Unix: Development Tools

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Unix Development Tools

• CSRU3130, Spring 2008
• Ellen Zhang

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Today

• A command a day find
• A Script a day
• Make
• gdb
• Profiling
• memcheck

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A command a day find

• find command process a set of files and/or
  directories in a file subtree you can specify
• where to search (pathname)
• what type of file to search for (-type
  directories, data files, links)
• how to process the files (-exec run a process
  against a selected file)
• the name of the file(s) (-name)
• perform logical operations on selections (-o and
  -a)

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Examples

• Search for file with a specific name in a set of
  files (-name)
• find . -name "rc.conf" -print
• Apply a unix command to all files found
• find . -name "rc.conf" -exec chmod or '' \
• Search for a string in a selection of files
• find . -exec grep "www.athabasca" '' \ -print
• search in current directory and all sub
  directories. All files containing string will
  have their path printed to standard output.

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More ways to find files

• Find all files under the root, that is regular
  file and modified seven or fewer days ago
• find / -type f -mtime -7 -print

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xargs

• find / -type f -mtime -7 xargs tar -rf
  weekly_incremental.tar
• xargs, a handy utility that coverts a stream of
  input into command line arguments for the
  supplied command
• Examples
• find / -name core xargs /bin/rm -f
• find / -name core -exec /bin/rm -f '' \ same
  thing
• find / -name core -delete same if using Gnu find

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GCC overview

• GNU Compiler Collection includes front ends for
  C, C, Objective-C, Fortran, Java, and Ada, as
  well as libraries for these languages (libstdc,
  libgcj,...).
• gcc is the "GNU" C Compiler, and g is the GNU
  C compiler

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Compiling
Object files
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Things can go wrong

• Preprocessor processing directives, ifdef,
  define, include,
• some header files not found
• Illegal directives
• Compiler
• variable/function undeclared,
• syntax errors (missing semicolon, etc), etc.
• Linker undefined reference to foo'
• collect2 ld returned 1 exit status

http//www.network-theory.co.uk/docs/gccintro/inde
x.html
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Warning messages are important

• include ltstdio.hgt
• int main (void)
• printf ("Two plus two is f\n", 4)
• return 0
• Any problem ?

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What you need to know about compiler ?

• gcc o myProg main.c
• ./myProg
• Two plus two is -0.693111
• Why ?
• printf does not check arguments type, an integer
  4 is printed out as a float
• Integers and floating-point numbers are stored in
  different formats in memory, and generally occupy
  different numbers of bytes, leading to a spurious
  result.

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This can be avoided

• Always turn on warning options, and make sure
  your program compile cleanly (i.e., compile
  successfully and without warning message)
• gcc -Wall main.c -o main
• main.c In function main' main.c6 warning
  double format, different type arg (arg 2)

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More about runtime errors after we learn some
basics about make
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Compiling larger program

• Example

• Compile green.o cc -c green.c
• Compile blue.o cc -c blue.c
• Link the parts together cc green.o blue.o

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Splitting your C program

• Be sure no two files have functions with same
  name in it, and no two files define same global
  variables.
• If you use global variables, be sure only one
  file defines them, and declare them in your .h as
  follows
• extern int globalvar
• To use functions from another file, make a .h
  file with function prototypes, and include those
  .h files within your .c files.
• At least one of the files must have a main()
  function.

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Dependency graph
Allow one to identify what needs to be compiled
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Dependency graph represented by makefile
Command to update targets

• Make
• read dependency info from makefile
• checks modification times of files
• whenever a file becomes "newer" than something
  that depends on it, make runs given command
  accordingly.

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Syntax of makefile

• target source file(s)command (must be preceded
  by a tab)
• A target given in the Makefile is a file which
  will be created or updated when any of its source
  files are modified.
• Command(s) given in subsequent line(s) (which
  must be preceded by a tab character) are executed
  in order to create the target file.

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Dependency on header file

• main.o data.h io.h main.c
• cc c main.c
• .h files are listed, but there are no references
  in their corresponding commands.
• the .h files are referred within main.c files
  through include data.h, etc.
• If you do not explicitly include these in your
  Makefile, your program will not be updated if you
  make a change to your header (.h) files.

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Running make

• The order in which dependencies are listed is
  important. If you simply type make and then
  return, make will attempt to create or update the
  first dependency listed.
• You can also specify one particular targets
  listed in the Makefile, and only that target (and
  its corresponding source files) would be made.

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Example of makefile

• SRC_FILESmain.c app.c bar.c lib.c
• OBJ_FILES(patsubst .c, .o, SRC_FILES)
• CFLAGS -c g
• LDFLAGS -g
• appexp (OBJ_FILES)
• gcc (LDFLAGS) o appexp OBJ_FILES
• .o.c
• gcc CFLAGS o _at_

• Make features
• Variables
• String manipulation functions

Pattern rule how to transform .c files into
corresponding .o files
_at_ Filename matched for the left side of the
rule filename matched for the right side
of the rule
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Example of makefile (contd)

• clean
• rm .o appexp
• MAIN_HDRSlib.h app.h
• LIB_HDRSlib.h
• main.o MAIN_HDRS
• app.o (MAIN_HDRS
• bar.oLIB_HDRS
• lib.o LIB_HDRS

• A target without source
• type make clean to delete
• all objects and executables

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Back to program debugging
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Runtime error

• Segmentation fault or bus error
• These runtime messages indicate a memory access
  error. Common causes include dereferencing a
  null pointer or uninitialized pointer
• out-of-bounds array access
• incorrect use of malloc, free and related
  functions
• use of scanf with invalid arguments

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Runtime error (contd)

• floating point exception
• caused by an arithmetic exception, such as
  division by zero, overflow, underflow or an
  invalid operation (e.g. taking the square root of
  -1). The operating system determines which
  conditions produce this error.
• Illegal instruction
• an illegal machine instruction is encountered. It
  occurs when code has been compiled for one
  specific architecture and run on another.

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GNU debugger gdb

• A debugger is a program that runs other programs,
  allowing the user to
• exercise control over these programs,
• examine variables when problems arise.

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Basic features of a debugger

• Allow one to step through the logic, e.g.,
• What statement or expression did the program
  crash on?
• If an error occurs while executing a function,
  what line of the program contains the call to
  that function, and what are the parameters?
• What are the values of program variables at a
  particular point during execution of the program?
  
• What is the result of a particular expression in
  a program?

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How to use gdb ?

• Compile your code with debug info
• -g option for compilation and linking
• Run your program under gdb
• gdb yourProg
• Now gdb load your program, and ready to execute
  the program

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gdb commands

• run command-line-arguments
• Starts your program as if you had typed
• a.out command-line arguments
• program will start running, and
• halts at the statement that causes runtime errors
  such as segmentation fault and bus error
• runs to the end if there is no runtime error

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Examining calling stack

• where
• Produces a backtrace - the chain of function
  calls that brought the program to its current
  place. The command backtrace is equivalent
• up
• go up one level in the calling stack
• down
• go down one level in the calling stack

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Display variable value

• print E
• prints the value of E in the current (stack)
  frame in the program, where E is a C expression
  (usually just a variable).
• display E
• Similar to print, except every time you execute a
  next or step, it will print out the expression
  based on the new variable values

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Changing data

• Change the data in a operating program
• (gdb) set op3

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Set breakpoints

• make the program stop at a given statement
• break place
• place can be function, filefunction,line,
  fileline, address (a physical address)
• Most common breakpoints are at the beginnings of
  functions, as in
• (gdb) break Traverse Breakpoint 2 at 0x2290
  file main.c, line 20
• break main stops at the beginning of execution

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Set breakpoints (contd)

• set breakpoints at a particular line in a file
• (gdb) break 20Breakpoint 2 at 0x2290 file
  main.c, line 20
• Conditional breakpoint
• (gdb) break operator if op2
• Break at function operator if op argument
  equals to 2
• When hitting a breakpoint, you'll get a message
  like this.
• Breakpoint 1, Traverse(head0x6110,
  NumNodes4)at main.c16(gdb)

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Executing program

• next (n)
• Executes the current line of the program and
  stops on the next statement to be executed. If
  the current line of the program contains a
  function call, it executes the function and stops
  at the next line.
• Step (s)
• Similar to next, except if current line is a
  function call, it executes next statement, and
  stops at the first line of the function

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Executing program

• finish
• Keeps doing nexts, without stepping, until
  reaching the end of the current function
• cont
• Continues regular execution of the program until
  a breakpoint is hit or the program stops
• Ctrlc
• Stop the current running program

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Other ways to run gdb

• Some process might hang around after running for
  a while
• Debugging an existing process
• gdb
• (gdb) attach 23548
• Debug the program as usual
• Release the process to continue normal operations
• (gdb) detach

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run gdb postmortem

• Core dumped recorded state of working memory
  of a program at a specific time, generally when
  the program has terminated abnormally a program
  when it encounters a run time error
• ulimit c unlimited
• ./testapp
• ..
• Segmentation fault (cored dumped)
• ls
• core.34523 testapp testapp.c
• gdb testapp core.34523

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Using GDB under Emacs

• Emacs provides a much better interface to gdb
  that saves a lot of typing and confusion.
• Executing Emacs command M-x gdb starts up a new
  window running gdb.
• If you set a breakpoint in main function and then
  run program with correct arguments, gdb will
  split the window in two, with your source code on
  the bottom and a gdb command line in the top.