The C Preprocessor: cpp

1
The C Preprocessor cpp

• Chapter 13

2
Whats CPP?

• Any source code line beginning with a are
  preprocessor directives
• The preprocessor runs before the compiler and
  follows the directives that are placed in the
  source, then passes the modified source on to the
  compiler.

3
Types of Directives

• include brings in the file specified and
  appends it to the source code.
• define
• Named constants identifier that is to be
  replaced with some other text.
• Macros More like a function, they actually
  allow you to use parameters. Work very well for
  very small functions. They have much less
  overhead than functions since theres no function
  call or return, just a simple text replacement.
  Macros are also type independent.
• Both named constants and macros should always
  have their names in all caps. It makes it much
  easier to see in your source what is and what
  isnt a preprocessor directive.

4
Includes

• System header files
• include ltstdio.hgt
• User defined header files
• include file.h
• path is relative from the directory your source
  code is in.
• What should be put in header files?
• defined macros constants
• function declarations (prototypes)
• inline function definitions
• type names defined with typedef
• includes with extreme caution
• When putting includes in header files, you should
  be careful not to have multiple header files
  including the same files (ie stdio.h). This
  would cause multiple copies of the included
  file(s) to be appended to your source code.
• Information that is only needed in one source
  file should not be put in separate header files.

5
Named Constants

• Some times you just need to use a constant value
  throughout your program.
• define MAX_SIZE 10
• Every occurrence of MAX_SIZE in your source will
  be replaced with the number 10.
• You can also use this to replace other kinds of
  text
• define PROMPT Please enter an integer
• define BEGIN define END

6
An Alternative to define

• C provides a way to specify a define as a flag to
  gcc. If you plan on changing your define
  regularly, this method keeps you from having to
  edit the source code every time. This comes in
  very handy for defining the platform youre
  running on or to turn on debugging.
• gcc Wall DPLATFORMFreeBSD sort.c
• define PLATFORM FreeBSD
• gcc Wall DDEBUG1 sort.c
• define DEBUG 1

7
Macros

• The following is a macro to compute absolute
  value of a number from the book.
• define ABS(n) ( (n) lt 0 ? (n) (n) )
• Lets assume we call dist ABS(x1-x2) in our
  code. The preprocessor expands this text to
• dist ( (x1-x2) lt 0 ? (x1-x2) (x1-x2) )
• Note that the parentheses are VERY important in
  the macro definition. Why?
• Macros are very hard to debug because you
  generally dont see the output of the
  preprocessor before it gets to the compiler. You
  can use gcc E to stop after the preprocessor and
  put the output to the screen.

8
Conditional Compilation

• One of the most useful reasons to use cpp is for
  conditional compilation. Lets say we want our
  program to be able to run on multiple platforms,
  but each platform requires some changes to your
  source code to be able to run properly. You
  could create a different version of your source
  for each platform that you want to run on, or you
  can use conditional compilation and let cpp
  change your source based on what platform youre
  running on.

9
Conditional Compilation

• In a previous slide, we defined a named constant
  called PLATFORM so lets use it to change our
  source code.
• if PLATFORMFreeBSD statements specific to
  FreeBSDendifif PLATFORMRedHat statements
  specific to RedHat Linuxendif
• The preprocessor if directive recognizes all of
  the standard C comparison operators and the
  logical operators like !, ,
• There is a else and elif directive as well

10
More Conditional Compilation

• ifdef
• Allows you to change your source code based on
  whether a named constant has been defined or not.
• ifdef DEBUG Debugging output statementsendif
  If DEBUG is defined regardless of what value it
  has, the output statements will be added to your
  source code.
• if defined(DEBUG) Debugging output
  statementsendifHas the same effect as the
  ifdef example above.
• ifndef
• The same as ifdef except will execute when the
  constant is NOT defined.

11
Predefined Macros

• _ _DATE_ _
• The current date
• _ _FILE_ _
• The source filename
• _ _LINE_ _
• The line number in the source file
• _ _STDC_ _
• 1 for ANSI compilers, 0 for non-ANSI
• _ _TIME_ _
• The current time