
<answer>
<para>This basically means that your program tried to
perform some sort of illegal operation on memory; Unix
is designed to protect the operating system and other
programs from rogue programs.</para>
<para>Common causes for this are:</para>
<itemizedlist>
<listitem>
<para>Trying to write to a <symbol>NULL</symbol>
pointer, eg</para>
<programlisting>char *foo = NULL;
strcpy(foo, "bang!");
</programlisting>
</listitem>
<listitem>
<para>Using a pointer that hasn't been initialised,
eg</para>
<programlisting>char *foo;
strcpy(foo, "bang!");
</programlisting>
<para>The pointer will have some random value that,
with luck, will point into an area of memory that
isn't available to your program and the kernel will
kill your program before it can do any damage. If
you're unlucky, it'll point somewhere inside your
own program and corrupt one of your data structures,
causing the program to fail mysteriously.</para>
</listitem>
<listitem>
<para>Trying to access past the end of an array,
eg</para>
<programlisting>int bar[20];
bar[27] = 6;
</programlisting>
</listitem>
<listitem>
<para>Trying to store something in read-only memory,
eg</para>
<programlisting>char *foo = "My string";
strcpy(foo, "bang!");
</programlisting>
<para>Unix compilers often put string literals like
<literal>"My string"</literal> into read-only areas
of memory.</para>
</listitem>
<listitem>
<para>Doing naughty things with
<function>malloc()</function> and
<function>free()</function>, eg</para>
<programlisting>char bar[80];
free(bar);
</programlisting>
<para>or</para>
<programlisting>char *foo = malloc(27);
free(foo);
free(foo);
</programlisting>
</listitem>
</itemizedlist>
<para>Making one of these mistakes will not always lead to
an error, but they are always bad practice. Some
systems and compilers are more tolerant than others,
which is why programs that ran well on one system can
crash when you try them on an another.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>Sometimes when I get a core dump it says
<errorname>bus error</errorname>. It says in my Unix
book that this means a hardware problem, but the
computer still seems to be working. Is this
true?</para>
</question>
<answer>
<para>No, fortunately not (unless of course you really do
have a hardware problem…). This is usually
another way of saying that you accessed memory in a way
you shouldn't have.</para>
</answer>
</qandaentry>
<qandaentry>
<question>
<para>This dumping core business sounds as though it could
be quite useful, if I can make it happen when I want to.
Can I do this, or do I have to wait until there's an
error?</para>
</question>
<answer>
<para>Yes, just go to another console or xterm, do</para>
<screen>&prompt.user; <userinput>ps</userinput>
</screen>
<para>to find out the process ID of your program, and
do</para>
<screen>&prompt.user; <userinput>kill -ABRT <replaceable>pid</replaceable></userinput>
</screen>
<para>where
<parameter><replaceable>pid</replaceable></parameter> is
the process ID you looked up.</para>
<para>This is useful if your program has got stuck in an
infinite loop, for instance. If your program happens to
trap <symbol>SIGABRT</symbol>, there are several other
signals which have a similar effect.</para>
</answer>
</qandaentry>
</qandaset>
</sect2>
</sect1>
<sect1>
<title>Make</title>
<sect2>
<title>What is <command>make</command>?</title>
<para>When you're working on a simple program with only one or
two source files, typing in</para>
<screen>&prompt.user; <userinput>cc file1.c file2.c</userinput>
</screen>
<para>is not too bad, but it quickly becomes very tedious when
there are several files—and it can take a while to
compile, too.</para>
<para>One way to get around this is to use object files and only
recompile the source file if the source code has changed. So
we could have something like:</para>
<screen>&prompt.user; <userinput>cc file1.o file2.o</userinput> … <userinput>file37.c</userinput> &hellip
</screen>
<para>if we'd changed <filename>file37.c</filename>, but not any
of the others, since the last time we compiled. This may
speed up the compilation quite a bit, but doesn't solve the
typing problem.</para>
<para>Or we could write a shell script to solve the typing
problem, but it would have to re-compile everything, making it
very inefficient on a large project.</para>
<para>What happens if we have hundreds of source files lying
about? What if we're working in a team with other people who
forget to tell us when they've changed one of their source
files that we use?</para>
<para>Perhaps we could put the two solutions together and write
something like a shell script that would contain some kind of
magic rule saying when a source file needs compiling. Now all
we need now is a program that can understand these rules, as
it's a bit too complicated for the shell.</para>
<para>This program is called <command>make</command>. It reads
in a file, called a <firstterm>makefile</firstterm>, that
tells it how different files depend on each other, and works
out which files need to be re-compiled and which ones don't.
For example, a rule could say something like <quote>if
<filename>fromboz.o</filename> is older than
<filename>fromboz.c</filename>, that means someone must have
changed <filename>fromboz.c</filename>, so it needs to be
re-compiled.</quote> The makefile also has rules telling
make <emphasis>how</emphasis> to re-compile the source file,
making it a much more powerful tool.</para>
<para>Makefiles are typically kept in the same directory as the
source they apply to, and can be called
<filename>makefile</filename>, <filename>Makefile</filename>
or <filename>MAKEFILE</filename>. Most programmers use the
name <filename>Makefile</filename>, as this puts it near the
top of a directory listing, where it can easily be
seen.
<footnote>
<para>They don't use the <filename>MAKEFILE</filename> form
as block capitals are often used for documentation files
like <filename>README</filename>.</para>
</footnote></para>
</sect2>
<sect2>
<title>Example of using <command>make</command></title>
<para>Here's a very simple make file:</para>
<programlisting>foo: foo.c
cc -o foo foo.c
</programlisting>
<para>It consists of two lines, a dependency line and a creation
line.</para>
<para>The dependency line here consists of the name of the
program (known as the <firstterm>target</firstterm>), followed
by a colon, then whitespace, then the name of the source file.
When <command>make</command> reads this line, it looks to see
if <filename>foo</filename> exists; if it exists, it compares
the time <filename>foo</filename> was last modified to the
time <filename>foo.c</filename> was last modified. If
<filename>foo</filename> does not exist, or is older than
<filename>foo.c</filename>, it then looks at the creation line
to find out what to do. In other words, this is the rule for
working out when <filename>foo.c</filename> needs to be
re-compiled.</para>
<para>The creation line starts with a <token>tab</token> (press
the <keycap>tab</keycap> key) and then the command you would
type to create <filename>foo</filename> if you were doing it
at a command prompt. If <filename>foo</filename> is out of
date, or does not exist, <command>make</command> then executes
this command to create it. In other words, this is the rule
which tells make how to re-compile
<filename>foo.c</filename>.</para>
<para>So, when you type <userinput>make</userinput>, it will
make sure that <filename>foo</filename> is up to date with
respect to your latest changes to <filename>foo.c</filename>.
This principle can be extended to
<filename>Makefile</filename>s with hundreds of
targets—in fact, on FreeBSD, it is possible to compile
the entire operating system just by typing <userinput>make
world</userinput> in the appropriate directory!</para>
<para>Another useful property of makefiles is that the targets
don't have to be programs. For instance, we could have a make
file that looks like this:</para>
<programlisting>foo: foo.c
cc -o foo foo.c
install:
cp foo /home/me
</programlisting>
<para>We can tell make which target we want to make by
typing:</para>
<screen>&prompt.user; <userinput>make <replaceable>target</replaceable></userinput>
</screen>
<para><command>make</command> will then only look at that target
and ignore any others. For example, if we type
<userinput>make foo</userinput> with the makefile above, make
will ignore the <action>install</action> target.</para>
<para>If we just type <userinput>make</userinput> on its own,
make will always look at the first target and then stop
without looking at any others. So if we typed
<userinput>make</userinput> here, it will just go to the
<action>foo</action> target, re-compile
<filename>foo</filename> if necessary, and then stop without
going on to the <action>install</action> target.</para>
<para>Notice that the <action>install</action> target doesn't
actually depend on anything! This means that the command on
the following line is always executed when we try to make that
target by typing <userinput>make install</userinput>. In this
case, it will copy <filename>foo</filename> into the user's
home directory. This is often used by application makefiles,
so that the application can be installed in the correct
directory when it has been correctly compiled.</para>
<para>This is a slightly confusing subject to try and explain.
If you don't quite understand how <command>make</command>
works, the best thing to do is to write a simple program like
<quote>hello world</quote> and a make file like the one above
and experiment. Then progress to using more than one source
file, or having the source file include a header file. The
<command>touch</command> command is very useful here—it
changes the date on a file without you having to edit
it.</para>
</sect2>
<sect2>
<title>FreeBSD Makefiles</title>
<para>Makefiles can be rather complicated to write. Fortunately,
BSD-based systems like FreeBSD come with some very powerful
ones as part of the system. One very good example of this is
the FreeBSD ports system. Here's the essential part of a
typical ports <filename>Makefile</filename>:</para>
<programlisting>MASTER_SITES= ftp://freefall.cdrom.com/pub/FreeBSD/LOCAL_PORTS/
DISTFILES= scheme-microcode+dist-7.3-freebsd.tgz
.include <bsd.port.mk>
</programlisting>
<para>Now, if we go to the directory for this port and type
<userinput>make</userinput>, the following happens:</para>
<procedure>
<step>
<para>A check is made to see if the source code for this
port is already on the system.</para>
</step>
<step>
<para>If it isn't, an FTP connection to the URL in
<symbol>MASTER_SITES</symbol> is set up to download the
source.</para>
</step>
<step>
<para>The checksum for the source is calculated and compared
it with one for a known, good, copy of the source. This
is to make sure that the source was not corrupted while in
transit.</para>
</step>
<step>
<para>Any changes required to make the source work on
FreeBSD are applied—this is known as
<firstterm>patching</firstterm>.</para>
</step>
<step>
<para>Any special configuration needed for the source is
done. (Many Unix program distributions try to work out
which version of Unix they are being compiled on and which
optional Unix features are present—this is where
they are given the information in the FreeBSD ports
scenario).</para>
</step>
<step>
<para>The source code for the program is compiled. In
effect, we change to the directory where the source was
unpacked and do <command>make</command>—the
program's own make file has the necessary information to
build the program.</para>
</step>
<step>
<para>We now have a compiled version of the program. If we
wish, we can test it now; when we feel confident about the
program, we can type <userinput>make install</userinput>.
This will cause the program and any supporting files it
needs to be copied into the correct location; an entry is
also made into a <database>package database</database>, so
that the port can easily be uninstalled later if we change
our mind about it.</para>
</step>
</procedure>
<para>Now I think you'll agree that's rather impressive for a
four line script!</para>
<para>The secret lies in the last line, which tells
<command>make</command> to look in the system makefile called
<filename>bsd.port.mk</filename>. It's easy to overlook this
line, but this is where all the clever stuff comes
from—someone has written a makefile that tells
<command>make</command> to do all the things above (plus a
couple of other things I didn't mention, including handling
any errors that may occur) and anyone can get access to that
just by putting a single line in their own make file!</para>
<para>If you want to have a look at these system makefiles,
they're in <filename>/usr/share/mk</filename>, but it's
probably best to wait until you've had a bit of practice with
makefiles, as they are very complicated (and if you do look at
them, make sure you have a flask of strong coffee
handy!)</para>
</sect2>
<sect2>
<title>More advanced uses of <command>make</command></title>
<para><command>Make</command> is a very powerful tool, and can
do much more than the simple example above shows.
Unfortunately, there are several different versions of
<command>make</command>, and they all differ considerably.
The best way to learn what they can do is probably to read the
documentation—hopefully this introduction will have
given you a base from which you can do this.</para>
<para>The version of make that comes with FreeBSD is the
<application>Berkeley make</application>; there is a tutorial
for it in <filename>/usr/share/doc/psd/12.make</filename>. To
view it, do</para>
<screen>&prompt.user; <userinput>zmore paper.ascii.gz</userinput>
</screen>
<para>in that directory.</para>
<para>Many applications in the ports use <application>GNU
make</application>, which has a very good set of
<quote>info</quote> pages. If you have installed any of these
ports, <application>GNU make</application> will automatically
have been installed as <command>gmake</command>. It's also
available as a port and package in its own right.</para>
<para>To view the info pages for <application>GNU
make</application>, you will have to edit the
<filename>dir</filename> file in the
<filename>/usr/local/info</filename> directory to add an entry
for it. This involves adding a line like</para>
<programlisting> * Make: (make). The GNU Make utility.
</programlisting>
<para>to the file. Once you have done this, you can type
<userinput>info</userinput> and then select
<guimenuitem>make</guimenuitem> from the menu (or in
<application>Emacs</application>, do <userinput>C-h
i</userinput>).</para>
</sect2>
</sect1>
<sect1 id="debugging">
<title>Debugging</title>
<sect2>
<title>The Debugger</title>
<para>The debugger that comes with FreeBSD is called
<command>gdb</command> (<application>GNU
debugger</application>). You start it up by typing</para>
<screen>&prompt.user; <userinput>gdb <replaceable>progname</replaceable></userinput>
</screen>
<para>although most people prefer to run it inside
<application>Emacs</application>. You can do this by:</para>
<screen><userinput>M-x gdb RET <replaceable>progname</replaceable> RET</userinput>
</screen>
<para>Using a debugger allows you to run the program under more
controlled circumstances. Typically, you can step through the
program a line at a time, inspect the value of variables,
change them, tell the debugger to run up to a certain point
and then stop, and so on. You can even attach to a program
that's already running, or load a core file to investigate why
the program crashed. It's even possible to debug the kernel,
though that's a little trickier than the user applications
we'll be discussing in this section.</para>
<para><command>gdb</command> has quite good on-line help, as
well as a set of info pages, so this section will concentrate
on a few of the basic commands.</para>
<para>Finally, if you find its text-based command-prompt style
off-putting, there's a graphical front-end for it <ulink
URL="../../ports/devel.html">xxgdb</ulink> in the ports
collection.</para>
<para>This section is intended to be an introduction to using
<command>gdb</command> and does not cover specialised topics
such as debugging the kernel.</para>
</sect2>
<sect2>
<title>Running a program in the debugger</title>
<para>You'll need to have compiled the program with the
<option>-g</option> option to get the most out of using
<command>gdb</command>. It will work without, but you'll only
see the name of the function you're in, instead of the source
code. If you see a line like:</para>
<screen>… (no debugging symbols found) …
</screen>
<para>when <command>gdb</command> starts up, you'll know that
the program wasn't compiled with the <option>-g</option>
option.</para>
<para>At the <command>gdb</command> prompt, type
<userinput>break main</userinput>. This will tell the
debugger to skip over the preliminary set-up code in the
program and start at the beginning of your code. Now type
<userinput>run</userinput> to start the program—it will
start at the beginning of the set-up code and then get stopped
by the debugger when it calls <function>main()</function>.
(If you've ever wondered where <function>main()</function>
gets called from, now you know!).</para>
<para>You can now step through the program, a line at a time, by
pressing <command>n</command>. If you get to a function call,
you can step into it by pressing <command>s</command>. Once
you're in a function call, you can return from stepping into a
function call by pressing <command>f</command>. You can also
use <command>up</command> and <command>down</command> to take
a quick look at the caller.</para>
<para>Here's a simple example of how to spot a mistake in a
program with <command>gdb</command>. This is our program
(with a deliberate mistake):</para>
<programlisting>#include <stdio.h>
int bazz(int anint);
main() {
int i;
printf("This is my program\n");
bazz(i);
return 0;
}
int bazz(int anint) {
printf("You gave me %d\n", anint);
return anint;
}
</programlisting>
<para>This program sets <symbol>i</symbol> to be
<literal>5</literal> and passes it to a function
<function>bazz()</function> which prints out the number we
gave it.</para>
<para>When we compile and run the program we get</para>
<screen>&prompt.user; <userinput>cc -g -o temp temp.c</userinput>
&prompt.user; <userinput>./temp</userinput>
This is my program
anint = 4231
</screen>
<para>That wasn't what we expected! Time to see what's going
on!</para>
<screen>&prompt.user; <userinput>gdb temp</userinput>
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
(gdb) <userinput>break main</> <lineannotation>Skip the set-up code</>
Breakpoint 1 at 0x160f: file temp.c, line 9. <lineannotation><command>gdb</command> puts breakpoint at <function>main()</></>
(gdb) <userinput>run</> <lineannotation>Run as far as <function>main()</></>
Starting program: /home/james/tmp/temp <lineannotation>Program starts running</>
Breakpoint 1, main () at temp.c:9 <lineannotation><command>gdb</command> stops at <function>main()</></>
(gdb) <userinput>n</> <lineannotation>Go to next line</>
This is my program <lineannotation>Program prints out</>
(gdb) <userinput>s</> <lineannotation>step into <function>bazz()</></>
bazz (anint=4231) at temp.c:17 <lineannotation><command>gdb</command> displays stack frame</>
(gdb)
</screen>
<para>Hang on a minute! How did <symbol>anint</symbol> get to be
<literal>4231</literal>? Didn't we set it to be
<literal>5</literal> in <function>main()</function>? Let's
move up to <function>main()</function> and have a look.</para>
<screen>(gdb) <userinput>up</> <lineannotation>Move up call stack</>
#1 0x1625 in main () at temp.c:11 <lineannotation><command>gdb</command> displays stack frame</>
(gdb) <userinput>p i</> <lineannotation>Show us the value of <symbol>i</></>
$1 = 4231 <lineannotation><command>gdb</command> displays <literal>4231</></>
</screen>
<para>Oh dear! Looking at the code, we forgot to initialise
<symbol>i</symbol>. We meant to put</para>
<programlisting><lineannotation>…</>
main() {
int i;
i = 5;
printf("This is my program\n");
<lineannotation>&hellip</>
</programlisting>
<para>but we left the <literal>i=5;</literal> line out. As we
didn't initialise <symbol>i</symbol>, it had whatever number
happened to be in that area of memory when the program ran,
which in this case happened to be
<literal>4231</literal>.</para>
<note>
<para><command>gdb</command> displays the stack frame every
time we go into or out of a function, even if we're using
<command>up</command> and <command>down</command> to move
around the call stack. This shows the name of the function
and the values of its arguments, which helps us keep track
of where we are and what's going on. (The stack is a
storage area where the program stores information about the
arguments passed to functions and where to go when it
returns from a function call).</para>
</note>
</sect2>
<sect2>
<title>Examining a core file</title>
<para>A core file is basically a file which contains the
complete state of the process when it crashed. In <quote>the
good old days</quote>, programmers had to print out hex
listings of core files and sweat over machine code manuals,
but now life is a bit easier. Incidentally, under FreeBSD and
other 4.4BSD systems, a core file is called
<filename><replaceable>progname</replaceable>.core</filename> instead of just
<filename>core</filename>, to make it clearer which program a
core file belongs to.</para>
<para>To examine a core file, start up <command>gdb</command> in
the usual way. Instead of typing <command>break</command> or
<command>run</command>, type</para>
<screen>(gdb) <userinput>core <replaceable>progname</replaceable>.core</userinput>
</screen>
<para>If you're not in the same directory as the core file,
you'll have to do <userinput>dir
/path/to/core/file</userinput> first.</para>
<para>You should see something like this:</para>
<screen>&prompt.user; <userinput>gdb a.out</userinput>
GDB is free software and you are welcome to distribute copies of it
under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.13 (i386-unknown-freebsd), Copyright 1994 Free Software Foundation, Inc.
(gdb) <userinput>core a.out.core</userinput>
Core was generated by `a.out'.
Program terminated with signal 11, Segmentation fault.
Cannot access memory at address 0x7020796d.
#0 0x164a in bazz (anint=0x5) at temp.c:17
(gdb)
</screen>
<para>In this case, the program was called
<filename>a.out</filename>, so the core file is called
<filename>a.out.core</filename>. We can see that the program
crashed due to trying to access an area in memory that was not
available to it in a function called
<function>bazz</function>.</para>
<para>Sometimes it's useful to be able to see how a function was
called, as the problem could have occurred a long way up the
call stack in a complex program. The <command>bt</command>
command causes <command>gdb</command> to print out a
back-trace of the call stack:</para>
<screen>(gdb) <userinput>bt</userinput>
#0 0x164a in bazz (anint=0x5) at temp.c:17
#1 0xefbfd888 in end ()
#2 0x162c in main () at temp.c:11
(gdb)
</screen>
<para>The <function>end()</function> function is called when a
program crashes; in this case, the <function>bazz()</function>
function was called from <function>main()</function>.</para>
</sect2>
<sect2>
<title>Attaching to a running program</title>
<para>One of the neatest features about <command>gdb</command>
is that it can attach to a program that's already running. Of
course, that assumes you have sufficient permissions to do so.
A common problem is when you are stepping through a program
that forks, and you want to trace the child, but the debugger
will only let you trace the parent.</para>
<para>What you do is start up another <command>gdb</command>,
use <command>ps</command> to find the process ID for the
child, and do</para>
<screen>(gdb) <userinput>attach <replaceable>pid</replaceable></userinput>
</screen>
<para>in <command>gdb</command>, and then debug as usual.</para>
<para><quote>That's all very well,</quote> you're probably
thinking, <quote>but by the time I've done that, the child
process will be over the hill and far away</quote>. Fear
not, gentle reader, here's how to do it (courtesy of the
<command>gdb</command> info pages):</para>
<screen><lineannotation>&hellip</lineannotation>
if ((pid = fork()) < 0) /* _Always_ check this */
error();
else if (pid == 0) { /* child */
int PauseMode = 1;
while (PauseMode)
sleep(10); /* Wait until someone attaches to us */
<lineannotation>&hellip</lineannotation>
} else { /* parent */
<lineannotation>&hellip</lineannotation>
</screen>
<para>Now all you have to do is attach to the child, set
<symbol>PauseMode</symbol> to <literal>0</literal>, and wait
for the <function>sleep()</function> call to return!</para>
</sect2>
</sect1>
<sect1 id="emacs">
<title>Using Emacs as a Development Environment</title>
<sect2>
<title>Emacs</title>
<para>Unfortunately, Unix systems don't come with the kind of
everything-you-ever-wanted-and-lots-more-you-didn't-in-one-gigantic-package
integrated development environments that other systems
have.
<footnote>
<para>At least, not unless you pay out very large sums of
money.</para>
</footnote>
However, it is possible to set up your own environment. It
may not be as pretty, and it may not be quite as integrated,
but you can set it up the way you want it. And it's free.
And you have the source to it.</para>
<para>The key to it all is Emacs. Now there are some people who
loathe it, but many who love it. If you're one of the former,
I'm afraid this section will hold little of interest to you.
Also, you'll need a fair amount of memory to run it—I'd
recommend 8MB in text mode and 16MB in X as the bare minimum
to get reasonable performance.</para>
<para>Emacs is basically a highly customisable
editor—indeed, it has been customised to the point where
it's more like an operating system than an editor! Many
developers and sysadmins do in fact spend practically all
their time working inside Emacs, leaving it only to log
out.</para>
<para>It's impossible even to summarise everything Emacs can do
here, but here are some of the features of interest to
developers:</para>
<itemizedlist>
<listitem>
<para>Very powerful editor, allowing search-and-replace on
both strings and regular expressions (patterns), jumping
to start/end of block expression, etc, etc.</para>
</listitem>
<listitem>
<para>Pull-down menus and online help.</para>
</listitem>
<listitem>
<para>Language-dependent syntax highlighting and
indentation.</para>
</listitem>
<listitem>
<para>Completely customisable.</para>
</listitem>
<listitem>
<para>You can compile and debug programs within
Emacs.</para>
</listitem>
<listitem>
<para>On a compilation error, you can jump to the offending
line of source code.</para>
</listitem>
<listitem>
<para>Friendly-ish front-end to the <command>info</command>
program used for reading GNU hypertext documentation,
including the documentation on Emacs itself.</para>
</listitem>
<listitem>
<para>Friendly front-end to <command>gdb</command>, allowing
you to look at the source code as you step through your
program.</para>
</listitem>
<listitem>
<para>You can read Usenet news and mail while your program
is compiling.</para>
</listitem>
</itemizedlist>
<para>And doubtless many more that I've overlooked.</para>
<para>Emacs can be installed on FreeBSD using <ulink
URL="../../ports/editors.html">the Emacs
port</ulink>.</para>
<para>Once it's installed, start it up and do <userinput>C-h
t</userinput> to read an Emacs tutorial—that means
hold down the <keycap>control</keycap> key, press
<keycap>h</keycap>, let go of the <keycap>control</keycap>
key, and then press <keycap>t</keycap>. (Alternatively, you
can you use the mouse to select <guimenuitem>Emacs
Tutorial</guimenuitem> from the <guimenu>Help</guimenu>
menu).</para>
<para>Although Emacs does have menus, it's well worth learning
the key bindings, as it's much quicker when you're editing
something to press a couple of keys than to try and find the
mouse and then click on the right place. And, when you're
talking to seasoned Emacs users, you'll find they often
casually throw around expressions like <quote><literal>M-x
replace-s RET foo RET bar RET</literal></quote> so it's
useful to know what they mean. And in any case, Emacs has far
too many useful functions for them to all fit on the menu
bars.</para>
<para>Fortunately, it's quite easy to pick up the key-bindings,
as they're displayed next to the menu item. My advice is to
use the menu item for, say, opening a file until you
understand how it works and feel confident with it, then try
doing C-x C-f. When you're happy with that, move on to
another menu command.</para>
<para>If you can't remember what a particular combination of
keys does, select <guimenuitem>Describe Key</guimenuitem> from
the <guimenu>Help</guimenu> menu and type it in—Emacs
will tell you what it does. You can also use the
<guimenuitem>Command Apropos</guimenuitem> menu item to find
out all the commands which contain a particular word in them,
with the key binding next to it.</para>
<para>By the way, the expression above means hold down the
<keysym>Meta</keysym> key, press <keysym>x</keysym>, release
the <keysym>Meta</keysym> key, type
<userinput>replace-s</userinput> (short for
<literal>replace-string</literal>—another feature of
Emacs is that you can abbreviate commands), press the
<keysym>return</keysym> key, type <userinput>foo</userinput>
(the string you want replaced), press the
<keysym>return</keysym> key, type bar (the string you want to
replace <literal>foo</literal> with) and press
<keysym>return</keysym> again. Emacs will then do the
search-and-replace operation you've just requested.</para>
<para>If you're wondering what on earth the
<keysym>Meta</keysym> key is, it's a special key that many
Unix workstations have. Unfortunately, PC's don't have one,
so it's usually the <keycap>alt</keycap> key (or if you're
unlucky, the <keysym>escape</keysym> key).</para>
<para>Oh, and to get out of Emacs, do <command>C-x C-c</command>
(that means hold down the <keysym>control</keysym> key, press
<keysym>x</keysym>, press <keysym>c</keysym> and release the
<keysym>control</keysym> key). If you have any unsaved files
open, Emacs will ask you if you want to save them. (Ignore
the bit in the documentation where it says
<command>C-z</command> is the usual way to leave
Emacs—that leaves Emacs hanging around in the
background, and is only really useful if you're on a system
which doesn't have virtual terminals).</para>
</sect2>
<sect2>
<title>Configuring Emacs</title>
<para>Emacs does many wonderful things; some of them are built
in, some of them need to be configured.</para>
<para>Instead of using a proprietary macro language for
configuration, Emacs uses a version of Lisp specially adapted
for editors, known as Emacs Lisp. This can be quite useful if
you want to go on and learn something like Common Lisp, as
it's considerably smaller than Common Lisp (although still
quite big!).</para>
<para>The best way to learn Emacs Lisp is to download the <ulink
URL="ftp://prep.ai.mit.edu:pub/gnu/elisp-manual-19-2.4.tar.gz">Emacs
Tutorial</ulink></para>
<para>However, there's no need to actually know any Lisp to get
started with configuring Emacs, as I've included a sample
<filename>.emacs</filename> file, which should be enough to
get you started. Just copy it into your home directory and
restart Emacs if it's already running; it will read the
commands from the file and (hopefully) give you a useful basic
setup.</para>
</sect2>
<sect2>
<title>A sample <filename>.emacs</filename> file</title>
<para>Unfortunately, there's far too much here to explain it in
detail; however there are one or two points worth
mentioning.</para>
<itemizedlist>
<listitem>
<para>Everything beginning with a <literal>;</literal> is a comment
and is ignored by Emacs.</para>
</listitem>
<listitem>
<para>In the first line, the
<literal>-*- Emacs-Lisp -*-</literal> is so that
we can edit the <filename>.emacs</filename> file itself
within Emacs and get all the fancy features for editing
Emacs Lisp. Emacs usually tries to guess this based on
the filename, and may not get it right for
<filename>.emacs</filename>.</para>
</listitem>
<listitem>
<para>The <keysym>tab</keysym> key is bound to an
indentation function in some modes, so when you press the
tab key, it will indent the current line of code. If you
want to put a <token>tab</token> character in whatever
you're writing, hold the <keysym>control</keysym> key down
while you're pressing the <keysym>tab</keysym> key.</para>
</listitem>
<listitem>
<para>This file supports syntax highlighting for C, C++,
Perl, Lisp and Scheme, by guessing the language from the
filename.</para>
</listitem>
<listitem>
<para>Emacs already has a pre-defined function called
<function>next-error</function>. In a compilation output
window, this allows you to move from one compilation error
to the next by doing <command>M-n</command>; we define a
complementary function,
<function>previous-error</function>, that allows you to go
to a previous error by doing <command>M-p</command>. The
nicest feature of all is that <command>C-c C-c</command>
will open up the source file in which the error occurred
and jump to the appropriate line.</para>
</listitem>
<listitem>
<para>We enable Emacs's ability to act as a server, so that
if you're doing something outside Emacs and you want to
edit a file, you can just type in</para>
<screen>&prompt.user; <userinput>emacsclient <replaceable>filename</replaceable></userinput>
</screen>
<para>and then you can edit the file in your
Emacs!
<footnote>
<para>Many Emacs users set their <systemitem
class=environvar>EDITOR</systemitem> environment to
<literal>emacsclient</literal> so this happens every
time they need to edit a file.</para>
</footnote></para>
</listitem>
</itemizedlist>
<example>
<title>A sample <filename>.emacs</filename> file</title>
<programlisting>;; -*-Emacs-Lisp-*-
;; This file is designed to be re-evaled; use the variable first-time
;; to avoid any problems with this.
(defvar first-time t
"Flag signifying this is the first time that .emacs has been evaled")
;; Meta
(global-set-key "\M- " 'set-mark-command)
(global-set-key "\M-\C-h" 'backward-kill-word)
(global-set-key "\M-\C-r" 'query-replace)
(global-set-key "\M-r" 'replace-string)
(global-set-key "\M-g" 'goto-line)
(global-set-key "\M-h" 'help-command)
;; Function keys
(global-set-key [f1] 'manual-entry)
(global-set-key [f2] 'info)
(global-set-key [f3] 'repeat-complex-command)
(global-set-key [f4] 'advertised-undo)
(global-set-key [f5] 'eval-current-buffer)
(global-set-key [f6] 'buffer-menu)
(global-set-key [f7] 'other-window)
(global-set-key [f8] 'find-file)
(global-set-key [f9] 'save-buffer)
(global-set-key [f10] 'next-error)
(global-set-key [f11] 'compile)
(global-set-key [f12] 'grep)
(global-set-key [C-f1] 'compile)
(global-set-key [C-f2] 'grep)
(global-set-key [C-f3] 'next-error)
(global-set-key [C-f4] 'previous-error)
(global-set-key [C-f5] 'display-faces)
(global-set-key [C-f8] 'dired)
(global-set-key [C-f10] 'kill-compilation)
;; Keypad bindings
(global-set-key [up] "\C-p")
(global-set-key [down] "\C-n")
(global-set-key [left] "\C-b")
(global-set-key [right] "\C-f")
(global-set-key [home] "\C-a")
(global-set-key [end] "\C-e")
(global-set-key [prior] "\M-v")
(global-set-key [next] "\C-v")
(global-set-key [C-up] "\M-\C-b")
(global-set-key [C-down] "\M-\C-f")
(global-set-key [C-left] "\M-b")
(global-set-key [C-right] "\M-f")
(global-set-key [C-home] "\M-<")
(global-set-key [C-end] "\M->")
(global-set-key [C-prior] "\M-<")
(global-set-key [C-next] "\M->")
;; Mouse
(global-set-key [mouse-3] 'imenu)
;; Misc
(global-set-key [C-tab] "\C-q\t") ; Control tab quotes a tab.
(setq backup-by-copying-when-mismatch t)
;; Treat 'y' or <CR> as yes, 'n' as no.
(fset 'yes-or-no-p 'y-or-n-p)
(define-key query-replace-map [return] 'act)
(define-key query-replace-map [?\C-m] 'act)
;; Load packages
(require 'desktop)
(require 'tar-mode)
;; Pretty diff mode
(autoload 'ediff-buffers "ediff" "Intelligent Emacs interface to diff" t)
(autoload 'ediff-files "ediff" "Intelligent Emacs interface to diff" t)
(autoload 'ediff-files-remote "ediff"
"Intelligent Emacs interface to diff")
(if first-time
(setq auto-mode-alist
(append '(("\\.cpp$" . c++-mode)
("\\.hpp$" . c++-mode)
("\\.lsp$" . lisp-mode)
("\\.scm$" . scheme-mode)
("\\.pl$" . perl-mode)
) auto-mode-alist)))
;; Auto font lock mode
(defvar font-lock-auto-mode-list
(list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'lisp-mode 'perl-mode 'scheme-mode)
"List of modes to always start in font-lock-mode")
(defvar font-lock-mode-keyword-alist
'((c++-c-mode . c-font-lock-keywords)
(perl-mode . perl-font-lock-keywords))
"Associations between modes and keywords")
(defun font-lock-auto-mode-select ()
"Automatically select font-lock-mode if the current major mode is
in font-lock-auto-mode-list"
(if (memq major-mode font-lock-auto-mode-list)
(progn
(font-lock-mode t))
)
)
(global-set-key [M-f1] 'font-lock-fontify-buffer)
;; New dabbrev stuff
;(require 'new-dabbrev)
(setq dabbrev-always-check-other-buffers t)
(setq dabbrev-abbrev-char-regexp "\\sw\\|\\s_")
(add-hook 'emacs-lisp-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) nil)
(set (make-local-variable 'dabbrev-case-replace) nil)))
(add-hook 'c-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) nil)
(set (make-local-variable 'dabbrev-case-replace) nil)))
(add-hook 'text-mode-hook
'(lambda ()
(set (make-local-variable 'dabbrev-case-fold-search) t)
(set (make-local-variable 'dabbrev-case-replace) t)))
;; C++ and C mode...
(defun my-c++-mode-hook ()
(setq tab-width 4)
(define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key c++-mode-map "\C-ce" 'c-comment-edit)
(setq c++-auto-hungry-initial-state 'none)
(setq c++-delete-function 'backward-delete-char)
(setq c++-tab-always-indent t)
(setq c-indent-level 4)
(setq c-continued-statement-offset 4)
(setq c++-empty-arglist-indent 4))
(defun my-c-mode-hook ()
(setq tab-width 4)
(define-key c-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key c-mode-map "\C-ce" 'c-comment-edit)
(setq c-auto-hungry-initial-state 'none)
(setq c-delete-function 'backward-delete-char)
(setq c-tab-always-indent t)
;; BSD-ish indentation style
(setq c-indent-level 4)
(setq c-continued-statement-offset 4)
(setq c-brace-offset -4)
(setq c-argdecl-indent 0)
(setq c-label-offset -4))
;; Perl mode
(defun my-perl-mode-hook ()
(setq tab-width 4)
(define-key c++-mode-map "\C-m" 'reindent-then-newline-and-indent)
(setq perl-indent-level 4)
(setq perl-continued-statement-offset 4))
;; Scheme mode...
(defun my-scheme-mode-hook ()
(define-key scheme-mode-map "\C-m" 'reindent-then-newline-and-indent))
;; Emacs-Lisp mode...
(defun my-lisp-mode-hook ()
(define-key lisp-mode-map "\C-m" 'reindent-then-newline-and-indent)
(define-key lisp-mode-map "\C-i" 'lisp-indent-line)
(define-key lisp-mode-map "\C-j" 'eval-print-last-sexp))
;; Add all of the hooks...
(add-hook 'c++-mode-hook 'my-c++-mode-hook)
(add-hook 'c-mode-hook 'my-c-mode-hook)
(add-hook 'scheme-mode-hook 'my-scheme-mode-hook)
(add-hook 'emacs-lisp-mode-hook 'my-lisp-mode-hook)
(add-hook 'lisp-mode-hook 'my-lisp-mode-hook)
(add-hook 'perl-mode-hook 'my-perl-mode-hook)
;; Complement to next-error
(defun previous-error (n)
"Visit previous compilation error message and corresponding source code."
(interactive "p")
(next-error (- n)))
;; Misc...
(transient-mark-mode 1)
(setq mark-even-if-inactive t)
(setq visible-bell nil)
(setq next-line-add-newlines nil)
(setq compile-command "make")
(setq suggest-key-bindings nil)
(put 'eval-expression 'disabled nil)
(put 'narrow-to-region 'disabled nil)
(put 'set-goal-column 'disabled nil)
;; Elisp archive searching
(autoload 'format-lisp-code-directory "lispdir" nil t)
(autoload 'lisp-dir-apropos "lispdir" nil t)
(autoload 'lisp-dir-retrieve "lispdir" nil t)
(autoload 'lisp-dir-verify "lispdir" nil t)
;; Font lock mode
(defun my-make-face (face colour &optional bold)
"Create a face from a colour and optionally make it bold"
(make-face face)
(copy-face 'default face)
(set-face-foreground face colour)
(if bold (make-face-bold face))
)
(if (eq window-system 'x)
(progn
(my-make-face 'blue "blue")
(my-make-face 'red "red")
(my-make-face 'green "dark green")
(setq font-lock-comment-face 'blue)
(setq font-lock-string-face 'bold)
(setq font-lock-type-face 'bold)
(setq font-lock-keyword-face 'bold)
(setq font-lock-function-name-face 'red)
(setq font-lock-doc-string-face 'green)
(add-hook 'find-file-hooks 'font-lock-auto-mode-select)
(setq baud-rate 1000000)
(global-set-key "\C-cmm" 'menu-bar-mode)
(global-set-key "\C-cms" 'scroll-bar-mode)
(global-set-key [backspace] 'backward-delete-char)
; (global-set-key [delete] 'delete-char)
(standard-display-european t)
(load-library "iso-transl")))
;; X11 or PC using direct screen writes
(if window-system
(progn
;; (global-set-key [M-f1] 'hilit-repaint-command)
;; (global-set-key [M-f2] [?\C-u M-f1])
(setq hilit-mode-enable-list
'(not text-mode c-mode c++-mode emacs-lisp-mode lisp-mode
scheme-mode)
hilit-auto-highlight nil
hilit-auto-rehighlight 'visible
hilit-inhibit-hooks nil
hilit-inhibit-rebinding t)
(require 'hilit19)
(require 'paren))
(setq baud-rate 2400) ; For slow serial connections
)
;; TTY type terminal
(if (and (not window-system)
(not (equal system-type 'ms-dos)))
(progn
(if first-time
(progn
(keyboard-translate ?\C-h ?\C-?)
(keyboard-translate ?\C-? ?\C-h)))))
;; Under UNIX
(if (not (equal system-type 'ms-dos))
(progn
(if first-time
(server-start))))
;; Add any face changes here
(add-hook 'term-setup-hook 'my-term-setup-hook)
(defun my-term-setup-hook ()
(if (eq window-system 'pc)
(progn
;; (set-face-background 'default "red")
)))
;; Restore the "desktop" - do this as late as possible
(if first-time
(progn
(desktop-load-default)
(desktop-read)))
;; Indicate that this file has been read at least once
(setq first-time nil)
;; No need to debug anything now
(setq debug-on-error nil)
;; All done
(message "All done, %s%s" (user-login-name) ".")
</programlisting>
</example>
</sect2>
<sect2>
<title>Extending the Range of Languages Emacs Understands</title>
<para>Now, this is all very well if you only want to program in
the languages already catered for in the
<filename>.emacs</filename> file (C, C++, Perl, Lisp and
Scheme), but what happens if a new language called
<quote>whizbang</quote> comes out, full of exciting
features?</para>
<para>The first thing to do is find out if whizbang comes with
any files that tell Emacs about the language. These usually
end in <filename>.el</filename>, short for <quote>Emacs
Lisp</quote>. For example, if whizbang is a FreeBSD port, we
can locate these files by doing</para>
<screen>&prompt.user; <userinput>find /usr/ports/lang/whizbang -name "*.el" -print</userinput>
</screen>
<para>and install them by copying them into the Emacs site Lisp
directory. On FreeBSD 2.1.0-RELEASE, this is
<filename>/usr/local/share/emacs/site-lisp</filename>.</para>
<para>So for example, if the output from the find command
was</para>
<screen>/usr/ports/lang/whizbang/work/misc/whizbang.el
</screen>
<para>we would do</para>
<screen>&prompt.root; <userinput>cp /usr/ports/lang/whizbang/work/misc/whizbang.el /usr/local/share/emacs/site-lisp</userinput>
</screen>
<para>Next, we need to decide what extension whizbang source
files have. Let's say for the sake of argument that they all
end in <filename>.wiz</filename>. We need to add an entry to
our <filename>.emacs</filename> file to make sure Emacs will
be able to use the information in
<filename>whizbang.el</filename>.</para>
<para>Find the <symbol>auto-mode-alist entry</symbol> in
<filename>.emacs</filename> and add a line for whizbang, such
as:</para>
<programlisting><lineannotation>…</>
("\\.lsp$" . lisp-mode)
("\\.wiz$" . whizbang-mode)
("\\.scm$" . scheme-mode)
<lineannotation>…</>
</programlisting>
<para>This means that Emacs will automatically go into
<function>whizbang-mode</function> when you edit a file ending
in <filename>.wiz</filename>.</para>
<para>Just below this, you'll find the
<symbol>font-lock-auto-mode-list</symbol> entry. Add
<function>whizbang-mode</function> to it like so:</para>
<programlisting>;; Auto font lock mode
(defvar font-lock-auto-mode-list
(list 'c-mode 'c++-mode 'c++-c-mode 'emacs-lisp-mode 'whizbang-mode 'lisp-mode 'perl-mode 'scheme-mode)
"List of modes to always start in font-lock-mode")
</programlisting>
<para>This means that Emacs will always enable
<function>font-lock-mode</function> (ie syntax highlighting)
when editing a <filename>.wiz</filename> file.</para>
<para>And that's all that's needed. If there's anything else
you want done automatically when you open up a
<filename>.wiz</filename> file, you can add a
<function>whizbang-mode hook</function> (see
<function>my-scheme-mode-hook</function> for a simple example
that adds <function>auto-indent</function>).</para>
</sect2>
</sect1>
<sect1>
<title>Further Reading</title>
<itemizedlist>
<listitem>
<para>Brian Harvey and Matthew Wright
<emphasis>Simply Scheme</emphasis>
MIT 1994.<!-- <br> -->
ISBN 0-262-08226-8</para>
</listitem>
<listitem>
<para>Randall Schwartz
<emphasis>Learning Perl</emphasis>
O'Reilly 1993<!-- <br> -->
ISBN 1-56592-042-2</para>
</listitem>
<listitem>
<para>Patrick Henry Winston and Berthold Klaus Paul Horn
<emphasis>Lisp (3rd Edition)</emphasis>
Addison-Wesley 1989<!-- <br> -->
ISBN 0-201-08319-1</para>
</listitem>
<listitem>
<para>Brian W. Kernighan and Rob Pike
<emphasis>The Unix Programming Environment</emphasis>
Prentice-Hall 1984<!-- <br> -->
ISBN 0-13-937681-X</para>
</listitem>
<listitem>
<para>Brian W. Kernighan and Dennis M. Ritchie
<emphasis>The C Programming Language (2nd Edition)</emphasis>
Prentice-Hall 1988<!-- <br> -->
ISBN 0-13-110362-8</para>
</listitem>
<listitem>
<para>Bjarne Stroustrup
<emphasis>The C++ Programming Language</emphasis>
Addison-Wesley 1991<!-- <br> -->
ISBN 0-201-53992-6</para>
</listitem>
<listitem>
<para>W. Richard Stevens
<emphasis>Advanced Programming in the Unix Environment</emphasis>
Addison-Wesley 1992<!-- <br> -->
ISBN 0-201-56317-7</para>
</listitem>
<listitem>
<para>W. Richard Stevens
<emphasis>Unix Network Programming</emphasis>
Prentice-Hall 1990<!-- <br> -->
ISBN 0-13-949876-1</para>
</listitem>
</itemizedlist>
</sect1>
</article>