C Program Structure (and tools)

1
C Program Structure (and tools)

• Today well talk generally about C development
  (plus a few platform specifics)
• Well develop, submit, and grade code in Windows
• Its also helpful to become familiar with Linux
• E.g., on shell.cec.wustl.edu
• For example, running code through two different
  compilers can catch a lot more easy to make
  errors

2
Writing a C Program
Visual Studio
Eclipse
emacs
editor
1 source file 1 compilation unit
C source files (ASCII text) .cpp
Also .C .cxx .cc
Programmer (you)
Also .H .hxx .hpp
C header files (ASCII text) .h
readme (ASCII text)
3
What Goes Into a C Program?

• Declarations data types, function signatures,
  classes
• Allows the compiler to check for type safety,
  correct syntax
• Usually kept in header (.h) files
• Included as needed by other files (to keep
  compiler happy)
• class Simple typedef
  unsigned int UINT32
  
• public
• Simple (int i) int usage
  (char program_name)
• void print_i ()
• private struct
  Point2D
• int i_ double
  x_
• double
  y_
• Definitions static variable initialization,
  function implementation
• The part that turns into an executable program
• Usually kept in source (.cpp) files
• void Simpleprint_i ()

4
A Very Simple C Program

• include ltiostreamgt // precompiler directive
• using namespace std // compiler directive
• // definition of function named main
• int main (int, char )
• cout ltlt hello, world! ltlt endl
• return 0

5
What is include ltiostreamgt ?

• include tells the precompiler to include a file
• Usually, we include header files
• Contain declarations of structs, classes,
  functions
• Sometimes we include template definitions
• Varies from compiler to compiler
• Advanced topic well cover later in the semester
• ltiostreamgt is the C label for a standard header
  file for input and output streams

6
What is using namespace std ?

• The using directive tells the compiler to include
  code from libraries that have separate namespaces
• Similar idea to packages in other languages
• C provides a namespace for its standard library
• Called the standard namespace (written as std)
• cout, cin, and cerr standard iostreams, and much
  more
• Namespaces reduce collisions between symbols
• Rely on the scoping operator to match symbols
  to them
• If another library with namespace mylib defined
  cout we could say stdcout vs. mylibcout
• Can also apply using more selectively
• E.g., just using stdcout

7
What is int main (int, char) ... ?

• Defines the main function of any C program
• Who calls main?
• The runtime environment, specifically a function
  often called something like crt0 or crtexe
• What about the stuff in parentheses?
• A list of types of the input arguments to
  function main
• With the function name, makes up its signature
• Since this version of main ignores any inputs, we
  leave off names of the input variables, and only
  give their types
• What about the stuff in braces?
• Its the body of function main, its definition

8
Whats cout ltlt hello, world! ltlt endl ?

• Uses the standard output iostream, named cout
• For standard input, use cin
• For standard error, use cerr
• ltlt is an operator for inserting into the stream
• A member operator of the ostream class
• Returns a reference to stream on which its called
• Can be applied repeatedly to references
  left-to-right
• hello, world! is a C-style string
• A 14-postion character array terminated by \0
• endl is an iostream manipulator
• Ends the line, by inserting end-of-line
  character(s)
• Also flushes the stream

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What about return 0 ?

• The main function should return an integer
• By convention it should return 0 for success
• And a non-zero value to indicate failure
• The program should not exit any other way
• Letting an exception propagate uncaught
• Dividing by zero
• Dereferencing a null pointer
• Accessing memory not owned by the program
• Indexing an array out of range can do this
• Dereferencing a stray pointer can do this

10
A Slightly Bigger C Program

• include ltiostreamgt
• using namespace std
• int main (int argc, char argv)
• for (int i 0 i lt argc i)
• cout ltlt argvi ltlt endl
• return 0

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int argc, char argv

• A way to affect the programs behavior
• Carry parameters with which program was called
• Passed as parameters to main from crt0
• Passed by value (well discuss what that means)
• argc
• An integer with the number of parameters (gt1)
• argv
• An array of pointers to C-style character strings
• Its array-length is the value stored in argc
• The name of the program is kept in argv0

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for (int i 0 i lt argc i)

• Standard (basic) C for loop syntax
• Initialization statement done once at start of
  loop
• Test expression done before running each time
• Expression to increment after running each time
• int i 0
• Declares integer i (scope is the loop itself)
• Initializes i to hold value 0 (not an
  assignment!)
• i lt argc
• Tests whether or not were still inside the
  array!
• Reading/writing memory we dont own can crash the
  program (if were really lucky!)
• i
• increments the array position (why prefix?)

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cout ltlt argvi ltlt endl

• Body of the for loop
• I strongly prefer to use braces with for, if,
  while, etc., even w/ single-statement body
• Avoids maintenance errors when adding/modifying
  code
• Ensures semantics/indentation say same thing
• argvi
• An example of array indexing
• Specifies ith position from start of argv

14
Lifecycle of a C Program
xterm
An IDE
window
console/terminal/window
Makefile
WebCAT
make
E-mail
turnin/checkin
make utility
Runtime/utility libraries (binary) .lib .a .dll
.so
Eclipse
compile
link
Visual Studio
C source code
Programmer (you)
debugger
precompiler
gcc, etc.
link
compiler
linker
executable program
compiler
object code (binary, one per compilation unit) .o
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Development Environment Studio

• Well follow a similar format most days in the
  course
• Around 30 minutes of lecture and discussion
• Then about 60 minutes of studio time
• Except for open studio/lab days, reviews before
  the midterm and final, and the day of the midterm
  itself
• In the studios, please work in groups of 2 or 3
• Exercises are posted on the course web page
• Record your answers to the exercises, and e-mail
  your answers to the course account when youre
  done
• Well migrate throughout the studio to answer
  questions
• Use studio time to develop skills and
  understanding
• A good chance to explore ideas you can use for
  the labs
• Exams will test understanding of the studio
  material
• Youre encouraged to try variations beyond the
  exercises