1. C Programming
CISC3130, Spring 2011
Dr. Zhang 1

2. Midterm Review
Program: your C/C++ program, system command (ls, mv,…), a bash script, a PERL script, …
a static entity, a file exists in the disk
Process: a dynamic entity, representing each program that is being executed in the system
Program, user, states (running, waiting for I/O, ready), opened files, memory (current values of variables) …
Operating system schedules multiple processes in the system to best utilize system resources (CPU, disk, networking…)
Related command: ps, at, nice,
To run a program in background: use &

3. Midterm Review When running a program,
Standard input/output/error: can be redirected
Command line arguments: to pass arguments to a program
e.g., ls -l ~
e.g., headtail 6 6 file.txt
Return value: 0 for successful execution, 1 for failure or negative results
In C/C++ program, the main( ) function returns value
In Bash, you use exit 1 to return 1 to caller …

4. Standard input, output, error
On startup, each program (bash script, C/C++ program) has access to three files:
Keyboard input
Putty window, terminal 4

5. input/output redirection
Review of the syntax
./a.out > my_output
./a.out < my_input
./a.out >> existing_output
./a.out < input > my_output 2> error_msg
To discard output, redirect it to /dev/null
What’s happening ?
Shell opens/creates the files, and “starts” the program with its input/output/error redirected.
Will study how it is done (through system calls) later … 5

6. Simple example: can you write C++ version?
A very simple C program
#include <stdio.h>
main() {
char yourName[256];
printf ("Your name ?\n");
if (fgets (yourName,256,stdin)==NULL)
fprintf (stderr,"No input");
else
printf("hello, %s\n", yourName); } 6

7. Standard input Standard input (0):
Default: the keyboard
How to read standard input from a file ?
using <
How to read standard input from the script ?
Here document, <<
In bash script, use “read x” to read from standard input a value for variable x
In C++: use cin

8. Standard output Standard output (1):
Default: terminal (window); can be redirect using >, or >>
In C++: cout
In C ? Will study later in this class
Bash script: many commands write to standard output
echo command
printf command 8 8

9. Command line arguments
Strings passed after the command name; separated by spaces
e.g. ls –l ~, rm –i *.o
e.g. echo Hello world
e.g. pick `cat students.txt`
e.g. mv a.c b.c
String enclosed by quotes are treated as single argument
mv b.c “test b.c”,
mv b.c ‘test b.c’

10. Command line arguments (cont’d)
Commands interpret the arguments
Typically, dash (-) denotes an option
e.g. the “-l” options for ls means “long listing”
to remove a file named “-raw” ?
rm “-raw” does not work
rm ./-raw

11. Command line arguments (cont’d)
Access command line arguments
Bash: $1,$2,… $#, $*
Example: display_argument
#!/bin/bash
set -x
i=1
while [ $i -lt $# ] do
echo ${$i}
i=`expr $i + 1`
done

12. Script: reverse
Functions: display command line arguments in reverse order, i.e., last word first, to standard output
$ reverse apple banana orange
orange banana apple

13. Next C/C++ programming with a focus on
Standard I/O
File system access
System calls
To review Bash and Unix/Linux command
One new command every Tuesday
One bash new feature/exercise every Friday

14. C vs. C++: similarities Built-in data types: int, double, char, etc.
Preprocessor (handles #include, #define, etc.)
Control structures: if, while, do, for, etc. 14

15. C vs. C++: similarities Operators: Arithmetic: + - * / ++ --
Assignment: =
Comparison: == != < > >= <=
if (i<=10)
while (count!=20)
Arithmetic/assignment: += -= *= /=
counter += 1; // count = count+1;
temp /= 10.0; // temp = temp/10.0;
Logic: && || !
if (i<10 && !found) 15

16. C vs. C++: Similarities (cont’d)
There must be a function named main ()
Function definitions are done the same way
Can split code into files (object modules) and link modules together

17. C vs. C++: Differences C does not support any function overloading
C does not have classes/objects
All code is in functions (subroutines)
C structures can not have methods
C does not support any function overloading
You cannot have two functions with same name
In C++, compare (int first, int second) and compare(float first, float second) …
C does not have reference variables
remember in C++,
double calculate_Area (Circle & myCircle);

18. C vs. C++: differences (cont.)
C input/output (I/O) is based on library functions
printf, scanf , fopen, fclose, fread, fwrite,…
In C++: cin, cout, cerr
C does not have new or delete
Use malloc, and free functions to handle dynamic memory allocation/deallocation
C does not have string class
In C++:
string s("What we have here is a failure to communicate");
s=s+”!”;
string sub = s.substr(21);
cout << "The original string is " << s << endl;
cout << "The substring is " << sub << endl;    

19. C string
A “string” is an array of characters, terminated with null (‘\0’)
char myStr[10]=“Hello”;
myStr[0]=‘H’;
myStr[1]=‘e’;
myStr[2]=‘l’;
myStr[3]=‘l’;
myStr[4]=‘o’;
myStr[5]=‘\0’; H e l l o \0

20. Handling strings in C char myStr[10]=“Hello”;
Library functions for operations on string:
strcpy, strlen, strtok, strcat, …
All using the ‘\0’ to tell where does a string ends …
strcat (myStr, “!”);// append ! To the last non-null char
str_len = strlen (myStr); // str_len is set to 5, the actual // length of the string, not the size of the array H e l l o \0

21. Typical C Program includes Defines, data type
#include <stdio.h>
#include <stdlib.h>
#define MAX 1000
typedef chra bigstring[MAX};
char * reverse (char *s ){
char buf[MAX};
int Ii, len;
len=strlen(s);
printf (“receiving %s\n”,s);
for (i=0;i<len;i++)
buf[i] = s[len-i-l];
buf[i] =‘\0’;
strcpy (s,buf);
return s; }
void main (int argc, char ** argv){
if (argc==2)
printf (“%s\n”,reverse (argv[1]));
includes
Defines, data type
Definitions, global variable
declarations
Function definitions
main()

22. C Libraries Standard I/O: printf, scanf, fopen, fread …
String functions: strcpy, strspn, strtok, …
Math: sin, cos, sqrt, exp, abs, pow, log
System calls: fork, exec, signal, kill,…

23. Simple C Program
# include <stdio.h> int main (void) { printf (“Hello World \n”); return (0); }

24. Another Program
#include <stdio.h> void printhello (int n ){ int i; for (i=0;i<n;i++) printf (“Hello world\n”); } void main(){ printhello(5);

25. Compiling on Unix We can use GNU compiler named gcc
gcc –o reverse reverse.cc
Tells compiler to
Create executable file with the
Name reverse
Tells compiler the name of input
file

26. Quick I/O Primer: printf
int printf (const char *, … );
… means “variable number of arguments”, the first argument is required (a string).
Given a simple string, printf just prints the string (to standard output).

27. Simple printf
printf (“Hi – I am a string\n”); printf (“I\thave\ttabs\n”); char s[100]; strcpy(s, “printf is fun!\n”); printf (s);

28. Arguing with printf
You can tell printf to embed some values in the string – these values are determined at run-time
printf (“here is an integer: %d\n”, i);
%d is replaced by value of the argument , i.e., i

29. More integer arguments
printf (“%d + %d = %d\n”, x, y, x+y); for (j=99;j>=0;j--) printf (%d bottles of beer on the wall\n”,j); printf (“%d is my favorite number\n”, 17);

30. printf is dumb
%d is replaced by the value of the parameter when treated as an integer, even if the parameter is not an integer variable
printf (“print an int %d\n”, “Hi Dave”);
print an int
printf ("print an int %d\n",12.3);
print an int
printf ("print chars as unsigned int %u %u %u\n",'a','b','c');
print chars as unsigned int

31. Other formats
%d is a format – “treat the corresponding parameter as a signed integer”
%u means unsigned integer
%x means print as hexadecimal
%s means “treat it as a string”
%c is for character (char)
%f is for floating point numbers
%%: print a single ‘%’
e.g. printf (“%f%% of the population:\n”, 12.4);

32. Fun with printf char * s = “Hi Dave”;
printf (“the string \”%s\” is %d characters long\n”, s, strlen(s));
printf (“The square root of 10 is %f\n”,sqrt (10));

33. Controlling the output
There are formatting options that you can use to control field width, precision, etc.
printf (“square root of 10 is %20.15f\n”,sqrt(10);
square root of 10 is

34. Lining things up int i; for (i=0; i<5; i++)
printf (“%2d %f %20.15f\n”,I, sqrt(i),sqrt(i));
…..

35. Alas, we must move on
There are more formats and format options for printf
The man page for printf includes a complete description
man 3 printf

36. Input - scanf scanf provides input from standard input
scanf use pointers, or addresses

37. Remember Memory Every C variable is stored in memory
Every memory location has an address
In C, you can use variables called pointers to refer to variables by their address in memory

38. scanf int scanf (const char * format, … );
Remember “…” means “variable number of arguments”
Uses format string to determine what kind of variable(s) it should read
The arguments are the addresses of the variables
scanf (%d %d”,&x,&y);
x,y should be two integer variables

39. A simple example of scanf
float x; printf (“Enter a number\n”); scanf (“%f”,&x); printf (“Square root of %f is %f\n”,x,sqrt(x));

40. scanf and strings
Using %s in a scanf tells scanf to read the next word from input – not a line of input
char s[100];
printf (“Type in your name\n”);
scanf (“%s”,&s);
printf (“Your name is %s\n”,s);

41. Reading a line You can use function fgets to read an entire line
char *fgets (char *s, int size, FILE * stream);
size is the maximum # of chars
FILE is a file handle, for now, remember
stdin (a constant): standard input
Read a line (i.e., read character until newline is met or until reach maximum #) from specified file, and save to the string pointed to by s 41

42. Example of fgets
char s[101]; printf (“Type in your name\n”); fgets (s,100,stdin); printf (“Your name is %s\n”,s);

43. A Real C Program A Program that accepts one command line argument
Treats the command line argument as a string, and reverses the letters in the string
Prints out the reversed string
For example
$ ./reverse apple
elppa
$./reverse spring_song
gons_gnirps

44. reverse.c (1)
#include <stdio.h> /* printf */ #include <stdlib.h> /* malloc, free */ /* MAX is the size of largest string we can handle */ #define MAX 1000 typedef char bigstring[MAX];

45. reverse.c (2)
/* reverses a string in place returns a pointer to the string */ char * reverse (char * s){ bigstring buf; int I, len; len = strlen (s); // fine the length of the string for (i=0;i<len;i++) buf[i] = s[len-i-1]; buf[i] = ‘\0’; // null terminate ! strcpy (s,buf); // put back to s return (s); }

46. reverse.c (3)
void main (int argc, char ** argv) { if (argc<2){ printf (“Invalid usage: must supply s string\n”); exit(1); } printf (“%s\n”,reverse (argv[1]));

47. Using dynamic allocation
char * reverse (char * s) {
char * buf;
int I, len;
len = strlen (s); // fine the length of the string
buf = malloc (len+1);
for (i=0;i<len;i++)
buf[i] = s[len-i-1];
buf[i] = ‘\0’; // null terminate !
strcpy (s,buf); // put back to s
free (buf);
return (s); }

48. Compiling on Unix We can use GNU compiler named gcc
gcc –o reverse reverse.cc
Tells compiler to
Create executable file with the
Name reverse
Tells compiler the name of input
file

49. Running the program $ ./reverse hidave evadih
$ ./reverse This is a long string
sihT
$./reverse “This is a long string”
gnirts gnol a si sihT