SEG3460 Tutorial 3

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SEG3460 Tutorial 3

• Multi-module programming in C

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From Source Code to ExecutablePreprocessing,
Compiling and Linking

• Preprocessing handles all directives
• E.g.
• Header file inclusion (include)
• Defining constant variables or functions
  (define)
• The above examples are basically copy-and-paste
  operations
• Compiling
• Convert the source code of each module into
  object files (.o)
• Under Unix gt gcc c module1.c An object file
  module1.o would then be created
• Linking
• Link all the modules and required libraries
  together to create an executable
• Under Unixgt gcc -o a.out module1.o module2.o

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Header FilesFunction Prototype

• Function prototype tell the compiler what types
  of arguments would an unknown function take and
  what types of value it returns
• For example, look at retstring.cinclude
  "retstring.h" char return_string(int i, double
  d) char string string (char )
  malloc(80) sprintf(string, "d .2lf", i,
  d) return string
• Then, what should be wrote in retstring.h, such
  that other modules that use this function can
  compile successfully?
• Only need to write the function prototype char
  return_string(int, double)
• Why retstring.c included retstring.h?

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Header FilesFunction Prototype (2)

• If we want to use return_string() in another C
  file, we include retstring.h. For example, here
  is a program called print12.c
• Compile them and link themUNIXgt gcc -c
  print12.c UNIXgt gcc -c retstring.c UNIXgt gcc -o
  print12 print12.o retstring.o UNIXgt print12 1
  2.33
• What if we didn't include retstring.h, like
  print12a.c?
• We would get a compiler warning UNIXgt gcc -c
  print12a.c print12a.c In function main'
  print12a.c7 warning assignment makes pointer
  from integer without a cast
• This warning comes because C implicitly assumes
  that all functions return integers (and take
  integer arguments) if you dont explicitly
  declare them
• A warning would not stop you to compile, but
  indicate you probably doing something wrong!!

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Header FilesUsing Standard Library

• C comes with a bunch of header files and
  libraries that are standard
• Suppose you want to use the log10 function in
  standard math library
• Type man log10 in console to see the usage
• First, look at a bad example badlog1.c, which use
  log10 but doesnt include math.h
• Compile itUNIXgt gcc -o badlog1 badlog1.c
  Undefined first referenced
  symbol in file
  log10 /var/tmp/cca007Fp1.o
  ld fatal Symbol referencing errors. No output
  written to badlog1

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Header FilesUsing Standard Library

• Add -lm as told in man-pageUNIXgt gcc -o
  badlog1 badlog1.c -lm UNIXgt badlog1 0.000000
• Include math.h, see goodlog1.cUNIXgt gcc -o
  goodlog1 goodlog1.c -lm UNIXgt goodlog1 4.477121
  
• Note that including standard library need angle
  brackets

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Header FilesShare variables across modules

• To define a global variable to use in multiple C
  files
• The way to do this is to define the variable in
  one file, and then declare it as an extern
  variable in another.
• For example, look at share1.c and share2.c
• Compile and runUNIXgt gcc -c share1.c UNIXgt gcc
  -c share2.c UNIXgt gcc -o share share1.o share2.o
  UNIXgt share 45
• extern here is to distinguish declaration from
  definition
• What if we didn't define GV as a regular global
  variable in share2.c? See share3.cUNIXgt gcc -c
  share3.c UNIXgt gcc -o share share1.o share3.o
  Undefined first
  referenced symbol
  in file GV
  share1.o ld fatal Symbol
  referencing errors. No output written to share

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Header FilesShare variables across modules (2)

• If we define GV twice, then the compiler will
  give us another error.
• Here, share4.c is identical to share2.c UNIXgt
  cp share2.c share4.c UNIXgt gcc -c share4.c
  UNIXgt gcc -o share share1.o share2.o share4.o
  ld fatal symbol GV' is multiply defined
  (file share2.o and file share4.o) ld fatal
  File processing errors. No output written to
  share
• Typically, it is recommended to put the extern
  declarations into a header file, and then the
  programs simply need to include the header file.
• For exmaple, twoshare1.c and twoshare2.c share
  the variables GV1 and GV2 through the header file
  twoshare.h
• Compile and runUNIXgt gcc -c twoshare1.c UNIXgt
  gcc -c twoshare2.c UNIXgt gcc -o twoshare
  twoshare1.o twoshare2.o UNIXgt twoshare 45 99

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Header FilesShare data types across modules

• You can also define types in header files, and
  then multiple C files can use the same type.
• For example, typeshare1.c and typeshare2.c share
  the definition of the type Person through the
  header file typeshare.h
• Compile and runUNIXgt gcc -c typeshare1.c UNIXgt
  gcc -c typeshare2.c UNIXgt gcc -o typeshare
  typeshare1.o typeshare2.o UNIXgt typeshare Jim
  Plank

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Make

• Make help you compile
• By default, it assumes that you have a file
  called makefile or Makefile in the current
  directory
• To override this default setting, add f flag
• The makefile needs to be in specific format,
• E.g.hw hw.o gcc -o hw hw.o hw.o hw.c gcc
  -c hw.c
• The first line is the target (left) and its
  dependencies.
• In the example tells make that the exectable hw
  is made from the object file hw.o.
• The second line is the command to compile the
  target.
• In the example, it tells make to compile hw using
  the command gcc -o hw hw.o

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Make (2)

• Two special cases all and clean
• The all specification says that if you type make
  all it will make all the object files and
  executables
• The clean specification says that if you type
  make clean, it will remove all of the executables
  
• See an example makefile