## A User's Guide to FreeBSD Programming Tools

James
Raynard

    jraynard@freebsd.org

August 17, 1997
1997
James Raynard

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 not 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

## Introduction

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 `sed and `awk. If that is not enough, there are many more compilers and interpreters in the Ports collection. FreeBSD is very compatible with standards such as POSIX and ANSI 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.``

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.

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!

## Introduction to Programming

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 commands as they are usually called. One way uses an interpreter, the other a compiler. 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.

### Interpreters

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.

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.

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 (`sh, `csh) is itself an interpreter, and many people do in fact write shell scripts to help with various housekeeping 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.``

### Interpreters available with FreeBSD

Here is a list of interpreters that are available as FreeBSD packages, with a brief discussion of some of the more popular interpreted languages.

To get one of these packages, all you need to do is to click on the hotlink for the package, then run

    $ pkg_add package name

as root. Obviously, you will need to have a fully functional FreeBSD 2.1.0 or later system for the package to work!

BASIC

  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, BASIC has been the first programming language for many programmers. It's also the foundation for Visual Basic.

  The Bywater Basic Interpreter and the Phil Cockroft's Basic Interpreter (formerly Rabbit Basic) are available as FreeBSD FreeBSD packages

Lisp

  A language that was developed in the late 1950s as an alternative to the number-crunching 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 List Processing. Very popular in AI (Artificial Intelligence) circles.

  Lisp is an extremely powerful and sophisticated language, but can be rather large and unwieldy.

  FreeBSD has GNU Common Lisp available as a package.

Perl

  Very popular with system administrators for writing scripts; also often used on World Wide Web servers for writing CGI scripts.

  Version 4, which is probably still the most widely-used version, comes with FreeBSD; the newer Perl Version 5 is available as a package.

Scheme

  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.

  FreeBSD has packages of the Elk Scheme Interpreter, the MIT Scheme Interpreter and the SCM Scheme Interpreter.

Icon

  The Icon Programming Language.

Logo

  Brian Harvey's LOGO Interpreter.

Python

  The Python Object-Oriented Programming Language

### Compilers

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. If you run it in the shell, you may get a core dump.

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.

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.

As the edit-compile-run-debug cycle is rather tedious when using separate programs, many commercial compiler makers have produced Integrated Development Environments (IDEs for short). FreeBSD does not have an IDE as such; however it is possible to use Emacs for this purpose. This is discussed in .

## Compiling with `cc`

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 `cc or `gcc. 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.``

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.

1. Pre-process your source code to remove comments and do other tricks like expanding macros in C.

2. Check the syntax of your code to see if you have obeyed the rules of the language. If you have not, it will complain!

3. Convert the source code into assembly language—this is very close to machine code, but still understandable by humans. Allegedly. To be strictly accurate, `cc converts the source code into its own, machine-independent p-code instead of assembly language at this stage.`

4. Convert the assembly language into machine code—yep, we are talking bits and bytes, ones and zeros here.

5. 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.

6. If you are trying to produce an executable from several source code files, work out how to fit them all together.

7. Work out how to produce something that the system's run-time loader will be able to load into memory and run.

8. Finally, write the executable on the file system.

The word compiling is often used to refer to just steps 1 to 4—the others are referred to as linking. Sometimes step 1 is referred to as pre-processing and steps 3-4 as assembling.

Fortunately, almost all this detail is hidden from you, as `cc is a front end that manages calling all these programs with the right arguments for you; simply typing`

    $ cc foobar.c

will cause `foobar.c to be compiled by all the steps above. If you have more than one file to compile, just do something like`

    $ cc foo.c bar.c

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. In case you didn't know, a binary sort is an efficient way of sorting things into order and a bubble sort isn't.

There are lots and lots of options for `cc, which are all in the man page. Here are a few of the most important ones, with examples of how to use them.`

`-o filename`

  The output name of the file. If you do not use this option, `cc will produce an executable called `a.out. The reasons for this are buried in the mists of history.``

      $ cc foobar.c               executable is a.out
      $ cc -o foobar foobar.c     executable is foobar

`-c`

  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 `Makefile`.

      $ cc -c foobar.c

  This will produce an object file (not an executable) called `foobar.o`. This can be linked together with other object files into an executable.

`-g`

  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 very useful; the disadvantage is that all this extra information makes the program much bigger. Normally, you compile with `-g` while you are developing a program and then compile a release version without `-g` when you're satisfied it works properly.

      $ cc -g foobar.c

  This will produce a debug version of the program. Note, we didn't use the `-o` flag to specify the executable name, so we will get an executable called `a.out`. Producing a debug version called `foobar` is left as an exercise for the reader!

`-O`

  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 `-O` to specify a higher level of optimisation, but this often exposes bugs in the compiler's optimiser. For instance, the version of `cc` that comes with the 2.1.0 release of FreeBSD is known to produce bad code with the `-O2` option in some circumstances.

  Optimisation is usually only turned on when compiling a release version.

      $ cc -O -o foobar foobar.c

  This will produce an optimised version of `foobar`.

The following three flags will force `cc` to check that your code complies to the relevant international standard, often referred to as the ANSI standard, though strictly speaking it is an ISO standard.

`-Wall`

  Enable all the warnings which the authors of `cc` believe are worthwhile. Despite the name, it will not enable all the warnings `cc` is capable of.

`-ansi`

  Turn off most, but not all, of the non-ANSI C features provided by `cc`. Despite the name, it does not guarantee strictly that your code will comply to the standard.

`-pedantic`

  Turn off all `cc`'s non-ANSI C features.

Without these flags, `cc` 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 portable code.

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?

    $ cc -Wall -ansi -pedantic -o foobar foobar.c

This will produce an executable `foobar` after checking `foobar.c` for standard compliance.

`-llibrary`

  Specify a function library to be used during when linking.

  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.

  The rule is that if the library is called `libsomething.a`, you give `cc` the argument `-lsomething`. For example, the math library is `libm.a`, so you give `cc` the argument `-lm`. A common gotcha with the math library is that it has to be the last library on the command line.

      $ cc -o foobar foobar.c -lm

  This will link the math library functions into `foobar`.

  If you are compiling C++ code, you need to add `-lg++`, or `-lstdc++` if you are using FreeBSD 2.2 or later, to the command line argument to link the C++ library functions. Alternatively, you can run `c++` instead of `cc`, which does this for you. `c++` can also be invoked as `g++` on FreeBSD.

      $ cc -o foobar foobar.cc -lg++     For FreeBSD 2.1.6 and earlier
      $ cc -o foobar foobar.cc -lstdc++  For FreeBSD 2.2 and later
      $ c++ -o foobar foobar.cc

  Each of these will both produce an executable `foobar` from the C++ source file `foobar.cc`. Note that, on Unix systems, C++ source files traditionally end in `.C`, `.cxx` or `.cc`, rather than the MS-DOS style `.cpp` (which was already used for something else). `gcc` 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 `.cpp` with impunity!

### Common `cc` Queries and Problems

Q. I am trying to write a program which uses the `sin()` function and I get an error like this. What does it mean?

    /var/tmp/cc0143941.o: Undefined symbol `_sin' referenced from text segment

A. When using mathematical functions like `sin()`, you have to tell `cc` to link in the math library, like so:

    $ cc -o foobar foobar.c -lm

Q. All right, I wrote this simple program to practice using `-lm`. All it does is raise 2.1 to the power of 6.

    #include <stdio.h>

    int main() {
    	float f;

    	f = pow(2.1, 6);
    	printf("2.1 ^ 6 = %f\n", f);
    	return 0;
    }

and I compiled it as:

    $ cc temp.c -lm

like you said I should, but I get this when I run it:

    $ ./a.out
    2.1 ^ 6 = 1023.000000

This is not the right answer! What is going on?

A. 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 int, which is definitely not what you want here.

Q. So how do I fix this?

A. The prototypes for the mathematical functions are in `math.h`. If you include this file, the compiler will be able to find the prototype and it will stop doing strange things to your calculation!

    #include <math.h>
    #include <stdio.h>

    int main() {
    ...

After recompiling it as you did before, run it:

    $ ./a.out
    2.1 ^ 6 = 85.766121

If you are using any of the mathematical functions, always include `math.h` and remember to link in the math library.

Q. I compiled a file called `foobar.c` and I cannot find an executable called `foobar`. Where's it gone?

A. Remember, `cc` will call the executable `a.out` unless you tell it differently. Use the `-o filename` option:

    $ cc -o foobar foobar.c

Q. OK, I have an executable called `foobar`, I can see it when I run `ls`, but when I type in `foobar` at the command prompt it tells me there is no such file. Why can it not find it?

A. Unlike MS-DOS, 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 `./foobar`, which means run the file called `foobar` in the current directory, or change your PATH environment variable so that it looks something like

    bin:/usr/bin:/usr/local/bin:.

The dot at the end means look in the current directory if it is not in any of the others.

Q. I called my executable `test`, but nothing happens when I run it. What is going on?

A. Most Unix systems have a program called `test` in `/usr/bin` and the shell is picking that one up before it gets to checking the current directory. Either type:

    $ ./test

or choose a better name for your program!

Q. I compiled my program and it seemed to run all right at first, then there was an error and it said something about core dumped. What does that mean?

A. The name core dump 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 `core`, which the programmer could then pore over to find out what went wrong.

Q. Fascinating stuff, but what I am supposed to do now?

A. Use `gdb` to analyse the core (see ).

Q. When my program dumped core, it said something about a segmentation fault. What's that?

A. 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.

Common causes for this are:

- Trying to write to a NULL pointer, eg

      char *foo = NULL;
      strcpy(foo, "bang!");

- Using a pointer that hasn't been initialised, eg

      char *foo;
      strcpy(foo, "bang!");

  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.

- Trying to access past the end of an array, eg

      int bar[20];
      bar[27] = 6;

- Trying to store something in read-only memory, eg

      char *foo = "My string";
      strcpy(foo, "bang!");

  Unix compilers often put string literals like `"My string"` into read-only areas of memory.

- Doing naughty things with `malloc()` and `free()`, eg

      char bar[80];
      free(bar);

  or

      char *foo = malloc(27);
      free(foo);
      free(foo);

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.

Q. Sometimes when I get a core dump it says bus error. It says in my Unix book that this means a hardware problem, but the computer still seems to be working. Is this true?

A. 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.

Q. 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?

A. Yes, just go to another console or xterm, do

    $ ps

to find out the process ID of your program, and do

    $ kill -ABRT pid

where `pid` is the process ID you looked up.

This is useful if your program has got stuck in an infinite loop, for instance. If your program happens to trap SIGABRT, there are several other signals which have a similar effect.

## Make

### What is `make`?

When you're working on a simple program with only one or two source files, typing in

    $ cc file1.c file2.c

is not too bad, but it quickly becomes very tedious when there are several files—and it can take a while to compile, too.

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:

    $ cc file1.o file2.o … file37.c &hellip

if we'd changed `file37.c`, 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.

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.

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?

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.

This program is called `make`. It reads in a file, called a makefile, 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 if `fromboz.o` is older than `fromboz.c`, that means someone must have changed `fromboz.c`, so it needs to be re-compiled. The makefile also has rules telling make how to re-compile the source file, making it a much more powerful tool.

Makefiles are typically kept in the same directory as the source they apply to, and can be called `makefile`, `Makefile` or `MAKEFILE`. Most programmers use the name `Makefile`, as this puts it near the top of a directory listing, where it can easily be seen. They don't use the `MAKEFILE` form as block capitals are often used for documentation files like `README`.

### Example of using `make`

Here's a very simple make file:

    foo: foo.c
    	cc -o foo foo.c

It consists of two lines, a dependency line and a creation line.

The dependency line here consists of the name of the program (known as the target), followed by a colon, then whitespace, then the name of the source file. When `make` reads this line, it looks to see if `foo` exists; if it exists, it compares the time `foo` was last modified to the time `foo.c` was last modified. If `foo` does not exist, or is older than `foo.c`, it then looks at the creation line to find out what to do. In other words, this is the rule for working out when `foo.c` needs to be re-compiled.

The creation line starts with a tab (press the tab key) and then the command you would type to create `foo` if you were doing it at a command prompt. If `foo` is out of date, or does not exist, `make` then executes this command to create it. In other words, this is the rule which tells make how to re-compile `foo.c`.

So, when you type `make`, it will make sure that `foo` is up to date with respect to your latest changes to `foo.c`. This principle can be extended to `Makefile`s with hundreds of targets—in fact, on FreeBSD, it is possible to compile the entire operating system just by typing `make world` in the appropriate directory!

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:

    foo: foo.c
    	cc -o foo foo.c

    install:
    	cp foo /home/me

We can tell make which target we want to make by typing:

    $ make target

`make` will then only look at that target and ignore any others. For example, if we type `make foo` with the makefile above, make will ignore the install target.

If we just type `make` on its own, make will always look at the first target and then stop without looking at any others. So if we typed `make` here, it will just go to the foo target, re-compile `foo` if necessary, and then stop without going on to the install target.

Notice that the install 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 `make install`. In this case, it will copy `foo` 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.

This is a slightly confusing subject to try and explain. If you don't quite understand how `make` works, the best thing to do is to write a simple program like hello world 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 `touch` command is very useful here—it changes the date on a file without you having to edit it.

### FreeBSD Makefiles

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 `Makefile`:

    MASTER_SITES=   ftp://freefall.cdrom.com/pub/FreeBSD/LOCAL_PORTS/
    DISTFILES=      scheme-microcode+dist-7.3-freebsd.tgz

    .include <bsd.port.mk>

Now, if we go to the directory for this port and type `make`, the following happens:

1. A check is made to see if the source code for this port is already on the system.

2. If it isn't, an FTP connection to the URL in MASTER_SITES is set up to download the source.

3. 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.

4. Any changes required to make the source work on FreeBSD are applied—this is known as patching.

5. 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).

6. The source code for the program is compiled. In effect, we change to the directory where the source was unpacked and do `make`—the program's own make file has the necessary information to build the program.

7. 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 `make install`. 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 package database, so that the port can easily be uninstalled later if we change our mind about it.

Now I think you'll agree that's rather impressive for a four line script!

The secret lies in the last line, which tells `make` to look in the system makefile called `bsd.port.mk`. It's easy to overlook this line, but this is where all the clever stuff comes from—someone has written a makefile that tells `make` 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!

If you want to have a look at these system makefiles, they're in `/usr/share/mk`, 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!)

### More advanced uses of `make`

`Make` is a very powerful tool, and can do much more than the simple example above shows. Unfortunately, there are several different versions of `make`, 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.

The version of make that comes with FreeBSD is the Berkeley make; there is a tutorial for it in `/usr/share/doc/psd/12.make`. To view it, do

    $ zmore paper.ascii.gz

in that directory.

Many applications in the ports use GNU make, which has a very good set of info pages. If you have installed any of these ports, GNU make will automatically have been installed as `gmake`. It's also available as a port and package in its own right.

To view the info pages for GNU make, you will have to edit the `dir` file in the `/usr/local/info` directory to add an entry for it. This involves adding a line like

     * Make: (make).                 The GNU Make utility.

to the file. Once you have done this, you can type `info` and then select make from the menu (or in Emacs, do `C-h i`).

## Debugging

### The Debugger

The debugger that comes with FreeBSD is called `gdb` (GNU debugger). You start it up by typing

    $ gdb progname

although most people prefer to run it inside Emacs. You can do this by:

    M-x gdb RET progname RET

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.

`gdb` 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.

Finally, if you find its text-based command-prompt style off-putting, there's a graphical front-end for it xxgdb in the ports collection.

This section is intended to be an introduction to using `gdb` and does not cover specialised topics such as debugging the kernel.

### Running a program in the debugger

You'll need to have compiled the program with the `-g` option to get the most out of using `gdb`. 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:

    … (no debugging symbols found) …

when `gdb` starts up, you'll know that the program wasn't compiled with the `-g` option.

At the `gdb` prompt, type `break main`. 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 `run` 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 `main()`. (If you've ever wondered where `main()` gets called from, now you know!).

You can now step through the program, a line at a time, by pressing `n`. If you get to a function call, you can step into it by pressing `s`. Once you're in a function call, you can return from stepping into a function call by pressing `f`. You can also use `up` and `down` to take a quick look at the caller.

Here's a simple example of how to spot a mistake in a program with `gdb`. This is our program (with a deliberate mistake):

    #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;
    }

This program sets i to be `5` and passes it to a function `bazz()` which prints out the number we gave it.

When we compile and run the program we get

    $ cc -g -o temp temp.c
    $ ./temp
    This is my program
    anint = 4231

That wasn't what we expected! Time to see what's going on!

    $ gdb temp
    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) break main				Skip the set-up code
    Breakpoint 1 at 0x160f: file temp.c, line 9.	gdb puts breakpoint at main()
    (gdb) run					Run as far as main()
    Starting program: /home/james/tmp/temp		Program starts running

    Breakpoint 1, main () at temp.c:9		gdb stops at main()
    (gdb) n						Go to next line
    This is my program				Program prints out
    (gdb) s						step into bazz()
    bazz (anint=4231) at temp.c:17			gdb displays stack frame
    (gdb)

Hang on a minute! How did anint get to be `4231`? Didn't we set it to be `5` in `main()`? Let's move up to `main()` and have a look.

    (gdb) up					Move up call stack
    #1  0x1625 in main () at temp.c:11		gdb displays stack frame
    (gdb) p i					Show us the value of i
    $1 = 4231					gdb displays 4231

Oh dear! Looking at the code, we forgot to initialise i. We meant to put

    …
    main() {
    	int i;

    	i = 5;
    	printf("This is my program\n");
    &hellip

but we left the `i=5;` line out. As we didn't initialise i, it had whatever number happened to be in that area of memory when the program ran, which in this case happened to be `4231`.

`gdb` displays the stack frame every time we go into or out of a function, even if we're using `up` and `down` 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).

### Examining a core file

A core file is basically a file which contains the complete state of the process when it crashed. In the good old days, 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 `progname.core instead of just `core`, to make it clearer which program a core file belongs to.`

To examine a core file, start up `gdb` in the usual way. Instead of typing `break` or `run`, type

    (gdb) core progname.core

If you're not in the same directory as the core file, you'll have to do `dir /path/to/core/file` first.

You should see something like this:

    $ gdb a.out
    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) core a.out.core
    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)

In this case, the program was called `a.out`, so the core file is called `a.out.core`. 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 `bazz`.

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 `bt` command causes `gdb` to print out a back-trace of the call stack:

    (gdb) bt
    #0  0x164a in bazz (anint=0x5) at temp.c:17
    #1  0xefbfd888 in end ()
    #2  0x162c in main () at temp.c:11
    (gdb)

The `end()` function is called when a program crashes; in this case, the `bazz()` function was called from `main()`.

### Attaching to a running program

One of the neatest features about `gdb` 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.

What you do is start up another `gdb`, use `ps` to find the process ID for the child, and do

    (gdb) attach pid

in `gdb`, and then debug as usual.

That's all very well, you're probably thinking, but by the time I've done that, the child process will be over the hill and far away. Fear not, gentle reader, here's how to do it (courtesy of the `gdb` info pages):

    &hellip
    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 */
    	&hellip
    } else {			/* parent */
    	&hellip

Now all you have to do is attach to the child, set PauseMode to `0`, and wait for the `sleep()` call to return!

## Using Emacs as a Development Environment

### Emacs

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. At least, not unless you pay out very large sums of money. 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.

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.

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.

It's impossible even to summarise everything Emacs can do here, but here are some of the features of interest to developers:

- Very powerful editor, allowing search-and-replace on both strings and regular expressions (patterns), jumping to start/end of block expression, etc, etc.

- Pull-down menus and online help.

- Language-dependent syntax highlighting and indentation.

- Completely customisable.

- You can compile and debug programs within Emacs.

- On a compilation error, you can jump to the offending line of source code.

- Friendly-ish front-end to the `info` program used for reading GNU hypertext documentation, including the documentation on Emacs itself.

- Friendly front-end to `gdb`, allowing you to look at the source code as you step through your program.

- You can read Usenet news and mail while your program is compiling.

And doubtless many more that I've overlooked.

Emacs can be installed on FreeBSD using the Emacs port.

Once it's installed, start it up and do `C-h t` to read an Emacs tutorial—that means hold down the control key, press h, let go of the control key, and then press t. (Alternatively, you can you use the mouse to select Emacs Tutorial from the Help menu).

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 `M-x replace-s RET foo RET bar RET` 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.

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.

If you can't remember what a particular combination of keys does, select Describe Key from the Help menu and type it in—Emacs will tell you what it does. You can also use the Command Apropos menu item to find out all the commands which contain a particular word in them, with the key binding next to it.

By the way, the expression above means hold down the Meta key, press x, release the Meta key, type `replace-s` (short for `replace-string`—another feature of Emacs is that you can abbreviate commands), press the return key, type `foo` (the string you want replaced), press the return key, type bar (the string you want to replace `foo` with) and press return again. Emacs will then do the search-and-replace operation you've just requested.

If you're wondering what on earth the Meta key is, it's a special key that many Unix workstations have. Unfortunately, PC's don't have one, so it's usually the alt key (or if you're unlucky, the escape key).

Oh, and to get out of Emacs, do `C-x C-c` (that means hold down the control key, press c, press x and release the control 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 `C-z` 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).

### Configuring Emacs

Emacs does many wonderful things; some of them are built in, some of them need to be configured.

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!).

The best way to learn Emacs Lisp is to download the Emacs Tutorial

However, there's no need to actually know any Lisp to get started with configuring Emacs, as I've included a sample `.emacs` 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.

### A sample `.emacs` file

Unfortunately, there's far too much here to explain it in detail; however there are one or two points worth mentioning.

- Everything beginning with a `; is a comment and is ignored by Emacs.`

- In the first line, the `-*- Emacs-Lisp -*-` is so that we can edit the `.emacs` 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 `.emacs`.

- The tab 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 tab character in whatever you're writing, hold the control key down while you're pressing the tab key.

- This file supports syntax highlighting for C, C++, Perl, Lisp and Scheme, by guessing the language from the filename.

- Emacs already has a pre-defined function called `next-error`. In a compilation output window, this allows you to move from one compilation error to the next by doing `M-n`; we define a complementary function, `previous-error`, that allows you to go to a previous error by doing `M-p`. The nicest feature of all is that `C-c C-c` will open up the source file in which the error occurred and jump to the appropriate line.

- 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

      $ emacsclient filename

  and then you can edit the file in your Emacs! Many Emacs users set their EDITOR environment to `emacsclient` so this happens every time they need to edit a file.

#### A sample `.emacs` file

    ;; -*-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) ".")

### Extending the Range of Languages Emacs Understands

Now, this is all very well if you only want to program in the languages already catered for in the `.emacs` file (C, C++, Perl, Lisp and Scheme), but what happens if a new language called whizbang comes out, full of exciting features?

The first thing to do is find out if whizbang comes with any files that tell Emacs about the language. These usually end in `.el`, short for Emacs Lisp. For example, if whizbang is a FreeBSD port, we can locate these files by doing

    $ find /usr/ports/lang/whizbang -name "*.el" -print

and install them by copying them into the Emacs site Lisp directory. On FreeBSD 2.1.0-RELEASE, this is `/usr/local/share/emacs/site-lisp`.

So for example, if the output from the find command was

    /usr/ports/lang/whizbang/work/misc/whizbang.el

we would do

    $ cp /usr/ports/lang/whizbang/work/misc/whizbang.el /usr/local/share/emacs/site-lisp

Next, we need to decide what extension whizbang source files have. Let's say for the sake of argument that they all end in `.wiz`. We need to add an entry to our `.emacs` file to make sure Emacs will be able to use the information in `whizbang.el`.

Find the auto-mode-alist entry in `.emacs` and add a line for whizbang, such as:

    …
    ("\\.lsp$" . lisp-mode)
    ("\\.wiz$" . whizbang-mode)
    ("\\.scm$" . scheme-mode)
    …

This means that Emacs will automatically go into `whizbang-mode` when you edit a file ending in `.wiz`.

Just below this, you'll find the font-lock-auto-mode-list entry. Add `whizbang-mode` to it like so:

    ;; 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")

This means that Emacs will always enable `font-lock-mode` (ie syntax highlighting) when editing a `.wiz` file.

And that's all that's needed. If there's anything else you want done automatically when you open up a `.wiz` file, you can add a `whizbang-mode hook` (see `my-scheme-mode-hook` for a simple example that adds `auto-indent`).

## Further Reading

- Brian Harvey and Matthew Wright Simply Scheme MIT 1994. ISBN 0-262-08226-8

- Randall Schwartz Learning Perl O'Reilly 1993 ISBN 1-56592-042-2

- Patrick Henry Winston and Berthold Klaus Paul Horn Lisp (3rd Edition) Addison-Wesley 1989 ISBN 0-201-08319-1

- Brian W. Kernighan and Rob Pike The Unix Programming Environment Prentice-Hall 1984 ISBN 0-13-937681-X

- Brian W. Kernighan and Dennis M. Ritchie The C Programming Language (2nd Edition) Prentice-Hall 1988 ISBN 0-13-110362-8

- Bjarne Stroustrup The C++ Programming Language Addison-Wesley 1991 ISBN 0-201-53992-6

- W. Richard Stevens Advanced Programming in the Unix Environment Addison-Wesley 1992 ISBN 0-201-56317-7

- W. Richard Stevens Unix Network Programming Prentice-Hall 1990 ISBN 0-13-949876-1