Wiley Beginning Linux Programming, 4th Edition
Contents
1. code examples in the book you ll need to use an editor There are many to choose from on a typical Linux system The vi editor is popular with many users Both of the authors like Emacs so we suggest you take the time to learn some of the features of this powerful editor Almost all Linux distributions have Emacs as an optional package you can install or you can get it from the GNU website at http www gnu org or a version for graphical environments at the XEmacs site at http www xemacs org To learn more about Emacs you can use its online tutorial To do this start the editor by running the emacs command and then type Ctrl H followed by t for the tutorial Emacs also has its entire manual available When in Emacs type Ctrl H and then i for information Some versions of Emacs may have menus that you can use to access the manual and tutorial fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 7 F Chapter 1 Getting Started The C Compiler On POSIX compliant systems the C compiler is called c89 Historically the C compiler was simply called cc Over the years different vendors have sold UNIX like systems with C compilers with differ ent facilities and options but often still called cc When the POSIX standard was prepared it was impossible to define a standard cc command with which all these vendors would be compatible Instead the committee decided to create a new standard command for the C compiler c89 When this command is p2. give a complete list of all options and commands that a utility supports In either case the manual page is a good place to start The GNU software suite and some other free software use an online documentation system called info You can browse full documentation online using a special program info or via the info command of fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 15 F Chapter 1 Getting Started the emacs editor The benefit of the info system is that you can navigate the documentation using links and cross references to jump directly to relevant sections For the documentation author the info system has the benefit that its files can be automatically generated from the same source as the printed typeset documentation Try It Out Manual Pages and info Let s look for documentation of the GNU C compiler gcc 1 First take a look at the manual page man gcc GCC 1 NAME GNU GCC 1 gcc GNU project C and C compiler SYNOPSIS gcc c S E std standard g pg Olevel Wwarn pedantic Idir Ldir Dmacro defn Umacro foption mmachine option er o outfile infile Only the most useful options are listed here see below for the remainder g accepts mostly the same options as gcc ESCRIPTION When you invoke GCC it normally does preprocessing com pilation assembly and linking The overall options allow you to stop this proce
3. or C Applications Standards Language standards supported by GCC Invoking GCC Command options supported by gcc C Implementation How GCC implements the ISO C specification C Extensions GNU extensions to the C language family C Extensions GNU extensions to the C language Objective C GNU Objective C runtime features Compatibility Binary Compatibility zz Info gcc info gz Top 39 lines Top Welcome to Info version 4 8 Type for help m for menu item You re presented with a long menu of options that you can select to move around a complete text ver sion of the documentation Menu items and a hierarchy of pages allow you to navigate a very large doc ument On paper the GNU C documentation runs to many hundreds of pages The info system also contains its own help page in info form pages of course If you type Ctrl H you ll be presented with some help that includes a tutorial on using info The info program is avail able with many Linux distributions and can be installed on other UNIX systems Summary In this introductory chapter we ve looked at Linux programming and the things Linux holds in com mon with proprietary UNIX systems We ve noted the wide variety of programming systems available to UNIX developers We ve also presented a simple program and library to demonstrate the basic C tools comparing them with their Windows equivalents 16
4. places where system programs are stored These include Q bin Binaries programs used in booting the system m usr bin User binaries standard programs available to users m usr local bin Local binaries programs specific to an installation An administrator s login such as root may use a PATH variable that includes directories where system administration programs are kept such as sbin and usr sbin Optional operating system components and third party applications may be installed in subdirectories of opt and installation programs might add to your PATH variable by way of user install scripts It s not a good idea to delete directories from PATH unless you are sure that you understand what will result if you do Note that Linux like UNIX uses the colon character to separate entries in the PATH variable rather than the semicolon that MS DOS and Windows use UNIX chose first so ask Microsoft why Windows is different not why UNIX is different Here s a sample PATH variable usr local bin bin usr bin home neil bin usr X11R6 bin Here the PATH variable contains entries for the standard program locations the current directory a user s home directory and the X Window System Remember Linux uses a forward slash to separate directory names in a filename rather than the backslash of Windows Again UNIX got there first Text Editors To write and enter the
5. programmer and we assume knowledge of the C programming language Linux Programs Linux applications are represented by two special types of files executables and scripts Executable files are programs that can be run directly by the computer they correspond to Windows exe files Scripts are collections of instructions for another program an interpreter to follow These correspond to Windows bat or cmd files or interpreted BASIC programs Linux doesn t require executables or scripts to have a specific filename or any extension whatsoever File system attributes which we discuss in Chapter 2 are used to indicate that a file is a program that may be run In Linux you can replace scripts with compiled programs and vice versa without affecting other programs or the people who call them In fact at the user level there is essentially no difference between the two When you log in to a Linux system you interact with a shell program often bash that runs programs in the same way that the Windows command prompt does It finds the programs you ask for by name by 5 fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 6 F Chapter 1 Getting Started searching for a file with the same name in a given set of directories The directories to search are stored in a shell variable PATH in much the same way as with Windows The search path to which you can add is configured by your system administrator and will usually contain some standard
6. task Examples include libraries of screen handling functions the curses and ncurses libraries and database access routines the dbm library We show you some libraries in later chapters 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 10 F Chapter 1 Getting Started Standard system libraries are usually stored in lib and usr lib The C compiler or more exactly the linker needs to be told which libraries to search because by default it searches only the standard C library This is a remnant of the days when computers were slow and CPU cycles were expensive It s not enough to put a library in the standard directory and hope that the compiler will find it libraries need to follow a very specific naming convention and need to be mentioned on the command line A library filename always starts with 1ib Then follows the part indicating what library this is like c for the C library or m for the mathematical library The last part of the name starts with a dot and specifies the type of the library m a for traditional static libraries Q so for shared libraries see the following The libraries usually exist in both static and shared formats as a quick 1s usr 1ib will show You can instruct the compiler to search a library either by giving it the full path name or by using the 1 flag For example gcc o fred fred c usr lib libm a tells the compiler to compile file fred c call the resulting program file frea and se
7. we hope to convey a flavor of Linux and therefore UNIX programming Although programming in C is in many ways the same whatever the platform UNIX and Linux devel opers have a special view of program and system development The UNIX operating system and hence Linux encourages a certain programming style Following are a few characteristics shared by typical UNIX programs and systems Q Simplicity Many of the most useful UNIX utilities are very simple and as a result small and easy to understand KISS Keep It Small and Simple is a good technique to learn Larger more complex systems are guaranteed to contain larger more complex bugs and debugging is a chore that we d all like to avoid Q Focus It s often better to make a program perform one task well than to throw in every feature along with the kitchen sink A program with feature bloat can be difficult to use and difficult to maintain Programs with a single purpose are easier to improve as better algorithms or inter faces are developed In UNIX small utilities are often combined to perform more demanding tasks when the need arises rather than trying to anticipate a user s needs in one large program Reusable Components Make the core of your application available as a library Well documented libraries with simple but flexible programming interfaces can help others to develop variations or apply the techniques to new application areas Examples include the dbm dat
8. 11 shared libraries are added when the X Window System is installed You can see which shared libraries are required by a program by running the utility 1dd For example if you try running it on your example application you get the following ldd program linux gate so 1 gt 0xffffe000 libc so 6 gt lib libc so 6 0xb7db4000 lib ld linux so 2 0xb7efc000 In this case you see that the standard C library 1ibc is shared so The program requires major Version 6 Other UNIX systems will make similar arrangements for access to shared libraries Refer to your system documentation for details In many ways shared libraries are similar to dynamic link libraries used under Windows The so libraries correspond to DLL files and are required at run time and the a libraries are similar to LIB files included in the program executable Getting Help 14 The vast majority of Linux systems are reasonably well documented with respect to the system program ming interfaces and standard utilities This is true because since the earliest UNIX systems programmers have been encouraged to supply a manual page with their applications These manual pages which are sometimes provided in a printed form are invariably available electronically The man command provides access to the online manual pages The pages vary considerably in quality and detail Some may simply refer the reader to other more thorough documentation whereas others
9. 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 1 F Getting Started In this chapter you discover what Linux is and how it relates to its inspiration UNIX You take a guided tour of the facilities provided by a Linux development system and write and run your first program Along the way you ll be looking at QO UNIX Linux and GNU Programs and programming languages for Linux How to locate development resources Static and shared libraries The UNIX philosophy Q Q Q Q An Introduction to UNIX Linux and GNU In recent years Linux has become a phenomenon Hardly a day goes by without Linux cropping up in the media in some way We ve lost count of the number of applications that have been made available on Linux and the number of organizations that have adopted it including some govern ment departments and city administrations Major hardware vendors like IBM and Dell now sup port Linux and major software vendors like Oracle support their software running on Linux Linux truly has become a viable operating system especially in the server market Linux owes its success to systems and applications that preceded it UNIX and GNU software This section looks at how Linux came to be and what its roots are What Is UNIX The UNIX operating system was originally developed at Bell Laboratories once part of the telecommunications giant AT amp T Designed in the 1970s for Digital Equipment PDP computers UNIX has become a very popula
10. abase library which is a suite of reusable functions rather than a single database management program Q Filters Many UNIX applications can be used as filters That is they transform their input and produce output As you ll see UNIX provides facilities that allow quite complex applications to be developed from other UNIX programs by combining them in novel ways Of course this kind of reuse is enabled by the development methods that we ve previously mentioned Q Open File Formats The more successful and popular UNIX programs use configuration files and data files that are plain ASCII text or XML If either of these is an option for your program development it s a good choice It enables users to use standard tools to change and search for configuration items and to develop new tools for performing new functions on the data files A good example of this is the ctags source code cross reference system which records symbol location information as regular expressions suitable for use by searching programs fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 3 F Chapter 1 Getting Started Q Flexibility You can t anticipate exactly how ingeniously users will use your program Try to be as flexible as possible in your programming Try to avoid arbitrary limits on field sizes or num ber of records If you can write the program so that it s network aware and able to run across a network as well as on a local machine Never assume that you kno
11. amming systems will also have header files that are stored in directories that get searched automatically by the appropriate compiler Examples include usr include X11 for the X Window System and usr include c for GNU C You can use header files in subdirectories or nonstandard places by specifying the 1 flag for include to the C compiler For example gcc I usr openwin include fred c will direct the compiler to look in the directory usr openwin include as well as the standard places for header files included in the red c program Refer to the manual page for the C compiler man gcc for more details It s often convenient to use the grep command to search header files for particular definitions and func tion prototypes Suppose you need to know the name of the defines used for returning the exit status from a program Simply change to the usr include directory and grep for a probable part of the name like this grep EXIT_ h stdlib h define EXIT_FAILURE 1 Failing exit status stdlib h define EXIT_SUCCESS 0 Successful exit status Here grep searches all the files in the directory with a name ending in h for the string EXIT_ In this example it has found among others the definition you need in the file stdlib h Library Files Libraries are collections of precompiled functions that have been written to be reusable Typically they con sist of sets of related functions to perform a common
12. arch the mathematical library in addition to the standard C library to resolve references to functions A similar result is achieved with the following command gcc o fred fred c lm The 1m no space between the 1 and the m is shorthand shorthand is much valued in UNIX circles for the library called 1ibm a in one of the standard library directories in this case usr 1ib An addi tional advantage of the 1m notation is that the compiler will automatically choose the shared library when it exists Although libraries are usually found in standard places in the same way as header files you can add to the search directories by using the L uppercase letter flag to the compiler For example gcc o x11fred L usr openwin lib x11lfred c 1X11 will compile and link a program called x11 red using the version of the library 1ibx11 found in the usr openwin 1ib directory Static Libraries 10 The simplest form of library is just a collection of object files kept together in a ready to use form When a program needs to use a function stored in the library it includes a header file that declares the function The compiler and linker take care of combining the program code and the library into a single executable program You must use the 1 option to indicate which libraries other than the standard C runtime library are required Static libraries also known as archives conventionally have names that end with a Examples are usr li
13. ays of UNIX people wanting to play games on the system often ran them as a out to avoid being caught by system adminis trators and some UNIX installations routinely delete all files called a out every evening Development System Roadmap For a Linux developer it can be important to know a little about where tools and development resources are located The following sections provide a brief look at some important directories and files Applications Applications are usually kept in directories reserved for them Applications supplied by the system for general use including program development are found in usr bin Applications added by system administrators for a specific host computer or local network are often found in usr local bin or opt Administrators favor opt and usr local because they keep vendor supplied files and later addi tions separate from the applications supplied by the system Keeping files organized in this way may help when the time comes to upgrade the operating system because only opt and usr local need be preserved We recommend that you compile your applications to run and access required files from the usr 1local hierarchy for system wide applications For development and personal applications it s best just to use a folder in your home directory Additional features and programming systems may have their own directory structures and program directories Chief among these is the X Window System which is
14. b libc aand usr 1ib 1ibX11 a for the standard C library and the X11 library respectively 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 11 F Chapter 1 Getting Started You can create and maintain your own static libraries very easily by using the ar for archive program and compiling functions separately with gcc c Try to keep functions in separate source files as much as possible If functions need access to common data you can place them in the same source file and use static variables declared in that file Try It Out Static Libraries In this example you create your own small library containing two functions and then use one of them in an example program The functions are called fred and bi11 and just print greetings 1 First create separate source files imaginatively called fred c and bi11 c for each function Here s the first include lt stdio h gt void fred int arg printf fred we passed d n arg And here s the second include lt stdio h gt void bill char arg printf bill we passed s n arg 2 You can compile these functions individually to produce object files ready for inclusion into a library Do this by invoking the C compiler with the c option which prevents the compiler from trying to create a complete program Trying to create a complete program would fail because you haven t defined a function called main gcc c bill c fred c Sls o bill o fred o 3 Nowwritea
15. commonly installed in the usr X11 or usr bin X11 directory Linux distributions typically use the X Org Foundation version of the X Window System based on Revision 7 X11R7 Other UNIX like systems may choose different versions of the X Window System installed in different locations such as usr openwin for Sun s Open Windows provided with Solaris 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 9 F Chapter 1 Getting Started The GNU compiler system s driver program gcc which you used in the preceding programming example is typically located in usr bin or usr local bin but it will run various compiler support applications from another location This location is specified when you compile the compiler itself and varies with the host computer type For Linux systems this location might be a version specific subdirectory of usr 1ib gcc On one of the author s machines at the time of writing it is usr lib gcc i586 suse linux 4 1 3 The separate passes of the GNU C C compiler and GNU specific header files are stored here Header Files For programming in C and other languages you need header files to provide definitions of constants and declarations for system and library function calls For C these are almost always located in usr include and subdirectories thereof You can normally find header files that depend on the particular incarnation of Linux that you are running in usr include sys and usr include linux Other progr
16. ed under the terms of the GNU General Public License GPL you can find a copy of the license at http www gnu org This license embodies the concept of copyleft a takeoff on copyright Copyleft is intended to prevent others from placing restrictions on the use of free software fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 4 F Chapter 1 Getting Started A few major examples of software from the GNU Project distributed under the GPL follow GCC The GNU Compiler Collection containing the GNU C compiler G A C compiler included as part of GCC GDB A source code level debugger GNU make A version of UNIX make Bison A parser generator compatible with UNIX yacc bash A command shell OCocoooo do GNU Emacs A text editor and environment Many other packages have been developed and released using free software principles and the GPL including spreadsheets source code control tools compilers and interpreters Internet tools graphical image manipulation tools such as the Gimp and two complete object based environments GNOME and KDE We discuss GNOME and KDE in Chapters 16 and 17 There is now so much free software available that with the addition of the Linux kernel it could be said that the goal of a creating GNU a free UNIX like system has been achieved with Linux To recognize the contribution made by GNU software many people now refer to Linux systems in general as GNU Linux You can learn more about the
17. eting This is just about the simplest example there is but if you can get this far with your system you should be able to compile and run the remainder of the examples in the book If this did not work for you make sure that the C compiler is installed on your system For example many Linux distributions have an install option called Software Development or something similar that you should select to make sure the necessary packages are installed Because this is the first program you ve run it s a good time to point out some basics The hello pro gram will probably be in your home directory If PATH doesn t include a reference to your home direc tory the shell won t be able to find he11lo Furthermore if one of the directories in PATH contains another program called hello that program will be executed instead This would also happen if sucha directory is mentioned in PATH before your home directory To get around this potential problem you can prefix program names with for example hel1lo This specifically instructs the shell to execute the program in the current directory with the given name The dot is an alias for the current directory If you forget the o name option that tells the compiler where to place the executable the compiler will place the program in a file called a out meaning assembler output Just remember to look for an a out if you think you ve compiled a program and you can t find it In the early d
18. free software concept at http www gnu org Linux Distributions As we have already mentioned Linux is actually just a kernel You can obtain the sources for the kernel to compile and install it on a machine and then obtain and install many other freely distributed software programs to make a complete Linux installation These installations are usually referred to as Linux sys tems because they consist of much more than just the kernel Most of the utilities come from the GNU Project of the Free Software Foundation As you can probably appreciate creating a Linux system from just source code is a major undertaking Fortunately many people have put together ready to install distributions often called flavors usually downloadable or on CD ROMs or DVDs that contain not just the kernel but also many other program ming tools and utilities These often include an implementation of the X Window System a graphical environment common on many UNIX systems The distributions usually come with a setup program and additional documentation normally all on the CD s to help you install your own Linux system Some well known distributions particularly on the Intel x86 family of processors are Red Hat Enterprise Linux and its community developed cousin Fedora Novell SUSE Linux and the free openSUSE variant Ubuntu Linux Slackware Gentoo and Debian GNU Linux Check out the DistroWatch site at http distrowatch com for details on many more Linux dist
19. m the library that it actually needs Including the header file which contains declarations for all of the func tions in the library doesn t cause the entire library to be included in the final program If you re familiar with Windows software development there are a number of direct analogies here illustrated in the following table Item UNIX Windows object module func o FUNC OBJ static library lib a LIB LIB program program PROGRAM EXE Shared Libraries One disadvantage of static libraries is that when you run many applications at the same time and they all use functions from the same library you may end up with many copies of the same functions in memory and indeed many copies in the program files themselves This can consume a large amount of valuable memory and disk space Many UNIX systems and Linux support shared libraries can overcome this disadvantage A complete discussion of shared libraries and their implementation on different systems is beyond the scope of this book so we ll restrict ourselves to the visible implementation under Linux Shared libraries are stored in the same places as static libraries but shared libraries have a different filename suffix On a typical Linux system the shared version of the standard math library is 1ib libm so When a program uses a shared library it is linked in such a way that it doesn t contain function code itself but references to shared code that will be made available at
20. ng system kernel itself forms only a small part of a usable development system Commercial UNIX systems traditionally come bundled with applications that provide system services and tools For Linux systems these additional pro grams have been written by many different programmers and have been freely contributed The Linux community together with others supports the concept of free software that is software that is free from restrictions subject to the GNU General Public License the name GNU stands for the recur sive GNU s Not Unix Although there may be a cost involved in obtaining the software it can thereafter be used in any way desired and is usually distributed in source form The Free Software Foundation was set up by Richard Stallman the author of GNU Emacs one of the best known text editors for UNIX and other systems Stallman is a pioneer of the free software concept and started the GNU Project an attempt to create an operating system and development environment that would be compatible with UNIX but not suffer the restrictions of the proprietary UNIX name and source code GNU may one day turn out to be very different from UNIX in the way it handles the hard ware and manages running programs but it will still support UNIX style applications The GNU Project has already provided the software community with many applications that closely mimic those found on UNIX systems All these programs so called GNU software are distribut
21. program that calls the function bi11 First it s a good idea to create a header file for your library This will declare the functions in your library and should be included by all applications that want to use your library It s a good idea to include the header file in the files f red c and bill c too This will help the compiler pick up any errors This is lib h It declares the functions fred and bill for users 11 fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 12 F Chapter 1 Getting Started void bill char void fred int 4 The calling program program c can be very simple It includes the library header file and calls one of the functions from the library include lt stdlib h gt include lib h int main bill Hello World exit 0 5 You can now compile the program and test it For now specify the object files explicitly to the compiler asking it to compile your file and link it with the previously compiled object module bill o gcc c program c gcc o program program o bill o program bill we passed Hello World 6 Now you ll create and use a library Use the ar program to create the archive and add your object files to it The program is called ar because it creates archives or collections of individual files placed together in one large file Note that you can also use ar to create archives of files of any type Like many UNIX utilities ar is a generic tool Sa
22. r crv libfoo a bill o fred o a bill o a fred o 7 The library is created and the two object files added To use the library successfully some sys tems notably those derived from Berkeley UNIX require that a table of contents be created for the library Do this with the ranlib command In Linux this step isn t necessary but it is harm less when you re using the GNU software development tools ranlib libfoo a 12 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 13 F Chapter 1 Getting Started Your library is now ready to use You can add to the list of files to be used by the compiler to create your program like this gcc o program program o libfoo a program bill we passed Hello World You could also use the 1 option to access the library but because it is not in any of the standard places you have to tell the compiler where to find it by using the L option like this gcc o program program o L lfoo The L option tells the compiler to look in the current directory for libraries The 1 00 option tells the compiler to use a library called 1ibfoo a ora shared library 1ibfoo so if one is present To see which functions are included in an object file library or executable program you can use the nm com mand If you take a look at program and 1ib a you see that the library contains both fred and bill but that program contains only bi11 When the program is created it includes only functions fro
23. r multiuser multitasking operating system for a wide variety of hardware platforms from PC workstations to multiprocessor servers and supercomputers 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 2 F Chapter 1 Getting Started A Brief History of UNIX Strictly UNIX is a trademark administered by The Open Group and it refers to a computer operating system that conforms to a particular specification This specification known as The Single UNIX Specification defines the names of interfaces to and behaviors of all mandatory UNIX operating sys tem functions The specification is largely a superset of an earlier series of specifications the P1003 or POSIX Portable Operating System Interface specifications developed by the IEEE Institute of Electrical and Electronic Engineers Many UNIX like systems are available commercially such as IBM s AIX HP s HP UX and Sun s Solaris Some have been made available for free such as FreeBSD and Linux Only a few systems currently con form to The Open Group specification which allows them to be marketed with the name UNIX In the past compatibility among different UNIX systems has been a real problem although POSIX was a great help in this respect These days by following a few simple rules it is possible to create applications that will run on all UNIX and UNIX like systems You can find more details on Linux and UNIX standards in Chapter 18 UNIX Philosophy In the following chapters
24. resent it will always take the same options independent of the machine On Linux systems that do try to implement the standards you might find that any or all of the com mands c89 cc and gcc refer to the system C compiler usually the GNU C compiler or gcc On UNIX systems the C compiler is almost always called cc In this book we use gcc because it s provided with Linux distributions and because it supports the ANSI standard syntax for C If you ever find yourself using a UNIX system without gcc we recommend that you obtain and install it You can find it at http www gnu org Wherever we use gcc in the book simply substitute the relevant command on your system Try It Out Your First Linux C Program In this example you start developing for Linux using C by writing compiling and running your first Linux program It might as well be that most famous of all starting points Hello World 1 Here s the source code for the file hello c include lt stdio h gt include lt stdlib h gt int main printf Hello World n exit 0 2 Now compile link and run your program gcc o hello hello c hello Hello World 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 8 F Chapter 1 Getting Started How It Works You invoked the GNU C compiler on Linux this will most likely be available as cc too that translated the C source code into an executable file called hello You ran the program and it printed a gre
25. ributions Programming Linux Many people think that programming Linux means using C It s true that UNIX was originally written in C and that the majority of UNIX applications are written in C but C is not the only option available to fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 5 F Chapter 1 Getting Started Linux programmers or UNIX programmers for that matter In the course of the book we ll mention a couple of the alternatives In fact the first version of UNIX was written in PDP 7 assembler language in 1969 C was conceived by Dennis Ritchie around that time and in 1973 he and Ken Thompson rewrote essentially the entire UNIX kernel in C quite a feat in the days when system software was written in assembly language A vast range of programming languages are available for Linux systems and many of them are free and available on CD ROM collections or from FTP archive sites on the Internet Here s a partial list of pro gramming languages available to the Linux programmer Ada C C Eiffel Forth Fortran Icon Java JavaScript Lisp Modula 2 Modula 3 Oberon Objective C Pascal Perl PostScript Prolog Python Ruby Smalltalk PHP Tcl Tk Bourne Shell We show how you can use a Linux shell bash to develop small to medium sized applications in Chapter 2 For the rest of the book we mainly concentrate on C We direct our attention mostly toward exploring the Linux programming interfaces from the perspective of the C
26. run time When the resulting program is loaded into memory to be executed the function references are resolved and calls are made to the shared library which will be loaded into memory if needed 13 fp 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 14 F Chapter 1 Getting Started In this way the system can arrange for a single copy of a shared library to be used by many applications at once and stored just once on the disk An additional benefit is that the shared library can be updated independently of the applications that rely on it Symbolic links from the 1ib libm so file to the actual library revision 1ib 1ibm so N where N represents a major version number 6 at the time of writing are used When Linux starts an application it can take into account the version of a library required by the application to prevent major new versions of a library from breaking older applications The following example outputs are taken from a SUSE 10 3 distribution Your output may differ slightly if you are not using this distribution For Linux systems the program the dynamic loader that takes care of loading shared libraries and resolv ing client program function references is called 1d so and may be made available as 1d linux so 2 or 1d 1sb so 2 or 1d 1sb so 3 The additional locations searched for shared libraries are configured in the file etc 1d so conf which needs to be processed by ldconfig if changed for example if X
27. ss at an intermediate stage For example the c option says not to run the linker Then the output consists of object files output by the assembler Other options are passed on to one stage of processing Some options control the preprocessor and others the com piler itself Yet other options control the assembler and linker most of these are not documented here since we rarely need to use any of them If you want you can read about the options that the compiler supports The manual page in this case is quite long but it forms only a small part of the total documentation for GNU C and C When reading manual pages you can use the spacebar to read the next page Enter or Return if your keyboard has that key instead to read the next line and q to quit altogether fp 15 47627c01 qxd WroxPro 9 28 07 8 56 PM Page 16 F Chapter 1 Getting Started 2 To get more information on GNU C you can try info info gcc File gcc info Node Top Next G and GCC Up DIR Introduction kkkxkxkkkkkkkx k This manual documents how to use the GNU compilers as well as their features and incompatibilities and how to report bugs It corresponds to GCC version 4 1 3 The internals of the GNU compilers including how to port them to new targets and some information about how to write front ends for new languages are documented in a separate manual Note Introduction gccint Top Menu G and GCC You can compile C
28. w everything that the user might want to do What Is Linux As you may already know Linux is a freely distributed implementation of a UNIX like kernel the low level core of an operating system Because Linux takes the UNIX system as its inspiration Linux and UNIX programs are very similar In fact almost all programs written for UNIX can be compiled and run on Linux Also some commercial applications sold for commercial versions of UNIX can run unchanged in binary form on Linux systems Linux was developed by Linus Torvalds at the University of Helsinki with the help of UNIX program mers from across the Internet It began as a hobby inspired by Andy Tanenbaum s Minix a small UNIX like system but has grown to become a complete system in its own right The intention is that the Linux kernel will not incorporate proprietary code but will contain nothing but freely distributable code Versions of Linux are now available for a wide variety of computer systems using many different types of CPUs including PCs based on 32 bit and 64 bit Intel x86 and compatible processors workstations and servers using Sun SPARC IBM PowerPC AMD Opteron and Intel Itanium and even some handheld PDAs and Sony s Playstations 2 and 3 If it s got a processor someone somewhere is trying to get Linux running on it The GNU Project and the Free Software Foundation Linux owes its existence to the cooperative efforts of a large number of people The operati
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