Preprocessor Commands

1
Preprocessor Commands

• Preprocessor commands are commands given directly
  to the compiler rather than instructions that
  form part of the program
• All preprocessor commands begin with the symbol
• We will look at the most common commands in the
  next few slides

2
include ltfilenamegt

• This directive tells the compiler to take code
  from the file with the name filename and paste it
  into the current file in the place of the
  directive
• It is typically used to include header files
• A header file contains function declarations

3
include filename

• This directive behaves in the same way as the
  previous slide except it is used with non
  standard file names
• For example if your files become to big, you can
  split them up and use the include directive to
  include them in your main file

4
define myconst 32

• The define directive is used to implement
  replacements
• The compiler will search through the code and
  wherever it sees the name following define it
  will replace it with the appropriate value
• The benefits of this is that you can alter the
  value in one place in the code and it makes code
  more legible

5
Example 1

• This function uses the define derivative to
  replace the word integer with i

• include ltstdio.hgt
• define integer i
• int main(void)
• printf(integer,42)

Digital Electronics EEE3017W R. Verrinder (2008)
5
6
Strings

• A string is a group of characters
• C does not have a specific data type to hold
  strings, it uses arrays of characters to store
  the string
• Most string functions are supported through
  library functions
• In order to mark the end of a string, C uses the
  \0 character or 0000 0000

7
Example 2

• Here is an example of a very basic function which
  prints out the word Cat

• include ltstdio.hgt
• int main(void)
• char mystring4
• mystring0C
• mystring1a
• mystring2t
• mystring3\0
• printf(mystring) //This prints the word Cat

Digital Electronics EEE3017W R. Verrinder (2008)
7
8
Null Terminator

• \0 is called the null terminator
• It is important that you leave one open character
  at the end of your string array for the null
  terminator
• You therefore always need one plus the number of
  characters in the string
• If you forget the null terminator, the program
  will print out all characters in memory until it
  encounters a null terminator

9
Example 3

• Here is an example of one function that is
  implemented in the string library. This function
  calculates the length of a string

• int len(const char instr)
• int count 0
• while (instrcount) count
• return count

Digital Electronics EEE3017W R. Verrinder (2008)
9
10
Library Functions

• C provides a set of standard library functions,
  called the ANSI C libraries
• These are broken down into logical groupings
• Library functions are used to implement much of
  the standard functionality used in high level
  programming languages

11
stdio.h

• This library is used for input and output
  functions
• It is a very vast library so we will only show a
  few functions here

12
int fprintf(stdout, char s, const char
format,)

• This function is very similar to printf except
  that it is used to print data to a file

13
int sprintf(char s, const char format,)

• This is very similar to fprintf, but the output
  is written into a string buffer s rather than a
  stream
• s must be large enough to hold the output
• The output is null terminated
• The function returns the string length excluding
  the null character

14
int scanf(const char format,)

• scanf is equivalent to fscanf(stdin, f, ) except
  data is taken in from a keyboard

15
int sscanf(char s, const char format,)

• This is like fscanf except that input is read
  from a string buffer s rather than a standard
  input

16
int fgetc(FILE stream)

• Returns next character from (input) stream
  stream, or EOF on end of file error

17
int putchar(int c)

• This function puts one character c on the screen
• It is equivalent to putc(c, stdout)

18
int puts(const char s)

• Write s (excluding terminating NULL) and newline
  to stdout. Returns non-negative on success, EOF
  on error