Crash Course in C
"A little learning is a dangerous thing." (Alexander Pope)
• The mechanics of program construction
• Structure of a C program
• Simple examples of C programs
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The Mechanics of Program Construction
Stages of Program Development
Architecture design
Detailed design
Edit source files
Refine design
Compile source files
Link Object files
Test and debug
executable program
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Writing C Code
• Once the program has been designed, it needs to be written in a
language that can be translated into a format that the computer can
understand
− English is a very loose language
− Computers are very rigid machines
− English description of the program must be converted to a computer
language (C, Ada, Fortran, Pascal, Java, Basic, Assembler, etc.)
• Writing a C program involves creating and editing C language text
files, called source files
• However, a computer can not execute, or run, a C source file
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Creating an Executable Program
source file source file source file source file
object file object file object file object file
link Runtime
Library
Executable
Code
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Compiling Source Files
• Once your program is written in C, it is ready to be translated into a
machine-readable language
• A compiler translates C statements into machine statements
− A compiler creates object code, which is an intermediary step between
source code and final executable code
− The compiler checks for syntax errors; e.g., Missing punctuation
− The compiler performs simple optimization on your code; e.g., eliminate a
redundant statement
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Linking Object Files
• The linker links together all object modules to form an executable
image of the program
• The output of the linker is an executable image, which can be loaded
into memory and executed
• The linker resolves any references to library functions
− If your program uses a library routine, like sqrt, the linker finds the object
code corresponding to this routine and links it within the final executable
image
• The linker is automatically invoked by the compiler
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Loading Your Program
• The loader loads your program into the computer's memory
• On most systems, this is performed automatically by the operating
system when you run the program
• Most embedded systems require you to explicitly run a loader program
to get the program into memory
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Mechanics of Program Construction - Review
• Let's review the steps involved in building a C program. This assumes
that you have already designed the program and defined it using the
steps above
1. Use an editor to create/edit the program and any data files
− Use meaningful names for files and variables; e.g., flightsim.c vs program3.c
2. Type in your program, complete with parentheses, braces, semicolons
3. Build (compile and link) your program. If compiler identifies errors (e.g.,
missing brace), edit your program and recompile.
4. Execute your program.
− Run your program using different test cases
− If program does not execute properly, edit and return to step 3
5. After program runs successfully, make printouts of source code and of test
results.
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Structure of a C program
• Program consists of one or more functions
• Function consists of header and body
− Header contains preprocessor statements and declarations
− Body, enclosed in {brackets} contains statements, each terminated by
semicolon
• One function must be one called main()
− The main function is where execution of the program begins
− The main function begins at the line containing main() and ends at the
closing brace on the last line of the source listing
− The body of the main function contains one or more statements that
describe the functionality of your program
• Program execution starts in main, and progresses, statement by
statement, until the last statement in main is completed
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Style of a C Program - Commenting and Indenting Your Code
• Properly commenting and indenting your code is an important part of
programming
− Enhance readability
− Allow others to understand more quickly
− Allow you to recall sooner
• Provide information at beginning of each source file that describes the
code, date modified, by whom
• Intersperse comments within the code to explain intent
• Style guide on the class webpage. Use it!
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Anatomy of a C Function
• A function, including the main function, contains the following
elements
− Function type fcn-type fcn-name (arg-declarations)
− Function name {
declarations;
− Left Parenthesis C-statements;
− Argument declarations* }
− Right parenthesis
− Left curly bracket
− Declarations*
− C statements*
− Right curly bracket
* = optional
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A Simple Example
• Let's define a function that computes the square of the number 5
1. File name: What shall we call the file? _______________
2. Function name: What shall we call the function? _______________
3. Function type: What type of value will the function return?
_____________
4. Executable statements: Write the statement to perform the computation ___
5. Return statement: Specify the output of the function __________________
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A Simple Example of C Code
/***************
* L. Fesq *
* This function *
* calculates the *
* square of 5 *
****************/
int main (void)
{
5 * 5;
return 0;
} /* end main */
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Critiquing our Code
• This function computes the square of 5, but what does it do with the
result?
• It would be helpful to store the result so we can use it again
• Use a variable to allocate memory to store the result, and to label it
with a relevant name:
int main (void)
{
int answer;
answer = 5 * 5;
return 0;
}
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Critiquing our Code - cont.
• If we try running our simple example, what happens? Will we be able
to see any results? How/why not?
• Add a line (or two) of code to help you see what is going on in this
program.
#include <stdio.h>
int main (void)
{
int answer;
answer = 5 * 5;
printf ("The square of 5 is %d\n", answer);
return 0;
}
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An Extension to our Simple Example
• The function would be more useful if it computes the square of any
integer number
• How can we provide input to the program?
#include <stdio.h>
int main (void)
{
int num;
int answer;
printf ("Enter the integer that you would like squared: \n");
scanf ("%d", &num);
answer = num * num;
printf ("The square of %d is %d\n", num, answer);
return 0;
}
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Another Extension to our Simple Example
• You can pass information from one function to another function when
running the program
• An argument allows you to supply information to a function, such as
the number that you want to square:
<fcn-type> <fcn-name> (<argument-type argument>)
1. How many arguments/inputs will be passed to the function? ____________
2. Argument type: What type of value will be passed? _____________
3. Argument name: What shall we call the argument? _____________
• Create a new function square, and have main call square with an
argument
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Sample Code
#include <stdio.h>
int square ( int num)
{
int answer;
answer = num * num;
printf ("The square of %d is %d\n", num, answer);
return 0;
}
int main (void)
{
square (5);
return 0;
}
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Sample Code for Returning a Value from a Function
#include <stdio.h>
int square ( int num)
{
int answer;
answer = num * num;
printf ("The square of %d is %d\n", num, answer);
return answer;
}
int main (void)
{
int final;
final = square (5);
return 0;
}
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Summary
• We now understand mechanics of program construction (post design)
− Create source file
− Type in C code
− Build: compile and link
− Execute: load and run
• Review structure of a C program - how do we write a C program?
− Program consists functions, one of which must be main
− Each function must follow Anatomy guidelines: fcn type, name, etc.
− You get points for style - comments, indent, etc.
• For Monday
− Read Chapter R4, especially 4.1 and 4.3. Skim other sections
− Read Sections C2.1-C2.5
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