Introduction to C
Creating your first C program
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Writing C Programs

The programmer uses a text editor to create
or modify files containing C code.

C code is also called source code

A file containing source code is called a
source file

After a source file has been created, the
programmer must invoke the C compiler.
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Using the C Compiler

Invoking the compiler is system
dependent.
o At UMBC, we have two C compilers
available, cc and gcc.
o For this class, we will use the gcc compiler
as it is the compiler available on the Linux
system
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Invoking the Compiler Example
gcc -ansi -Wall pgm.c
where pgm.c is the source file that
contains a program
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The Result : a.out

If there are no errors in pgm.c, this
command produces an executable file,
one that can be run or executed.
o Both the cc and the gcc compilers name
the executable file a.out
o To execute the program, type a.out at
the Unix prompt.
Although we call this “compiling a
program”, what actually happens is
more complicated.
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3 Stages of Compilation
1 Preprocessor - modifies the source
code
o Handles preprocessor directives
o Strips comments and whitespace from the
code
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3 Stages of Compilation
2 Compiler - translates the modified
source code into object code
o Parser - checks for errors
o Code Generator - makes the object code
o Optimizer - may change the code to be
more efficient
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3 Stages of Compilation
3 Linker - combines the object code of
our program with other object code to
produce the executable file.
The other object code can come from:
o The Run-Time Library - a collection of
object code with an index so that the linker
can find the appropriate code.
o other object files
o other libraries
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Compilation Diagram
Editor <
Source File myprog.c
Compiler
Preprocessor
Parser
Code Generator
Optimizer
Object File myprog.o
Other Obj’s Run-time library Other libraries
Linker
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Executable file a.out
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An algorithm for writing code
Write the algorithm
Write the code using emacs (pico, vi)
Try to compile the code
While there are still syntax errors
Fix errors
Try to compile the code
Test the program
Fix any semantic errors
Compile the code
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Incremental Approach
to Writing Code

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Tips about writing code.
Write your code in incomplete but working
pieces.
For instance: For your project
o Don’t write the whole program at once.
o Just write enough that you display the
prompt to the user on the screen.
o Get that part working first.
o Next write the part that gets the value from
the user, and then just print it out.
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Incremental Approach
to Writing Code (continued)
o Get that working.
o Next change the code so that you use the
value in a calculation and print out the
answer.
o Get that working.
o Make program modifications:
• perhaps additional instructions to the user
• a displayed program description for the user
• add more comments.
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o Get the final version working.
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A Simple C Program
/* Filename:
Author:
Date written:
Description:
hello.c
Brian Kernighan & Dennis Ritchie
?/?/1978
This program prints the greeting
“Hello, World!”
*/
#include <stdio.h>
main ( )
{
printf (“Hello, World!\n”) ;
}
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Anatomy of a C Program
program header comment
preprocessor directives
main ( )
{
statement(s)
}
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The Program Header Comment
All comments must begin with the
characters /* and end with the
characters */
 The program header comment always
comes first
 The program header comment should
include the filename, author, date
written and a description of the program

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Preprocessor Directive
Lines that begin with a # are called
preprocessor directives
 The #include <stdio.h> directive causes
the preprocessor to include a copy of
the standard input/output header file
stdio.h at this point in the code.
 This header file was included because it
contains information about the printf ( )
function that’s used in this program.
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main ( )

Every program has a function called
main, where execution begins

The parenthesis following main indicate
to the compiler that it is a function.
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Left Brace

A left curly brace -- { -- begins the body
of every function. A corresponding right
curly brace must end the function.

The style is to place these braces on
separate lines in column 1.
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printf (“Hello, World!\n”) ;
This is the function printf ( ) being
called with a single argument, namely
the string “Hello, World!\n”
 Even though a string may contain many
characters, the string itself should be
thought of as a single quantity.
 Notice that this line ends with a
semicolon. All statements in C end
with a semicolon.
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Right Brace

This right curly brace -- } --matches the
left curly brace above.

It ends the function main ( ).
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Good Programming Practices
C programming standards are available
on the Web -- see course homepage
 You will be expected to conform to
these standards for all programming
projects in this class and in CMSC 201
 These standards include:
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o Naming conventions
o Use of whitespace
o Use of Braces
o Comments
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Examples of Comment Styles
/* a comment */
/*** another comment ***/
/*****/
/*A comment can be written in this
* fashion to set it off from the
* surrounding code.
*/
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More Comments
/*******************************************\
* If you wish, you can put comments *
* in a box. This is typically used for *
* program header comments and for *
* function header comments
*
\*******************************************/
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Use of Whitespace
Use blank lines to separate major parts
of a source file or function
 Separate declarations from executable
statements with a blank line
 Preprocessor directives, main(), and the
braces are in column 1

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Use of Whitespace
(continued)
All executable statements are indented
one tab stop. How deep should my tabs
be ?
 Either 3 or 4 spaces. Choose
whichever you like and use that same
number consistently throughout your
program.
 2 spaces are not enough for good
readability, more than 4 causes the
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