For Dummies Linux All-in-One, 4th Edition
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1. To quit gdb type q and then press Enter gdb has a large number of commands but you need only a few to find the cause of an error quickly Table 1 4 lists the commonly used gdb commands Table 1 4 Common gdb Commands This Command Does the Following break NUM Sets a breakpoint at the specified line number NUM The debugger stops at breakpoints bt Displays a trace of all stack frames This command shows you the sequence of func tion calls so far clear FILENAME NUM Deletes the breakpoint at a specific line number NUM in the source file FILENAME For example clear xdraw c 8 clears the breakpoint at line 8 of file xdraw c continue Continues running the program being debugged Use this command after the pro gram stops due to a signal or breakpoint display EXPR Displays the value of an expression EXPR consisting of variables defined in the pro gram each time the program stops file FILE Loads the specified executable file FILE for debugging help NAME Displays help on the command named NAME info break Displays a list of current breakpoints including information on how many times each breakpoint is reached info files Displays detailed information about the file being debugged info func Displays all function names info local Displays information about local variables of the current function info prog Displays the execution status of the pr2. ar MBER X The LGPL is intended to allow use of libraries in your applications even if you don t distribute source code for your application The LGPL stipulates however that users must have access to the source code of the library you use and that users can make use of modified versions of those libraries The LGPL covers most Linux libraries including the C library libc a Thus when you build your application on Linux by using the GCC compiler your application links with code from one or more libraries that the LGPL covers If you want to distribute your application in only binary form you need to pay attention to LGPL One way to meet the intent of the LGPL is to provide the object code for your application and amakefile that relinks your object files with any updated Linux libraries the LGPL covers A better way to satisfy the LGPL is to use dynamic linking in which your application and the library are separate entities even though your applica tion calls functions in the library when it runs With dynamic linking users immediately get the benefit of any updates to the libraries without ever having to relink the application
3. c which in turn is called from the main function at line 7 of the dbgtst c file 4 Use the list command to inspect the lines of suspect source code In dbgtst you may start with line 16 of dbgtst c file as follows db list dbgtst c 16 return 0 void read_input char s printf Command gets s DIDHAKRWNEE 3 5 After looking at this listing you can tell that the problem may be the way read_input is called Then you list the lines around line 7 in dbgtst c where the read_input call originates gdb list dbgtst c 7 2 static char buf 256 3 void read_input char s 4 int main void 5 6 char input NULL Just a pointer no storage for string 7 read_input input 8 Process command 9 printf You typed s n input TOSA Se ad at N 11 return 0 gdb Exploring the Software Development Tools in Linux 549 At this point you can narrow the problem to the variable named input That variable is an array not a NULL which means zero pointer Fixing bugs in gdb Sometimes you can fix a bug directly in gdb For the example program in the preceding section you can try this fix immediately after the program dies after displaying an error message An extra buffer named buf is defined in the dbgtst program as follows static char buf 256 We can fix the problem of the uninitialized pointer by setting the variable input to buf The following session with gdb c
4. The capability to include a file in a makefile is unique to GNU make j NUM Specifies the number of commands that the make command can run simultaneously k Continues to build unrelated targets even if an error occurs when building one of the targets 1 LOAD Doesn t start a new job if load average is at least LOAD a floating point number Exploring the Software Development Tools in Linux 543 Option Meaning m Ignores the variable given but accepts that variable for compatibil ity with other versions of make n Prints the commands to execute but does not execute them 0o FILE Does not rebuild the file named FILE even if itis older than its dependents p Displays the make database of variables and implicit rules q Does not run anything but returns 0 zero if all targets are up to date 1 if anything needs updating or 2 if an error occurs r Gets rid of all built in rules R Gets rid of all built in variables and rules s Works silently without displaying the commands as they execute t Changes the timestamp of the files v Displays the version number of make and a copyright notice w Displays the name of the working directory before and after pro cessing the makefile W FILE Assumes that the specified file has been modified used with n to see what happens if you modify that file The GNU debugger Although make automates the process of building a
5. program that part of programming is the least of your worries when a program doesn t work cor rectly or when a program suddenly quits with an error message You need a debugger to find the cause of program errors Linux includes gdb the versatile GNU debugger with a command line interface Like any debugger gdb lets you perform typical debugging tasks such as the following Set a breakpoint so that the program stops at a specified line Watch the values of variables in the program Step through the program one line at a time Change variables in an attempt to correct errors The gdb debugger can debug C and C programs Book VIII Chapter 1 xnu ul Huimwesboig 544 Exploring the Software Development Tools in Linux ar Preparing to debug a program If you want to debug a program by using gdb you have to ensure that the compiler generates and places debugging information in the executable The debugging information contains the names of variables in your program and the mapping of addresses in the executable file to lines of code in the source file gdb needs this information to perform its functions such as stopping after executing a specified line of source code To make sure that the executable is properly prepared for debugging use the g option with GCC You can do this task by defining the variable CFLAGS in the makefile as CFLAGS g Running gdb The most common way to debug a program is to run gdb b
6. the execut able file lf you don t specify the name of an output file with the o option GCC saves the executable code in a file named a out Book VIII Chapter 1 xnu ul Huiwwesboig 53 4 Exploring the Software Development Tools in Linux MBER KO amp If you have too many source files to compile and link you can compile the files individually and generate object files that have the o extension That way when you change a source file you need to compile only that file you just link the compiled file to all the object files The following commands show how to separate the compile and link steps for the sample program gcc c area c gcc c circle c gcc o area area o circle o The first two commands run gcc with the c option compiling the source files The third gcc command links the object files into an executable named area Compiling C programs GNU CC is a combined C and C compiler so the gcc command also can compile C source files GCC uses the file extension to determine whether a file is C or C C files have a lowercase c extension whereas C files end with C or cpp Although the gcc command can compile a C file that command doesn t automatically link with various class libraries that C programs typically require Compiling and linking a C program by using the g command is easy because it runs gcc with appropriate options Suppose that you want to compile the following simple C p
7. use For most programming languages you have to go through the following steps to create a computer program 1 Use a text editor to type the sequence of commands from the program ming language This sequence of commands accomplishes your task This human readable version of the program is called the source file or source code You can create the source file with any application such as a word processor that can save a document in plain text form Always save your source code as plain text The filename depends on the type of programming language Word processors can sometimes put extra instructions in their documents that tell the computer to dis play the text in a particular font or other format Saving the file as plain text deletes any and all such extra instructions Trust me your program is much better off without such stuff Use a compiler program to convert that text file the source code from human readable form into machine readable object code Typically this step also combines several object code files into a single machine readable computer program something that the computer can run Use a special program called a debugger to track down any errors and find which lines in the source file might have caused the errors Go back to Step 1 and use the text editor to correct the errors and repeat the rest of the steps Exploring the Software Development Tools in Linux 531 These steps are referred to as t
8. you can t find the COPYING file turn to the back of this book to read the GPL The GPL has nothing to do with whether you charge for the software or distribute it for free its thrust is to keep the software free for all users GPL requires that the software is distributed in source code form and by stipulat ing that any user can copy and distribute the software in source code form to anyone else In addition everyone is reminded that the software comes with absolutely no warranty NING Understanding the Implications of GNU Licenses 551 The software that the GPL covers isn t in the public domain Software cov ered by GPL is always copyrighted and the GPL spells out the restrictions on the software s copying and distribution From a user s point of view of course GPL s restrictions aren t really restrictions the restrictions are ben efits because the user is guaranteed access to the source code If your application uses parts of any software the GPL covers your appli cation is considered a derived work which means that your application is also covered by the GPL and you must distribute the source code to your application Although the GPL covers the Linux kernel the GPL doesn t cover your appli cations that use the kernel services through system calls Those applica tions are considered normal use of the kernel If you plan to distribute your application in binary form as most commercial software is distri
9. 542 Exploring the Software Development Tools in Linux Even when you have a makefile with a default name such as Makefile you may want to build a specific target out of several targets defined in the makefile In that case you have to use the following syntax when you run make make target For example if the makefile contains the target named clean you can build that target with this command make clean Another special syntax overrides the value of a make variable For example GNU make uses the CFLAGS variable to hold the flags used when compiling C files You can override the value of this variable when you invoke make Here s an example of how you can define CFLAGS as the option g 02 make CFLAGS g 02 In addition to these options GNU make accepts several other command line options Table 1 3 lists the GNU make options Table 1 3 Options for GNU make Option Meaning b Ignores the variable given but accepts that variable for compatibil ity with other versions of make C DIR Changes to the specified directory before reading the makefile d Prints debugging information e Allows environment variables to override definitions of similarly named variables in the makefile f FILE Reads FILE as the makefile h Displays the list of make options i Ignores all errors in commands executed when building a target I DIR Searches the specified directory for included makefiles
10. Chapter 1 Programming in Linux In This Chapter Figuring out programming y Exploring the software development tools in Linux Compiling and linking programs with GCC Using make Debugging programs with gdb Understanding the implications of GNU GPL and LGPL iw comes loaded with all the tools you need to develop software All you have to do is install them In particular it has all the GNU soft ware development tools such as GCC C and C compiler GNU make and the GNU debugger This chapter introduces you to programming describes the software development tools and shows you how to use them Although I provide examples in the C and C programming languages the focus is not on showing you how to program in those languages but on showing you how to use various software development tools such as compilers make and debugger The chapter concludes with a brief explanation of how the Free Software Foundation s GNU General Public License GPL may affect any plans you might have to develop Linux software You need to know about the GPL because you use GNU tools and GNU libraries to develop software in Linux An Overview of Programming If you ve written computer programs in any programming language you can start writing programs on your Linux system quickly If you ve never written a computer program however you need two basic resources before you begin to write code a look at the basics of programming and a quick revi
11. NU make includes many features that make it easy for you to express the depen dencies and rules for building a target from its dependents If you need to compile a large number of C files by using GCC with the same options for example typing the options for each file is tedious You can avoid this repet itive task by defining a variable or macro in make as follows Define macros for name of compiler CXX g Define a macro for the GCC flags CXXFLAGS 02 g mcpu i686 A rule for building an object file form o form C form h CXX c CXXFLAGS form C In this example CXX and CXXFLAGS are make variables GNU make prefers to call them variables but most UNIX make utilities call them macros To use a variable anywhere in the makefile start with a dollar sign followed by the variable within parentheses GNU make replaces all occur rences of a variable with its definition thus it replaces all occurrences of CXXFLAGS with the string 02 g mcpu i686 GNU make has several predefined variables that have special meanings Table 1 2 lists these variables In addition to the variables listed in Table 1 2 GNU make considers all environment variables such as PATH and HOME to be predefined variables as well Table 1 2 Some Predefined Variables in GNU make Variable Meaning Member name for targets that are archives If the target is libDisp a image o for example is image o s Name of the target
12. age and you re left scratching your head because when you look at the makefile in a text editor you can t tell the differ ence between a tab and a space Now that you know the secret the fix is to replace the spaces at the beginning of the offending line with a single tab The benefit of using make is that it prevents unnecessary compilations After all you can run g or gcc from a shell script to compile and link all the files that make up your application but the shell script compiles everything even if the compilations are unnecessary GNU make on the other hand builds a target only if one or more of its dependents have changed since the last time the target was built make verifies this change by examining the time of the last modification of the target and the dependents make treats the target as the name of a goal to be achieved the target doesn t have to be a file You can have a rule such as this one clean rm f o Exploring the Software Development Tools in Linux 539 This rule specifies an abstract target named clean that doesn t depend on anything This dependency statement says that to create the target clean GNU make invokes the command rm f o which deletes all files that have the o extension namely the object files Thus the effect of creating the target named clean is to delete the object files Variables or macros In addition to the basic capability of building targets from dependents G
13. age during installation Debian Type apt get install gcc and then apt get install libc6 dev in a terminal window SUSE Choose Main Menu System YasT click Software on the left side of the window and then click Install and Remove Software Type gcc in the search field in YaST select the relevant packages from the search results and click Accept to install If you find any missing packages you can install them in a similar manner The next few sections briefly describe how to use these software develop ment tools to write applications for Linux GNU C and C compilers The most important software development tool in Linux is GCC the GNU C and C compiler In fact GCC can compile three languages C C and Objective C a language that adds object oriented programming capabilities to C You use the same gcc command to compile and link both C and C source files The GCC compiler supports ANSI standard C making it easy to port any ANSI C program to Linux In addition if you ve ever used a C com piler on other UNIX systems you should feel right at home with GCC Using GCC Use the gcc command to invoke GCC By default when you use the gcc command on a source file GCC preprocesses compiles and links to create an executable file However you can use GCC options to stop this process at an intermediate stage For example you might invoke gcc by using the c option to compile a source file and to generate an object
14. buted you must make sure that your application doesn t use any parts of any software the GPL covers Your application may end up using parts of other software when it calls functions in a library Most librar ies however are covered by a different GNU license which is described in the next section You have to watch out for only a few library and utility programs the GPL covers The GNU dbm gdbm database library is one of the prominent librar ies GPL covers The GNU bison parser generator tool is another utility the GPL covers If you allow bison to generate code the GPL covers that code Other alternatives for the GNU dbm and GNU bison aren t covered by GPL For a database library you can use the Berkeley database library db in place of gdbm For a parser generator you may use yacc instead of bison The GNU Lesser General Public License The text of the GNU Lesser General Public License LGPL is in a file named COPYING LIB If you have the kernel source installed a copy of COPYING LIB file is in one of the source directories To locate a copy of the COPYING LIB file on your Linux system type the following command in a terminal window find usr name COPYING print This command lists all occurrences of COPYING and COPYING LIB in your system The COPYING file contains the GPL whereas COPYING LIB has the LGPL Book VIII Chapter 1 xnury ul Huimwesboig 552 Understanding the Implications of GNU Licenses
15. computer programs while the CPU executes them If a program works on some data that data is also stored in memory The contents of memory aren t permanent they go away never to return when the computer is shut down or when a program is no longer running The hard drive also referred to as the hard disk or disk serves as the per manent storage space for computer programs and data The hard drive is organized into files which are in turn organized in hierarchical directories and subdirectories somewhat like organizing paper folders into the draw ers in a file cabinet Each file is essentially a block of storage capable of holding a variety of information For example a file may be a human readable text file or it may be a collection of computer instructions that makes sense only to the CPU When you create computer programs you work a lot with files An Overview of Programming 52 9 For a programmer the other two important items are the input and the output the way a program gets input from the user and displays output to the user The user provides input through the keyboard and mouse and output appears on the monitor However a program may also accept input from a file and send output to a file Role of the operating system The operating system is a special collection of computer programs whose pri mary purpose is to load and run other programs The operating system also acts as an interface between the software and
16. dies after displaying the Segmentation fault message For such a small program as this one you can probably find the cause by exam ining the source code In a real world application however you may not immediately know what causes the error That s when you have to use gdb to find the cause of the problem To use gdb to locate a bug follow these steps 1 Load the program under gdb For example type gdb dbgtst to load a program named dbgtst in gdb Book VIII Chapter 1 xnury Ul Hurwwesboig 548 Exploring the Software Development Tools in Linux 2 Start executing the program under gdb by typing the run command When the program prompts for input type some input text The program fails as it did previously Here s what happens with the dbgtst program gdb run Starting program home edulaney swdev dbgtst Command test Program received signal SIGSEGV Segmentation fault 0x400802b6 in gets from lib tls libc so 6 gdb 3 Use the where command to determine where the program died For the dbgt st program this command yields this output gdb where 0 0x400802b6 in gets from lib tls libc so 6 1 0x08048474 in read_input s 0x0 at dbgtst c 16 2 0x08048436 in main at dbgtst c 7 gdb The output shows the sequence of function calls Function call 0 the most recent one is to the gets C library function The gets call origi nates in the read_input function at line 16 of the file dbgt st
17. e bounty of Linux To protect its developers and users Linux is distributed under the GNU GPL General Public License which stipulates the distribution of the source code The GPL doesn t mean however that you can t write commercial software for Linux that you want to distribute either for free or for a price in binary form only You can follow all the rules and still sell your Linux applications in binary form When writing applications for Linux be aware of two licenses The GNU General Public License GPL which governs many Linux pro grams including the Linux kernel and GCC The GNU Library General Public License LGPL which covers many Linux libraries The following sections provide an overview of these licenses and some sug gestions on how to meet their requirements Don t take anything in this book as legal advice Instead you should read the full text for these licenses in the text files on your Linux system and then show these licenses to your legal counsel for a full interpretation and an assessment of applicability to your business The GNU General Public License The text of the GNU General Public License GPL is in a file named COPYING in various directories in your Linux system For example type the following command to find a copy of that file in your Linux system find usr name COPYING print After you find the file you can change to that directory and type more COPYING to read the GPL If
18. ething in a single source file These situations are where the GNU make utility comes to your rescue The make utility works by reading and interpreting a makefile a text file that describes which files are required to build a particular program as well as how to compile and link the files to build the program Whenever you change one or more files make determines which files to recompile and it issues the appropriate commands for compiling those files and rebuilding the program Makefile names By default GNU make looks for a makefile that has one of the following names in the order shown GNUmakefile makefile Makefile In UNIX systems using Makefile as the name of the makefile is custom ary because it appears near the beginning of directory listings where upper case names appear before lowercase names When you download software from the Internet you usually find a Makefile together with the source files To build the software you only have to type make at the shell prompt and make takes care of all the steps necessary to build the software If your makefile doesn t have a standard name such as Makefile you have to use the f option with make to specify the makefile name If your makefile is called myprogram mak for example you have to run make using the following command line make f myprogram mak The makefile For a program made up of several source and header files the makefile specifies the foll
19. ew of computers and their major parts This section offers an overview of com puter programming just enough to get you going A simplified view of a computer Before you get a feel for computer programming you need to understand where computer programs fit in with the overall scheme of computing 528 An Overview of Programming Figure 1 1 A simplified view of a computer and how it runs programs Figure 1 1 shows a simplified view of a computer highlighting the major parts that are important to a programmer Memory Transient storage where programs are loaded and executed by the CPU me Central Processing Unit CPU The computer s brain the microprocessor that executes the instructions contained in the program loaded into memory Hard Drive Permanent storage where programs are loaded and data is stored in files At the heart of a computer is the central processing unit CPU that performs the instructions contained in a computer program The specific piece of hardware that does the job which its makers call a microprocessor and the rest of us call a chip varies by system In a Pentium PC it s a Pentium in a Sun SPARC workstation it s a SPARC chip in an HP UNIX workstation it s a PA RISC chip These microprocessors have different capabilities but the same mission Tell the computer what to do Random Access Memory RAM or just memory serves as the storage for
20. file but not to per form the link step Using GCC to compile and link a few C source files is easy Suppose you want to compile and link a simple program made up of two source files To Exploring the Software Development Tools in Linux 533 accomplish this use the following program source the task that is stored in the file area c computes the area of a circle whose radius is specified at the command line include lt stdio h gt include lt stdlib h gt Function prototype double area_of_circle double r int main int argc char argv if arge lt 2 printf Usage s radius n argv 0 exit 1 else double radius atof argv 1 double area area_of_circle radius printf Area of circle with radius f f n radius area return 0 You need another file that actually computes the area of a circle Here s the listing for the circle c file which defines a function that computes the area of a circle include lt math h gt define SQUARE x x x double area_of_circle double r return 4 0 M_PI SQUARE r For such a simple program of course we could place everything in a single file but this example was contrived a bit to show you how to handle mul tiple files To compile these two files and to create an executable file named area use this command gcc o area area c circle c This invocation of GCC uses the o option to specify the name of
21. file without the extension Names of all dependent files with duplicate dependencies listed in their order of occurrence lt The name of the first dependent file continued Book VIII Chapter 1 xnury ul Huiumwesboig 540 Exploring the Software Development Tools in Linux Table 1 2 continued Variable Meaning Names of all dependent files with spaces between the names that are newer than the target e Complete name of the target If the target is LibDisp a image o for example is libDisp a AR ARFLAGS Names of all dependent files with spaces between the names Duplicates are removed from the dependent filenames Name of the archive maintaining program default value ar Flags for the archive maintaining program default value rv AS Name of the assembler program that converts the assembly lan guage to object code default value as ASFLAGS Flags for the assembler Ce Name of the C compiler default value cc CFLAGS Flags that are passed to the C compiler CO Name of the program that extracts a file from RCS the default value is co COF LAGS Flags for the RCS co program CPP Name of the C preprocessor default value CC E CPPI FLAGS Flags for the C preprocessor CXX Name of the C compiler default value g CXXI FLAGS Flags that are passed
22. files to o files uses the built in rule Defining the variable CFLAGS passes the flags to the C compiler The target named a11 is defined as the first target for a reason if you run GNU make without specifying any targets in the command line see the make syntax described in the following section the command builds the first target it finds in the makefile By defining the first target a11 as xdraw you can ensure that make builds this executable file even if you don t explic itly specify it as a target UNIX programmers traditionally use a11 as the name of the first target but the target s name is immaterial what matters is that it s the first target in the makefile How to run make Typically you run make by simply typing the following command at the shell prompt make When run this way GNU make looks for a file named GNUmakefile makefile or Makefile in that order If make finds one of these makefiles it builds the first target specified in that makefile However if make doesn t find an appropriate makefile it displays the following error message and exits make No targets specified and no makefile found Stop If your makefile happens to have a different name from the default names you have to use the f option to specify the makefile The syntax of the make command with this option is make f filename where filename is the name of the makefile Book VIII Chapter 1 xnury ul Huimwesboig
23. he edit compile debug cycle of programming because most programmers have to repeat this sequence several times before a program works correctly In addition to knowing the basic programming steps you also need to be familiar with the following terms and concepts Variables are used to store different types of data You can think of each variable as being a placeholder for data kind of like a mailbox with a name and room to store data The content of the variable is its value Expressions combine variables by using operators An expression may add several variables another may extract a part of a string Statements perform some action such as assigning a value to a variable or printing a string Flow control statements allow statements to execute in various orders depending on the value of some expression Typically flow control statements include for do while while and if then else statements Functions also called subroutines or routines allow you to group several statements and give the group a name This feature allows you to exe cute the same set of statements by invoking the function that represents those statements Typically a programming language provides many predefined functions to perform tasks such as opening and reading from a file Exploring the Software Development Tools in Linux Linux includes the following traditional UNIX software development tools Text editors such as vi and emacs fo
24. ntax This option disables some GNU C specific features such as the __asm__and___typeof__ keywords When used with g supports only ISO standard C Compiles and generates only the object file DMACRO Defines the macro with the string 1 as its value DMACRO DEFN Defines the macro as DEFN where DEFN is some text string E Runs only the C preprocessor fallow single precision Performs all math operations in single precision fpec struct return Returns all struct and union values in memory rather than in registers Returning values this way is less efficient but at least it s compatible with other compilers continued Book VIII Chapter 1 xnu ul Huimwesboig Table 1 1 continued 53 6 Exploring the Software Development Tools in Linux Option Meaning fPIC Generates position independent code PIC suitable for use in a shared library freg struct When possible returns struct and union values return registers g Generates debugging information The GNU debugger can use this information I DIRECTORY Searches the specified directory for files that you include by using the include preprocessor directive L DIRECTORY Searches the specified directory for libraries 1 LIBRARY Searches the specified library when linking mcpu cputype Optimizes code for a specific processor cputype can take many different
25. o gram being debugged continued Book VIII Chapter 1 xnury ul Huimwesboig 546 Exploring the Software Development Tools in Linux Table 1 4 continued info var Displays all global and static variable names kill Ends the program you re debugging list Lists a section of the source code make Runs the make utility to rebuild the execut able without leaving gdb next Advances one line of source code in the current function without stepping into other functions print EXPR Shows the value of the expression EXPR quit Quits gdb run Starts running the executable that is cur rently running set variable Sets the value of the variable VAR to VAR VALUE VALUE shell CMD Executes the UNIX command CMD without leaving gdb step Advances one line in the current function stepping into other functions if any watch VAR Shows the value of the variable named VAR whenever the value changes where Displays the call sequence Use this com mand to locate where your program died x F ADDR Examines the contents of the memory loca tion at address ADDR in the format speci fied by the letter F which can be o octal x hex d decimal u unsigned decimal t binary float a address i instruc tion c char or s string You can append a letter indicating the size of data type to the format letter Size letters are b byte h half
26. o run simulta neously A command processor or shell is a special program that allows the user to type commands and perform various tasks such as run any program look at a host of files or print a file In Windows Vista or Windows XP you can type commands in a Command Prompt window The term command line refers to the commands that a user types to the command processor Usually a command line contains a command and Book VIII Chapter 1 xnury ul Huiwwesboig 530 An Overview of Programming MBER KV amp one or more options the command is the first word in the line and the rest are the options specific behaviors for the computer Environment variables are essentially text strings with names For example the PATH environment variable refers to a string that contains the names of directories Operating systems use environment variables to provide useful information to processes To see a list of environ ment variables in a Windows system type set in the Command Prompt window In Linux you can type printenv to see the environment variables Basics of computer programming A computer program is a sequence of instructions for performing a spe cific task such as adding two numbers or searching for some text in a file Consequently computer programming involves creating that list of instruc tions telling the computer how to complete a specific task The exact instructions depend on the programming language that you
27. orrects the problem of the uninitialized pointer This example picks up immediately after the program runs and dies due to the segmentation fault gdb file dbgtst A program is being debugged already Kill it y or n y Load new symbol table from home edulaney sw dbgtst y or n y Reading symbols from home edulaney sw dbgtst done gdb list 1 include lt stdio h gt 2 static char buf 256 3 void read_input char s 4 int main void Dt 6 char input NULL Just a pointer no storage for string 7 read_input input 8 Process command 9 printf You typed s n input LOPE cee a FY gdb break 7 Breakpoint 2 at 0x804842b file dbgtst c line 7 gdb run Starting program home edulaney sw dbgtst Breakpoint 1 main at dbgtst c 7 7 read_input input gdb set var input buf gdb cont Continuing Command test You typed test Program exited normally gdb q As the preceding listing shows if the program is stopped just before read_ input is called and the variable named input is set to buf which is a valid array of characters the rest of the program runs fine After finding a fix that works in gdb you can make the necessary changes to the source files and make the fix permanent Book VIII Chapter 1 xnu ul Huiwwesboig 550 Understanding the Implications of GNU Licenses Understanding the Implications of GNU Licenses ANG You have to pay a price for th
28. owing The items that make creates usually the object files and the executable Using the term target to refer to any item that make has to create is common The files or other actions required to create the target Which commands to execute to create each target Book VIII Chapter 1 xnu ul Huiwwesboig 538 Exploring the Software Development Tools in Linux Suppose that you have a C source file named form c that contains the following preprocessor directive include form h Include header file The object file form o clearly depends on the source file form c and the header file form h In addition to these dependencies you must specify how make converts the form C file to the object file form o Suppose that you want make to invoke g because the source file is in C with these options c compile only g generate debugging information 02 optimize some In the makefile you can express these options with the following rule This a comment in the makefile The following lines indicate how form o depends on form C and form h and how to create form o form o form C form h g c g 02 form C In this example the first noncomment line shows form o as the target and form C and form h as the dependent files The line following the dependency indicates how to build the target from its dependents This line must start with a tab Otherwise the make command exits with an error mess
29. r editing the source code To find out more about vi see Book II Chapter 6 AC compiler for compiling and linking programs written in C the programming language of choice for writing UNIX applications though nowadays many programmers are turning to C and Java Linux includes the GNU C and C compilers Originally the GNU C Compiler was known as GCC which now stands for GNU Compiler Collection See a description at http gcc gnu org The GNU make utility for automating the software build process the process of combining object modules into an executable or a library The operating system can load and run an executable a library is a col lection of binary code that can be used by executables Book VIII Chapter 1 xnury ul Huiwwesboig 532 O Serhlbuy P ws Elly Exploring the Software Development Tools in Linux A debugger for debugging programs Linux includes the GNU debugger gdb A version control system to keep track of various revisions of a source file Linux comes with RCS Revision Control System and CVS Concurrent Versions System Nowadays most open source projects use CVS as their version control system but a recent version control system called Subversion is being developed as a replacement for CVS You can install these software development tools in any Linux distribution Xandros Usually the tools are installed by default Fedora Select the Development Tools pack
30. rogram stored in a file named hello C Using an uppercase C extension for C source files is customary include lt iostream gt int main using namespace std cout lt lt Hello from Linux lt lt endl To compile and link this program into an executable program named he11o use this command g o hello hello c The command creates the hello executable which you can run as follows hello The program displays the following output Hello from Linux 535 Exploring the Software Development Tools in Linux A host of GCC options controls various aspects of compiling C and C programs Exploring GCC options Here s the basic syntax of the gcc command gcc options filenames Each option starts with a hyphen and usually has a long name such as funsigned char or finline functions Many commonly used options are short however such as c to compile only and g to gener ate debugging information needed to debug the program by using the GNU debugger gdb You can view a summary of all GCC options by typing the following com mand in a terminal window man gcc Then you can browse through the commonly used GCC options Usually you don t have to provide GCC options explicitly because the default set tings are fine for most applications Table 1 1 lists some of the GCC options you may use Table 1 1 Common GCC Options Option Meaning ansi Supports only ANSI standard C sy
31. the hardware All operating systems include one or more command processors called shells in Linux that allow users to type commands and perform tasks such as running a program or printing a file Most operating systems also include a graphical user interface such as GNOME and KDE in Linux that allows the user to perform most tasks by clicking on screen icons Linux Windows whether the XP or Windows 7 version and various versions of UNIX are examples of operating systems The operating system gives a computer its personality For example you can run Windows Vista or Windows XP on a PC On that same PC you can also install and run Linux That means depending on the operating system installed on it the selfsame PC could be a Windows 7 Windows XP or Linux system Computer programs are built on top of the operating system That means a computer program must make use of the capabilities that the operating system includes For example computer programs read and write files by using the built in capabilities of the operating system And if the operating system can t make coffee no program can tell it to and still expect positive results Although the details vary most operating systems support a number of simi lar concepts As a programmer you need to be familiar with the following handful of concepts A process is a computer program that is currently running in the com puter Most operating systems allow multiple processes t
32. to the C compiler FC Name of the FORTRAN compiler default value 77 FFLAGS Flags for the FORTRAN compiler LDFLAGS Flags for the compiler when it s supposed to invoke the linker 1d RM Name of the command to delete a file Default value rm f A sample makefile You can write a makefile easily if you use the predefined variables of GNU make and its built in rules Consider for example a makefile that creates the executable xdraw from three C source files xdraw c xviewobj c and shapes c and two header files amp draw h and shapes h Assume that each source file includes one of the header files Given these facts here is what a sample makefile may look like Exploring the Software Development Tools in Linux 54 1 FEFE HEHE ETE HE AE HE EHE HEHE HE FE HE FE HE HE HE HE HEHE HEHE HE FE FE FE HE HE FE HE FE HE E HE E FE FE FE FE E TE HE ERE HERE Sample makefile Comments start with FE FE HE HE HE HETE HE HE HE FE FE FE HE HE FE FE FE HE FE FE FE HE E HE FE FE HE AE E FE FE FE HE E FE FE FE EEE HE HE HEEE E E E EH Use standard variables to define compile and link flags CFLAGS g 02 Define the target all all xdraw OBJS xdraw o xviewobj o shapes o xdraw OBJS Object files xdraw o Makefile xdraw c xdraw h xviewobj o Makefile xviewobj c xdraw h shapes o Makefile shapes c shapes h This makefile relies on GNU make s implicit rules The conversion of c
33. values some common ones are 1386 1486 1586 1686 pentium pentiumpro pentium2 pentium3 pentium4 o FILE Generates the specified output file used to designate the name of an executable file 00 two zeros Does not optimize O or O1 letter 0 Optimizes the generated code 02 letter 0 Optimizes even more 03 letter 0 Performs optimizations beyond those done for 02 Os letter 0 Optimizes for size to reduce the total amount of code pedantic Generates errors if any non ANSI standard extensions are used pg Adds extra code to the program so that when run this program generates information that the gprof pro gram can use to display timing details for various parts of the program shared Generates a shared object file typically used to create a shared library UMACRO Undefines the specified macros v Displays the GCC version number w Doesn t generate warning messages W1 OPTION Passes the OPTION string containing multiple comma separated options to the linker To create a shared library named 1ibXXX so 1 for example use the following flag W1 soname 1ibXXX so 1 Exploring the Software Development Tools in Linux 537 The GNU make utility When an application is made up of more than a few source files compiling and linking the files by manually typing the gcc command can get tiresome Also you don t want to compile every file whenever you change som
34. word 2 bytes w word 4 bytes and g giant 8 bytes Typically ADDR is the name of a variable or pointer Finding bugs by using gdb To understand how you can find bugs by using gdb you need to see an example The procedure is easiest to show with a simple example so the fol lowing dbgtst c is a contrived program that contains a typical bug Exploring the Software Development Tools in Linux 54 7 include lt stdio h gt static char buf 256 void read_input char s int main void char input NULL Just a pointer no storage for string read_input input Process command printf You typed s n input fe see return 0 void read_input char s printf Command gets s This program s main function calls the read_input function to get a line of input from the user The read_input function expects a character array in which it returns what the user types In this example however main calls read_input with an uninitialized pointer that s the bug in this simple program Build the program by using gcc with the g option gcc g o dbgtst dbgtst c Ignore the warning message about the gets function being dangerous I m trying to use the shortcoming of that function to show how you can use gdb to track down errors To see the problem with this program run it and type test at the Command prompt dbgtst Command test Segmentation fault The program
35. y using the fol lowing command gdb progname progname is the name of the program s executable file After progname runs gdb displays the following message and prompts you for a command GNU gdb 6 3 Copyright 2004 Free Software Foundation Inc GDB is free software covered by the GNU General Public License and you are welcome to change it and or 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 This GDB was configured as i586 suse linux gdb You can type gdb commands at the gdb prompt One useful command help displays a list of commands as the next listing shows gdb help List of classes of commands aliases Aliases of other commands breakpoints Making program stop at certain points data Examining data files Specifying and examining files internals Maintenance commands obscure Obscure features running Running the program stack Examining the stack status Status inquiries support Support facilities tracepoints Tracing of program execution without stopping the program user defined User defined commands Type help followed by a class name for a list of commands in that class Type help followed by command name for full documentation Command name abbreviations are allowed if unambiguous gdb Exploring the Software Development Tools in Linux 545
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