Unix Continuum of Tools

• Do something once: use the command line• Do something many times:

– Use an alias– Use a shell script

• Do something that is complex of with large amounts of data: Use a C program

• Examples– Command: pipes and redirection to search files– Shell script: apply a particular kind of filter using

different parameters at different times– C program: create a simulation program

C Advantages and Disadvantages

• Advantages– Super fast– Concise set of syntax– Portable: all systems can compile c– Very popular

• Disadvantages– Not object oriented meaning that procedures

and functions are written to operate on data constructs

History• 1972

– Dennis Ritchie creates the c language– The classic book by Kernigham and Ritchie defines

the syntax for this language.– 1990 ANSI C with some minor extensions– 1999 ANSI C with some more minor extensions

• 1979– C++ which is C with classes– Additional enhancements since then

• 1990s: Java– programming language based on C++ syntax

• 2002: C# – .net language with characteristics of Java and C++

Note: If you know Java, you know lots of C ("Just Like Java" or JLJ)

Making a C Program

• Create your program>vi hello.c

• Complile and link– The standard UNIX command: >cc <file>– The GNU command: >gcc <file>– Flags

• -o <file> creates an executable named <file> instead of a.out• -g create symbolic information for the gdb debugger• -l <library> include a run time library• -L <path> search path for libraries

• Debug: >gdb executable (we will cover this later)• To run: just type <executable> or ./<executable> if . is

not in the search path

Development Cycle

1. Design and code

2. Editor to enter the code: program.c

3. Compile: program.o

4. Link: program

5. If link errors, return to step 2

6. Execute program

7. If logic errors return to step 1

Note: Steps 3 and 4 is normally done by gcc in one step

Hello World Program/*

hello.c prints "hello world"   April 29, 2002: Kevin Sahr*/ #include <stdio.h> int main(int argc, char* argv[]){  

printf("hello world\n");   return 0; 

} /* main */

1. /* to */ are comments2. include merges source from

stdio.h3. main function runs first4. return 0 tells shell result ok,

return # indicates an error 5. printf is like java println,

but with more features6. the * is used to reference an

array called argv7. argc is the number of

command line arguments8. argv[0] has the program

name, command line arguments are in argv[1] .. argv[n]

9. { and } are JLJJLJ: each statement ends with ;

C Program Structure

• Comments– At top: File name, date, author, purpose– Throughout: clarify code as needed

• Preprocessor directives– C programs run through a preprocessor phase first– This phase: Include other source files, perform

modifications to the source file– Preprocessor directives do not end with ;

• Include the main function• Include other functions (Subject for a later week)

Comments

• C comments are /* … */– Do not nest comments– In labs, include the name of the file, your name, and

date at the top of every file you submit– Other comments will be optional, but use when the

code is not easy to follow

• The // format were introduced in C++– They presently are not part of C– They are proposed for the next version of C

Header files: stdio.h and stdlib.h

• Includes: analogous to Java Includes– stdio.h is a text file containing prototypes for

stream based I/O functions– stdlib.h another text file containing prototypes

to the C "standard library"

• Prototypes: – analogous to Java Interfaces– function bodies completed when linking

main() function

• main()– Every C program must have a main function– Analogous to Java "static void main(String[] args)"– Syntax: int main(int argc, char *argv[])

• argc: number of arguments• argv: an array of argument strings

– return statement returns to the caller• main() function returns to the shell• return 0; tells the shell that the run was successful• return #; returns an error code to the shell

Converting String Data• Commonly used functions

– Ascii to integer: atoi(string);

– Ascii to float: atof(string);

– String to double: strtod(char *start, char *end);

• ExampleInt main(int argc, char *args[])

{

int x = atoi(args[1]);

double y = atof(args[2]);

double z = strtod(args[2], NULL); // entire string

}